The miracle of life begins with fertilization, or the moment when sperm meets egg. Like a chain of dominoes, this phenomenon is a well-choreographed sequence of events. If there’s any interruption in the chain, fertilization will not occur.

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The first factor is timing: Once a month, a woman ovulates and her egg is viable for 24 to 72 hours as it travels down one of the fallopian tubes. During this window, sperm must enter the body. Millions of ejaculated sperm then swim through acidic vaginal fluids, push through cervical mucus, and pass through the uterus into a fallopian tube to reach and fertilize the ovum. Along the journey, each sperm begins to shed its outer layer of proteins until the tip of its head, called the acrosome, is revealed. When the sperm finally reach the egg, many of them release a barrage of powerful acrosomal enzymes to break down the egg’s tough zona pellucida layer. Once one sperm plunges through and reaches the egg’s inner membrane, the egg destroys its receptors and blocks any other sperm from entering.

When the sperm arrives at the center of the egg, the sperm’s chromosomes begin to swell and line up with those already present in the egg. Both the egg and sperm nuclei have merged their genetic messages by aligning the male and female chromosomes; it’s at this point that the egg graduates to a zygote, or preembryo. The zygote must still travel for another week down the fallopian tube to the uterus. During that time, it divides and grows into a fluid-filled ball of cells called a blastocyst, which will grow into an embryo. At that point, it will implant in the uterus.

ADDITIONAL FACTS

  1. The blastocyst attaches to a sticky protein surface inside the uterus. About a week later, the endometrial cells lining the uterine cavity completely cover the embryo.
  2. It takes 36 hours for the newly fertilized zygote to cleave into two new cell