Chapter 8

Making Babies: The Reproductive System

IN THIS CHAPTER

Understanding what the reproductive system does

Getting the gametes together

Looking at the male and female reproductive organs

Responding to the body’s changes during pregnancy

Like all animals, humans have an instinctive knowledge of mating. However, only humans have a need to understand the processes of mating and reproduction. This chapter gives you information about the anatomy and physiology of reproduction. You’re on your own for info about dating and mating rituals.

Surveying the Functions of the Reproductive System

The anatomy and physiology of the reproductive system is dedicated to supporting your role in continuing the human species (whether or not you choose to fulfill that role). Or to look at it another way, the reproductive system is dedicated to making sure your admirable characteristics are present in the next generation. Or to look at it from the “selfish-gene perspective,” your reproductive system is the means by which some genes replicate themselves and fight on in the never-ending battle for continuing existence.

Following is an overview of what the reproductive system is responsible for:

• Making gametes: The gametes are made within the organs of the female and male reproductive systems. Also called the sex cells, gametes are of two kinds: The ova (singular, ovum) are the female gametes, and the sperm (singular, sperm) are the male gametes. Specialized cells generate the gametes in a cell-division process called meiosis (see the “Crossing over with meiosis” section, later in the chapter). At the cell level, the processes are essentially identical in female and male bodies. At the tissue, organ, system, and organism levels, the processes are very different. (We fill you in on the various processes throughout this chapter.)
• Moving gametes into place: However differently ova and sperm are made, if the reproductive system is to succeed in its purpose, one ovum and one sperm must make their way to the same place at the same time under the right conditions for them to fuse. Many of the reproductive system’s tissues and organs chaperone the gametes from the place and time of their production to another place, where they’re most likely to encounter their destiny.
• Gestating and giving birth: Only the female reproductive system has organs for gestating a fetus and giving birth. (See the “Pausing for Pregnancy” section, later in the chapter, for a detailed discussion of pregnancy.)
• Nurturing the newborn: The female reproductive system has tissues and organs specialized for nourishing the newborn for the first few months of life, until the baby is capable of digesting other food.

Making Gametes

The process of meiosis includes the sequence of cell-level events that result in the formation of sex cells (gametes) from somatic cells. Meiosis is the only cellular process in the human life cycle that produces haploid cells.

Somatic cells are diploid, meaning each cell nucleus contains two complete copies of the DNA that came into being in the zygote. Sex cells (gametes) are haploid, meaning each cell nucleus contains one copy of the DNA of the mother (somatic) cell. When two gametes fuse to form the zygote, each contributes its DNA to the new zygote, which is, therefore, diploid.

In the following sections, we expand on the process of meiosis and how female and male gametes are produced. We also touch on what causes these sex cells to create a male or female zygote.

Crossing over with meiosis

A cell divides by mitosis for purposes of asexual reproduction and for growth, development, replacement, and repair. A cell divides by meiosis, on the other hand, to produce gametes — that is, for purposes of sexual reproduction. The process of meiosis is similar in its mechanics to the process of mitosis, but the two have several key distinctions.

The most obvious difference is that meiosis has two parts, called meiosis I and meiosis II. Each part proceeds in a sequence of events similar to that of mitosis (prophase, metaphase, anaphase, and telophase). As we explain in Chapter 2, in mitosis, the mother cell is diploid, and both daughter cells are also diploid, each having one complete and identical copy of the mother cell’s genome. In contrast, meiosis, when it functions optimally, results in four haploid daughter cells. What’s more, the four haploid genomes are all different.

The early stages of meiosis include a mechanism called crossing-over or recombination for exchanging genes between chromosomes. The result is that the cell that becomes the gamete (one of the four haploid products of meiosis) is carrying chromosomes that are completely unique and not identical to the mother cell’s chromosomes.

Note that replication of DNA does occur in meiosis, during the interphase that precedes the onset of meiosis I. After two sequential dichotomous divisions, the two complete copies are distributed among the four daughter cells.

