Any sixth grader can tell you how babies are made. But it's one thing to be fresh out of biology class and thinking you know it all, and quite another to be a grownup trying to create a new life. Unfortunately, many of us (even some of us who have had babies) are a bit fuzzy on certain key biological facts -- like where in the woman's body conception actually takes place, and what route the fertilized egg takes from there. For those of us who need a refresher, here's a review of how babies are made.

Every female baby is born with all the eggs she'll ever have -- 1 million to 2 million of them. By the time she hits puberty only 300,000 to 500,000 will remain in her ovaries. That should be more than enough because women release only 400 to 500 in a lifetime, beginning with the first period and ending with menopause -- a span of 30 to 40 years.

The eggs are nourished within the ovaries, and each month one fully matured egg is released. A mature egg has undergone a process of cell division that gives it 23 chromosomes -- just enough to match an equal number of chromosomes from the sperm. Once an egg is released from an ovary, it's pulled into one of the fallopian tubes leading to the uterus. But one of the 20 million to 200 million sperm released by the man has to catch up to that egg soon: If the egg doesn't encounter a healthy sperm, it will perish in 24 to 36 hours. In that case, it disintegrates and flows out with your period. If the egg does get fertilized, it leaves the fallopian tube and travels into the uterus. If everything is in good working order, it will implant there and begin to develop into a baby.

While women are born complete with all the eggs they'll ever need, men are making new sperm all the time -- some 100 million to 300 million a day. Sperm are stored in a man's testes at a cool three to four degrees below normal body temperature. Sperm need those climatic conditions to stay healthy. (That's why men who are trying to get their partners pregnant are advised to stay out of hot tubs.)

Although they're produced daily, individual sperm take about 72 days to develop from start to finish. Each mature sperm cell has a head with the requisite 23 chromosomes, a body with the necessary fuel for movement, and a tail to propel it forward. The life of a sperm begins inside tubules in the testes. These sperm then travel to the epididymis, a 20-foot-long series of ultra-thin tubes that "act as a school for sperm to perfect their swimming technique," according to Toni Weschler, MPH, author of Taking Charge of Your Fertility.

At ejaculation, the 20 million to 200 million sperm released by the man travel from the epididymis into the urethra, which runs through the penis. The rest is history. Sperm can live for as long as five days in a woman's cervical fluid (depending on where she is in her menstrual cycle). But the egg doesn't survive for much longer than a day, so obviously only the strongest and swiftest sperm will reach it in the fallopian tube. And only one will fertilize it.

Is one sexual position better than another for baby-making? Many experts suspect that the missionary position (man on top) affords the best opportunity, though no definitive studies have been done on this theory. But this position allows for the deepest penetration and, as a result, the sperm are deposited closer to the cervix. To boost your chances, some doctors advise you to elevate your hips with a pillow so your cervix is exposed to the maximum amount of semen. But Weschler advises that you not use a large pillow because the height may cause the sperm to puddle behind cervix -- actually making it harder for sperm to get into your fallopian tubes. Many fertility experts also recommend that you stay in bed for up to a half hour (again, with a pillow under your pelvis). In theory, this lets gravity help a few of the sperm reach their destination. However, the advice on using a pillow is not based on much hard evidence, so it's good to discuss this with your doctor.

If you're trying to get pregnant, some doctors recommend that you have intercourse no more often than once every other day in the time leading up to ovulation. A man's sperm count can actually decline if he has sex more frequently than that.

Millions of sperm are working frantically to find your egg -- and meeting enormous obstacles. Many will be killed by normal acids in the vagina. Millions more (about 90 percent) won't make it through the cervical mucus. Then the survivors have to forge their way through the cervix and uterus and into the correct fallopian tube. Reaching the egg by no means ends the quest: A chemical change must take place within the sperm to enable them to penetrate the egg. In a process called the acrosome reaction, enzymes are released from the sperm's head, which help it make a small opening in the egg's outer layer. Thousands of sperm will burrow through the next layers, but when one gets all the way through, another chemical reaction in the egg locks the rest out.

Within 12 hours, the head of the winning sperm disappears and the fertilized egg divides into two cells. After that, the number of cells doubles every 16 hours. Three to four days after fertilization, a bona fide embryo drops into the uterus, where it floats for several days until it attaches to the uterine lining and begins to develop into a fetus.

Some women begin to experience certain changes, such as breast tenderness, fluid retention, and possibly some vaginal spotting -- symptoms many women associate with being premenstrual. These changes are the result of rapidly rising estrogen and progesterone levels needed to nourish the growing baby. So while you feel like you're about to get your period, you may actually be pregnant. A home pregnancy test can tell you for sure.

-- Elaine Herscher is a senior editor at Consumer Health Interactive.

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Last updated September 22, 2009
Copyright © 2002 Consumer Health Interactive

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