Every month, during the childbearing years, a woman's body prepares an egg for possible fertilization. A woman's entire supply of eggs actually develops when she is a fetus. A newborn girl has approximately 2 million immature eggs at birth. By the time puberty arrives, the number has been reduced to about 300,000 to 400,000 eggs.
Each month the hypothalamus of the brain tells the pituitary gland to secrete follicle-stimulating hormone (FSH) which causes the ovaries to produce a mature egg. The egg matures within a small cavity in your ovary called a follicle. A few days later, when the egg is at the proper stage, the pituitary gland secretes the next hormone called lutenizing hormone (LH) which causes the follicle to swell and burst through the wall of your ovary, releasing the egg. This is called ovulation. You have two ovaries, but in any given cycle, ovulation occurs from just one of them. In the mean time, the hormones estrogen and progesterone are causing the lining of the uterus to thicken and prepare for a possible pregnancy. Finger-like structures at the junction between your ovary and fallopian tube, called fimbriae, catch the egg when ovulation occurs, keeping it on the right course. The egg moves slowly into your fallopian tube, which connects your ovary and uterus. Sperm must reach the mature egg while it is in the fallopian tube in order for fertilization to occur. Once an egg is released from the ovary it only lives about one day, making timing crucial. If you have intercourse before or during this time, you can become pregnant. The process begins when you and your partner have sexual intercourse. When he ejaculates, your partner releases into your vagina semen containing up to 1 billion sperm cells. Each sperm has a long, whip-like tail that propels it toward your egg.
Hundreds of millions of these sperm swim up through your reproductive system. With the help of your uterus and fallopian tubes, they travel from your vagina, up through the lower opening of your uterus (cervix), through your uterus and into your fallopian tube. Many sperm are lost along the way. Only a fraction of the sperm reach the egg’s position in the fallopian tube. Fertilization occurs when a single sperm makes this journey successfully and penetrates the wall of your egg. Your egg has a covering of nutrient cells called the corona radiata and a gelatinous shell called the zona pellucida. To fertilize your egg, your partner’s sperm must penetrate this covering. At this point, your egg is about 0.005 inch in diameter, too small to be seen. Many sperms may try to penetrate the wall of your egg but in the end, only one succeeds and enters the egg itself. After that, the membrane of the egg changes and all other sperm are locked out. Fertilization is now complete.
Occasionally, more than one follicle matures and more than one egg is released. This can result in multiple births if each of the eggs is fertilized by a sperm.
If the egg and the sperm are united, the fertilized egg (zygote) must continue down the Fallopian tube and into the uterus, finally implanting itself into the uterine lining. The fertilized egg must then divide and develop normally to become a healthy fetus. If fertilization does not occur, the uterine lining is sloughed off in the monthly menstrual cycle. The complete cycle repeats itself roughly every 28-32 days.
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