Producing female gametes: Ova

A mature ovum (see Figure 8-1) is one of the largest cells in the human body, about 120 micrometers in diameter (about 25 times larger than sperm) and visible without magnification. The ovum contains a haploid nucleus, ample cytoplasm, and all the types of organelles usually found in the somatic cell, all within a plasma membrane. The plasma membrane is enclosed within a glycoprotein membrane called the zona pellucida, which protects the zygote and pre-embryo until implantation.

FIGURE 8-1: The human ovum.

Oogenesis (the development of ova) in humans begins in embryonic and fetal development with specialized somatic cells called oogonia. A few of these cells head down the path of meiosis, producing cells called primary oocytes. However, this meiosis is suspended at the prophase I point until the female reaches puberty. At birth, the human female has about 700,000 oogonia and primary oocytes in suspended meiosis.

The primary oocyte undergoes meiosis I, producing two cells, called a secondary oocyte and the first polar body. Most of the primary oocyte’s cytoplasm moves to the secondary oocyte. The first polar body undergoes meiosis II, and its daughter cells degenerate.

The cells released from the ovary at ovulation are secondary oocytes. If the secondary oocyte isn’t fertilized, it degenerates without undergoing meiosis II.

When (or if) a sperm initiates fertilization, the secondary oocyte immediately undergoes meiosis II, producing the ovum (plus a second polar body, which degenerates). Following fertilization, the ovum contains the sperm nucleus, and after approximately 12 hours, the two haploid nuclei fuse, producing the zygote.

Developing male gametes: Sperm

A mature sperm has three parts: a head that measures about 5 x 3 micrometers, containing a haploid nucleus; a short middle section; and a long flagellum. The sperm is adapted for traveling light — it has very little cytoplasm. The head is covered by a structure that contains enzymes that break down the ovum’s membrane to allow entry. The middle contains mitochondria and little else. Mitochondria produce the energy that fuels the sperm’s highly active flagellum, which propels the sperm through the female reproductive tract.

The process of sperm development (spermatogenesis) from meiosis to maturation takes place inside the testes. Specialized cells called spermatogonia divide by mitosis to produce another generation of spermatogonia. Mature spermatogonia, called primary spermatocytes, divide by meiosis, producing four haploid gametes.

Similar to the case with females, males are born with spermatogonia in their seminiferous tubules, which remain dormant until puberty. During puberty, hormonal mechanisms pull the spermatogonia out of dormancy.

In contrast to oogenesis, which is cyclic, spermatogenesis is continuous beginning at puberty and continuing lifelong in some men. In contrast to the one-per-month gametogenesis in females, males produce astronomical numbers of sperm. Each ejaculation produces about 1 teaspoon of semen, which contains about 400 million sperm in a matrix of seminal fluid. Mature sperm can live in the epididymis and vas deferens for up to six weeks.

Merging ova and sperm to determine sex

An important difference between males and females is that in females, all chromosome pairs are made up of two identical-looking strands, whereas in males, the strands are different from each other. This difference is easily visible under a high-power microscope: One of the pair is “normal” length (about the same length as all the other chromosomes) and the other is markedly shorter than all other chromosomes. The first is called the X chromosome, and females have one set of them in all their somatic cells. The second is called the Y chromosome, and males have a mismatched pair (one X and one Y chromosome) in all their somatic cells. After meiosis in the female, all ova have one X chromosome. After meiosis in the male, each sperm has either an X or a Y. The fusion of an ovum with an X sperm produces a female (XX) zygote. The fusion of an ovum with a Y sperm produces a male (XY) zygote.

The Egg-citing Female Reproductive System

Among mammals, which include humans, the female body is specialized for reproduction to a much greater extent than the male body. The following sections introduce you to the various organs of the female reproductive system and walk you through the female menstrual cycle (which has to occur to prepare a woman’s body for pregnancy).

A woman’s reproductive organs

The organs of the female reproductive system are concentrated in the pelvic cavity. Many of the female reproductive organs are attached to the broad ligament, a sheet of tissue that supports the organs and connects the sides of the uterus to the walls and floor of the pelvis. The following sections go into detail about each of the woman’s reproductive organs.

