How babies are made

Getting pregnant: How babies are made

Reviewed by the BabyCenter Medical Advisory Board

You may think you know how to make a baby. But do you really understand all of the intricate details? Sure, you've got the basics covered — man meets woman, they both get naked and have sex, and nine months later, out pops Junior. But there's a lot more to it than that, not to mention a host of modern variations on that theme.

For instance, do you know exactly where sperm and eggs come from, or how these two microscopic miracle-makers find each other and create a new life? Or that you have only about a 20 percent chance of getting pregnant during each cycle, even if you time it just right? We'll spare you that awkward talk you probably got from your parents or middle school health teacher and instead give you just the facts — the fascinating biological facts about getting pregnant.

Inside the woman's body: How an egg is hatched

For women, the possibility of pregnancy begins in the ovaries, those two kiwi-shaped balls attached to either side of your uterus (see illustration above). The ovaries are like your permanent egg cartons, and they come fully stocked. Every infant girl is born with about 1 million eggs — more than a lifetime's supply — in her ovaries. You will never produce any more eggs. In fact, many of them begin dying off almost immediately. But don't worry, you have plenty left for your reproductive years. Altogether, you'll probably release about 400 eggs, beginning with your first period and ending when menopause arrives, usually between ages 45 and 55.

Sometime during the middle of your menstrual cycle, most likely between the 12th and 16th days, an egg reaches maturity in one of the two ovaries, is released and is quickly sucked up by the tulip-shaped opening of the nearest fallopian tube. These are two 4-inch canals leading from the ovaries to the uterus. This release, called ovulation, starts the conception clock ticking. The average egg lives only 24 hours, so it has to be fertilized soon if a baby is to be conceived. If your egg meets up with a healthy sperm on its way to the uterus, the two can join and begin the process of creating a new life. If not, it ends its journey at the uterus, where it either dissolves or is absorbed by the body. When pregnancy does not occur, the ovary eventually stops making estrogen and progesterone, hormones that would help maintain a pregnancy, and the thickened lining of the uterus is shed during your period.

Inside the man's body: The making of baby fertilizer

While women are busy maturing a single egg at the leisurely pace of about one a month, men are almost constantly at work producing millions of microscopic sperm, whose sole purpose in life is to swim for their lives and penetrate an egg. While women come complete with all of the eggs they'll ever need, men aren't born with ready-made sperm. They have to make them on a regular basis — from start to finish it takes about 64 to 72 days to create a new sperm cell. Since the average sperm lives only a few weeks in a man's body and around 30 million to 300 million are set free with each ejaculation, this fertilization factory is rocking 24/7.

Sperm production starts in the testicles, the two glands housed in the scrotal sac beneath the penis (see illustration above). The testicles hang outside of the body because they're quite sensitive to temperature. To produce healthy sperm, they have to stay a balmy 94 degrees Fahrenheit — about four degrees cooler than normal body temperature. Once the sperm is created, it's stored in a portion of the testicle known as the epididymis until it's scooped up and mixed with semen just prior to ejaculation.

Despite the millions of sperm produced and released in each ejaculate, only one can fertilize an egg — this is the case even for twins. The gender of the resulting embryo depends on which type of sperm burrows into the egg first. Sperm with a Y chromosome will make a boy baby, and sperm with an X chromosome will make a girl. Plenty of myths about how to conceive a boy or girl have been circulating for centuries, and some are backed by a bit of scientific evidence, but on the whole, a child's gender is determined randomly.

What happens while you're busy having sex

The big moment has arrived and you're actually having sex. In addition to all of the fun, your bodies are building up tension that you hope will end in orgasm, that wonderful, pleasurable release that also has an important biological function. In men, orgasm propels sperm-rich semen into the vagina and up against the cervix at roughly 10 miles per hour. This process, known as ejaculation, gives sperm a head start on their way to the egg. A woman's climax also aids conception. Some research shows that the wavelike contractions associated with the female O help pull the sperm farther into the cervix. So what's the take-home — or take-to-bed — message? Let go and have as much fun as you can. It can only help your chances of getting pregnant.

Many couples wonder if a particular sexual position is best for baby-making. No one knows for sure, but some experts believe the missionary position (man on top) or the rear-entry position (man behind woman, both facing the same direction) are best because they allow for deep penetration. But do whatever you like. The most important thing about sex is that you're both having a good time and you're doing it frequently enough that live sperm are in the woman's reproductive tract during ovulation. That means you should aim to make love at least every other day during the middle of your cycle.

During the afterglow, sperm fight to the finish

At this point, you can't do much except cross your fingers and hope, though some experts say the woman should stay on her back, with a pillow under her butt, for at least 20 or 30 minutes so gravity can help the sperm get to the waiting egg.

While you and your partner are enjoying a relaxing post-romp cuddle, a great deal of activity is taking place inside your body. Those millions of sperm have begun their quest to find your egg, and it's not an easy journey.
The first obstacle is the acid level in your vagina, which can be deadly to sperm. Then there's your cervical mucus, which can seem like an impenetrable net except on the one or two days when you're most fertile and it miraculously loosens up so a few of the strongest swimmers can get through. But that's not all — the sperm that survive still have a long road ahead. In all, they need to travel about seven inches from the cervix through the uterus to the fallopian tubes. When you consider that they travel at a rate of roughly an inch every 15 minutes, that's quite a trip.
The fastest swimmers may find the egg in as little as 45 minutes, while the slowest can take up to 12 hours. If they don't find an egg in the fallopian tubes at the time of intercourse, the sperm can wait there in a resting stage for up to 72 hours.

Only a few dozen sperm ever make it to the egg. The rest get trapped, lost — perhaps heading up the wrong fallopian tube — or die along the way. For the lucky few who get near the egg, the race isn't over. They have to work frantically to penetrate the egg's outer shell and get inside before the others. If the hardiest of the bunch makes it through, the egg changes instantaneously so that no other sperm can get in. It's like a protective shield that clamps down over the egg at the exact moment that first sperm is safely inside.

Now the real miracle begins. The egg will be fertilized within about 24 hours as the genetic material in the sperm combines with the genetic material in the egg to create a new cell that starts dividing rapidly. You're not actually pregnant until that bundle of new cells, known as the embryo, travels the rest of the way down the fallopian tube and attaches itself to the wall of your uterus.

However, you can have an ectopic pregnancy if the embryo implants somewhere other than the uterus, such as the fallopian tube. An ectopic pregnancy is not viable, and you either need to take medication to stop it from growing or have surgery to remove it and prevent rupture and damage to your fallopian tube.

That final leg of the trip can take another three days or so, but it may be a few more weeks until you miss a period and suspect that you're going to have a baby.

If you miss your period or notice one of the other signs of pregnancy, you can use a home pregnancy test to find out for sure if you have a little one on the way.

Taken from:  http://www.babycenter.com/0_getting-pregnant-how-babies-are-made_7056.bc?showAll=true