Ovaries

The ovaries are two almond-shaped structures approximately 2 inches (5 centimeters) wide, one on each side of the pelvic cavity. They house groups of cells called follicles.

The ovaries are the primary sex organs because they’re the site of oogenesis, the process of oocyte maturation. The ovaries also have a major role in endocrine signaling, especially the production and control of hormones related to sex and reproduction.

Beginning at the female’s puberty, the process of ovulation begins. The oogonia that have been dormant in her ovaries since early in her fetal development are hormonally activated, and secondary oocytes are released at a rate of approximately one per month from menarche (the first menstrual period) to menopause (the last menstrual period) — that is, from her early teen years to her late 40s or early 50s. The human female ovulates about 400 times during her lifetime.

Uterus

The uterus, or womb, nourishes and shelters the developing fetus during gestation. It’s a muscular organ about the size and shape of an upside-down pear. The walls of the uterus are thick and capable of stretching as a fetus grows.

The lining of the uterus, called the endometrium, is built and broken down in the menstrual cycle, which we cover in the “A woman’s (approximately) monthly cycle” section, later in the chapter. A portion of the endometrium (deciduas basalis) becomes part of the placenta during pregnancy.

The uterine cervix (neck) is a cylindrical muscular structure about 1 inch (2.5 centimeters) long that rests at the bottom of the uterus like a thimble. It controls the movement of biological fluids and other material (not to mention, occasionally, a baby) into and out of the uterus. Normally, the cervix is open ever so slightly to allow sperm to pass into the uterus. During childbirth, the cervix opens wide to allow the fetus to move out of the uterus.

Fallopian tubes

The fallopian tubes run from the ovary to the uterus. They’re not connected to the ovaries; they just kind of hang over them. These tubes transport the released ovum to the uterus during the monthly cycle, or the pre-embryo (early-stage fetus) to the uterus in the event of conception.

Vagina

The vagina is the part of the female body that receives the male penis during sexual intercourse and serves as a passageway for sperm to enter into the uterus and fallopian tubes. The vagina is about 3 to 4 inches (8 to 10 centimeters) in length. The uterine cervix marks the top of the vagina.

During childbirth, the vagina must accommodate the passage of a fetus weighing, on average, about 7 pounds (3 kilograms), so the vagina’s walls are made of stretchy tissues — some fibrous, some muscular, and some erectile. In their normal state, the vagina’s walls have many folds, much like the stomach’s lining. When the vagina needs to stretch, the folds flatten out, providing more volume.

Vulva

In females, the external genitalia comprise the labia majora, labia minora, and the clitoris. Together, these organs are called the vulva. The term labia (singular, labium) means “lips.” The labia of the vulva are loose flaps of flesh, just like the lips of the mouth. The labia protect the vagina’s opening and cover the pelvis’s bony structures.

Here are some details about the three parts of the vulva:

• Labia majora: These large folds of skin — one fold on each side — cover the smaller labia minora. The labia majora extend from the mons pubis (pubic mound) back toward the anus. The mons pubis contains fat deposits that cover the pubic bone. Following puberty, pubic hair covers the mons pubis and the labia majora.
• Labia minora: These hairless folds of skin lie underneath the labia majora and cover the opening of the vagina. The labia minora are attached near the vaginal opening and extend upward, forming the foreskin that covers the clitoris.
• Clitoris: This part of the vulva, located above the vagina’s opening and above the urethra, has a shaft and glans tip, just as a penis does, and it’s extremely sensitive to sexual stimulation. The clitoris contains erectile tissues that fill with blood during sexual stimulation. Because the tissue of the labia minora cap the clitoris, the swelling and reddening is also obvious in the labia minora. Stimulation of the clitoris can lead to orgasm in the female. Although females don’t ejaculate, females do experience a building and release of muscular tension. Female orgasm causes the muscle tissue that lines the vagina and uterus to contract, which helps to pull the sperm up through the reproductive tract.

Breasts

Like other female mammals, female humans have mammary glands (breasts) that produce a substance called milk for the nutrition of relatively helpless infants with high calorie requirements. Besides nutrition, breast milk boosts the infant’s immune system.

The breast contains about two dozen lobules that are filled with alveoli and contribute to ducts. The ducts merge at the nipple. Inside the alveoli are milk-producing cells. During puberty, the lobules and ducts form, and adipose tissue is deposited under the skin to protect the lobules and ducts and give shape to the breast. During pregnancy, hormones increase the number of milk-producing cells and increase the size of the lobules and ducts.

After the infant is born, the mother’s pituitary gland secretes the hormone prolactin, which causes the milk-producing cells to create milk, and lactation begins. The infant suckles the milk out of the ducts through the nipple. Lactation continues as long as a child nurses regularly.

The hormone oxytocin is strongly involved in milk release (also known as the let-down reflex). Stimulation of the nipple prompts the secretion of oxytocin from the mother’s pituitary gland. Oxytocin expels milk from the lobules by causing them to contract, just as it stimulates uterine contractions to expel the fetus. This hormone has also been strongly correlated with neuro-emotional phenomena, such as family bonding.

A woman’s (approximately) monthly cycle

The menstrual cycle (monthly cycle) consists of the ovarian cycle and the uterine cycle, both of which are approximately 28 days in duration (see Figure 8-2). These cycles run concurrently to prepare the ovum and the uterus, respectively, for pregnancy.

FIGURE 8-2: The menstrual cycle.

By convention, the first day of menstrual bleeding is counted as Day 1 of the menstrual cycle. Menstrual bleeding begins at a point in the cycle when the levels of estrogen and progesterone are at their lowest. However, the entire menstrual cycle is directed by several hormones, not just estrogen and progesterone.

The next sections outline the phases of a woman’s (monthly) menstrual cycle, up to menopause.

Looking at the ovarian cycle

The 28-day ovarian cycle is the most important part of the menstrual cycle because it’s responsible for producing the hormones that then control the uterine cycle (which we tell you about in the next section). From Day 1 to Day 13, triggered by low estrogen level, follicle-stimulating hormone (FSH) stimulates the development of a follicle and luteinizing hormone (LH) stimulates the maturation of an oocyte in one of the ovaries. When the follicle is developed enough, it begins to secrete estrogen. When the level of estrogen reaches the appropriate level, a negative feedback mechanism involving the hypothalamus slows the secretion of FSH and LH. When the follicle is fully mature and the oocyte is ready to be released, FSH and LH secretion peaks. On Day 14, the oocyte is released (ovulation). An oocyte lives for only 12 to 24 hours after ovulation.

At the time of ovulation, the anterior pituitary gland, which has been secreting FSH and LH simultaneously, secretes a surge of LH that causes the follicle from which the oocyte was released to become a corpus luteum (yellow body). The corpus luteum secretes the hormone progesterone, which triggers the hypothalamus. When the corpus luteum has secreted a sufficient amount of progesterone, the hypothalamus stops the anterior pituitary gland from secreting any more LH. At that point, the corpus luteum begins to shrink (about Day 17). When the corpus luteum is gone (about Day 26), the levels of estrogen and progesterone are at the lowest levels of the cycle (sometimes causing symptoms of premenstrual syndrome), and menstruation starts (about Day 29, or Day 1 of the new cycle).

Like any cycle, the whole process starts over. When the level of estrogen is low during menstruation, the hypothalamus detects the low level and secretes gonadotropin-releasing hormone (GnRH), which prompts the anterior pituitary gland to release its gonadotropic hormone — FSH — so another follicle is stimulated to develop a new oocyte that secretes estrogen. And now you’re back to the first paragraph of this section.

Clocking the uterine cycle

The 28-day uterine cycle, which aims to prepare the uterus for a possible pregnancy, overlaps with the ovarian cycle.

• Days 1 to 5: The first 5 days of the uterine cycle is when the level of estrogen and progesterone are lowest — the period of menstruation. The low level of sex hormones fails to prevent the tissues lining the uterus (the endometrium) from disintegrating and shedding. As the hormone levels drop, blood vessels spasm, cells undergo autolysis (self-destruction), tissues tear apart from the uterine wall, and blood vessels rupture, causing the bleeding that occurs during a period. The blood and tissue (menstrual flow) passes out of the uterus through the cervix and then out of the body through the vagina.
• Days 6 to 14: During this proliferative phase, estrogen production is highest. The developed follicle secretes estrogen, which makes the endometrium regenerate fresh tissue. The tissues lining the uterus and the glands in the uterine wall grow and develop an increased supply of blood. All these changes are preparation for nourishing an embryo and supporting a pregnancy, should the oocyte, which is released on Day 14, become fertilized and implant in the wall of the uterus.
• Days 15 to 28: During this secretory phase, the corpus luteum secretes an increasing level of progesterone, which further thickens the endometrium, and the glands of the uterus secrete a thick mucus. If the egg becomes fertilized, the thickened endometrium and mucus help to “trap” the fertilized egg so it implants properly in the uterus. If the egg doesn’t become fertilized within a day or two, the corpus luteum begins to shrink because it won’t be needed for a pregnancy. As the corpus luteum shrinks, the progesterone and estrogen levels decline, which causes the endometrium to “shred and shed” just before menstruation.

Winding down the cycle

Physiologically, menopause essentially reverses the hormonal pathway of adolescence. When a woman enters menopause, her ability to reproduce ends — ovulation stops, and she no longer can become pregnant. She may also experience hot flashes and sweat baths if faulty signals from the parasympathetic nervous system disrupt the body’s ability to accurately monitor its temperature. Other body processes slow down, including cellular metabolism and the replacement of the structural proteins in the skin, which leads to wrinkles. A woman’s bones may also weaken when the breakdown of bone tissue occurs faster than the buildup of bone tissue during bone remodeling.

Menopause is one of the unique aspects of human physiology. Note that reproductive cycling declines and stops as the female ages. That happens in many mammal, bird, and reptile species (although relatively few individuals in any species live long enough to experience the decline of their reproductive capabilities).

The unique part is that human females often live a substantial proportion of their life span beyond their reproductive capacity. (A woman in her 80s has lived around 40 percent of her life after menopause.) Much research into this phenomenon is concentrated at the interface of biology and culture. One theory holds that an adult female with no offspring of her own to feed tends, instead, to feed her grandchildren or other children in her community. Children with grannies eat better, the theory goes, improving their chances of surviving to reproductive age and pushing her genes into one more generation.

Furthering Fertilization: The Male Reproductive System

The male reproductive system produces sperm and moves them into the female reproductive system. On a rare occasion (relative to the astronomical number of sperm that the average human male produces), a sperm fertilizes an egg. All the billions and billions of other sperm a man produces in his lifetime have a limited life span — about six weeks.

The female gamete is released from the ovary as a secondary oocyte. Only when fertilization is initiated does the ovum (egg) come into being.

A man’s reproductive organs

The organs of the male reproductive system produce gametes, called sperm, and transfer them to the female reproductive system. In contrast to some other organ systems, and especially to the reproductive system of females, the male reproductive organs are located in an exposed location on the periphery of his body. The heart, lungs, kidneys, and the female’s ovaries and uterus are all located beneath protective layers of skin, muscle, and other connective tissue and suspended in a fluid matrix, all of which support homeostasis in these organs. The exposed location of the male reproductive organs is common in mammals but unusual in other vertebrates.

Testes and scrotum

The testes (singular, testis) are paired organs that produce sperm and hormones. Like the ovaries of the female reproductive system, the testes are the site of gamete production and, therefore, the primary sex organs.

The testes contain fibrous tissue that forms compartments in the testis. Inside the compartments are long, coiled seminiferous tubules, the site of spermatogenesis, the process of sperm development from meiosis to maturation. The walls of the seminiferous tubules are lined with thousands of spermatogonia (immature sperm). The seminiferous tubules also contain Sertoli cells that nourish the developing sperm and regulate how many of the spermatogonia are developing at any one time.

The epididymis (plural, epididymides) is a long, cordlike structure that lies atop each testis. The epididymis is continuous with (that is, “becomes”) the vas deferens, which is the final place of maturation for sperm. The vas deferens is a tube that connects the epididymis of each testis to the penis.

The testes are held within the scrotum beneath and outside of the abdomen. The scrotum contains smooth muscle that contracts when the scrotal skin senses cold temperatures and pulls the scrotum (and thereby the testicles) closer to the body to keep the sperm at the right temperature. The scrotum’s inside muscle layers are an outpouching of the pelvic cavity. The scrotum’s outside skin is continuous with the skin of the perineum and groin.

Prostate gland

Several other structures secrete the substances that make up the ejaculatory fluid that provides a matrix for the propulsion of sperm into the female reproductive tract. Among these are the prostate and the seminal vesicles. The prostate also contains some smooth muscles that help expel semen during ejaculation.

Penis

The penis consists of a shaft and the glans penis (tip). The tube-shaped urethra runs through the shaft of the penis, and the glans penis contains the urethral orifice. The semen is ejaculated through the urethra and urethral orifice. Foreskin (also called prepuce) covers the glans penis, although the foreskin of newborn males is often removed in a surgical procedure called circumcision.

During sexual arousal, erectile tissue enables the penis to be inserted into the female vagina, delivering the sperm to the vicinity of the secondary oocyte (if one is available).

Seminal fluid and ejaculation

The seminal vesicles, glands located at the juncture of the bladder and vas deferens, have ducts that allow the fluid they produce to sweep the sperm from the vas deferens into the urethra.

Next, the prostate gland adds its fluid, which contains mainly citric acid and a variety of enzymes that keep semen liquefied. The prostate gland surrounds the urethra just below where it exits from the urinary bladder.

The bulbourethral glands, also called Cowper’s glands, sit within the floor of the pelvis near the bulb of the penis on either side of the urethra. These two small glands have ducts leading directly to the urethra.

These three types of glands — the seminal vesicles, the prostate gland, and the bulbourethral glands — secrete fluids that have several functions:

• They’re slightly basic with a pH of 7.5, just the way sperm like their environment to be.
• They nourish the sperm by providing the sugar fructose so the sperm’s mitochondria can make enough energy to move its tail and travel all the way to the egg.
• They contain prostaglandins, which are chemicals that make the uterus reverse its downward contractions. When the uterus contracts, the sperm are pulled farther upward into the female’s reproductive tract.

As the glands add the secretions, forming the semen, pressure builds up on the structures of the male reproductive tract. When the pressure has reached its peak, the semen is expelled out of the urethra through the penis. Peristaltic waves and rhythmic contractions move the sperm through the vas deferens and urethra. The term for this discharge is ejaculation — part of orgasm in males, as is the contraction and relaxation of skeletal muscles at the base of the penis. As the muscles contract rhythmically, the semen comes out in spurts.

Pausing for Pregnancy

Pregnancy is established in two stages: fertilization of the secondary oocyte and implantation of the blastocyst in the uterus. The female body makes many and various adaptations to accommodate pregnancy and delivery, which we examine in the following sections.

Achieving fertilization

Ovulation sends a secondary oocyte from the follicle of the ovary into the uterine tube. Then, within an appropriate time, heterosexual intercourse results in the ejaculation of semen into the vagina. Some few million sperm make their way through the cervix, up through the uterus, and into the uterine tube to the vicinity of the waiting secondary oocyte.

To achieve fertilization, one sperm must penetrate the secondary oocyte’s membrane, which initiates meiosis II, and its nucleus must fuse with that of the ovum. At this point, the secondary oocyte is fertilized, the ovum is developed, and, with the fusion of the nuclei, the zygote comes into being.

The probability of any act of intercourse resulting in fertilization is actually quite low because many complicating factors exist. The timing of intercourse relative to ovulation is crucial. The released secondary oocyte is viable for only a matter of hours; sperm live a little longer in the female reproductive tract (12 to 72 hours). The environment within the female reproductive tract may be more or less hospitable to the sperm, depending on the female’s hormone levels and other physiological processes. Even when a single sperm has made contact with the secondary oocyte, fertilization is not assured.

Succeeding with implantation

Following fertilization, the zygote divides immediately. Several more cell division cycles take place as the pre-embryo moves down the uterine tube. Experts believe that many pre-embryos die at this stage, sometimes because of genetic or developmental abnormalities. Only if the pre-embryo arrives at the uterus and properly embeds itself into the endometrium is pregnancy established.

A successfully implanted pre-embryo, now called a blastocyst, begins immediately to take over its mother’s body. It begins to produce a hormone called human chorionic gonadotropin (hCG), which maintains the corpus luteum, elevates levels of progesterone and estrogen, and inhibits menstruation.

Adapting to pregnancy

The maternal body responds to pregnancy with many anatomical and physiological changes to accommodate the growth and development of the fetus. Most structures and processes revert to the nonpregnant form (more or less) after the end of the pregnancy.

Uterus

During pregnancy, the uterus grows to about five times its nonpregnant size and weight to accommodate not only the fetus but also the placenta, the umbilical cord, about a quart of amniotic fluid, and the fetal membranes. The size of the uterus usually reaches its peak at about 38 weeks gestation. During the last few weeks of pregnancy, the uterus has expanded to fill the abdominal cavity all the way up to the ribs. The size of the expanded uterus and the pressure of the full-grown fetus may make things difficult for the mother.

The placenta acts as a temporary endocrine gland during pregnancy, producing large amounts of estrogen and progesterone by 10 to 12 weeks. It serves to maintain the growth of the uterus, helps to control uterine activity, and is responsible for many of the changes in the maternal body.

Near the end of pregnancy, the uterine cervix softens. Enlarged and active mucus glands in the cervix produce the operculum, a mucus “plug” that protects the fetus and fetal membranes from infection. The mucus plug is expelled at the end of the pregnancy. Additional changes and softening of the cervix occur at the onset of labor.

Ovaries

Hormonal mechanisms prevent follicle development and ovulation in the ovaries during pregnancy.

Breasts

The breasts usually increase in size as pregnancy progresses and may feel inflamed or tender. The areolas of the nipples enlarge and darken. The areola’s sebaceous glands enlarge and tend to protrude. By the 16th week (second trimester), the breasts begin to produce colostrum, the precursor of breast milk.

Other organ systems

Pregnancy affects all organ systems as they support the growth and development of the fetus and maintain homeostasis in the female. Here are a few important physiological consequences of pregnancy:

• The other abdominal organs are displaced to the sides as the uterus grows.
• Decreased tone and mobility of smooth muscles slows peristalsis and enhances the absorption of nutrients. An increase in water uptake from the large intestines increases the risk for constipation. Relaxation of the cardiac sphincter may increase regurgitation and heartburn. Nausea and other gastric discomforts are common.
• Increases occur in blood volume, cardiac output, body core temperature, respiration rate, urine volume, and output from sweat glands.
• Immunity is partially suppressed.
• Spinal curvature is realigned to counterbalance the growing uterus. Slight relaxation and increased mobility of the pelvic joints prepares the pelvis for the passage of the infant. This can compromise the woman’s lower-body strength starting in the second trimester.

Going through labor and delivery

Labor is initiated by complex hormonal signaling between the maternal and fetal bodies. In the ideal labor and delivery process, powerful contractions of the smooth muscle of the uterus push the fully mature fetus (infant) past the cervix and down the birth canal without undue trauma to either the mother or the infant.

Labor occurs in three stages:

• Stage 1 begins with regular, effective uterine contractions. The uterine cervix begins to efface (become thinner and wider), and the amniotic membrane ruptures.

• Stage 2 involves continued uterine contractions pushing the fetus down through the birth canal. The cervix becomes fully effaced. The mother is aware of the passage of the fetus and may feel a strong urge to bear down and actively push the infant along.

  At the transition phase, the infant emerges head first from the birth canal, marking the end of this stage of labor.

  Just after delivery, the infant’s umbilical cord is cut and tied off. The infant is now totally separated from the mother and will soon have a stylish belly-button.

• Stage 3 starts after the baby is delivered. Uterine contractions continue so the placenta separates from the uterine wall. About 15 minutes after the baby is born, the placenta passes through the birth canal. Uterine contractions continue, during which the uterus contracts, eventually returning to near prepregnancy size.