Getting pregnant: how babies are made

Written by the BabyCentre UK team
Approved by the BabyCentre Medical Advisory Board

You may think you know how to make a baby: man meets woman, they make love and nine months later out pops a baby! But do you know exactly where sperm and eggs come from? Or how they find each other and combine to create a new life? Read on to discover the fascinating biological facts behind getting pregnant.


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


For women, the possibility of pregnancy begins in the ovaries; those two small oval organs attached to either side of your uterus (womb). Every baby girl is born with more than 200,000 in her ovaries, but altogether, you'll probably release about 400 eggs, beginning with your first period and ending when menopause arrives, usually between the ages of 45 and 55.

Each month, during the middle of your menstrual cycle (most likely between the 12th and 16th days), an egg reaches maturity in one of your ovaries. It's then released and is quickly sucked up by the tulip-shaped opening of the nearest Fallopian tube (these are two four-inch canals leading from the ovaries to the uterus). This release is known as ovulation. The average egg lives only 24 hours, so it has to be fertilised 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 disintegrates and is expelled a couple of weeks later during your period.


Inside the man's body: the making of a sperm


While women's bodies are busy maturing a single egg at the leisurely pace of about one a month, men's bodies are almost constantly at work producing millions of microscopic sperm, whose sole purpose in life is to swim towards and penetrate an egg. While women are born with all the eggs they'll ever need, men have to make sperm on a regular basis throughout their adult lives. From start to finish it takes about 64 to 72 days to create a new sperm cell and, as around 200 to 350 million are set free with each ejaculation, this sperm factory is kept pretty busy.

Sperm production starts in the testicles, the two glands housed in the scrotal sac beneath the penis. The testicles hang outside the body because they're quite sensitive to temperature. To produce healthy sperm they have to stay at a balmy 34ºC/ 94ºF-- 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 that are produced and released in each ejaculation, only one can fertilise each egg. The gender of baby 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. There are plenty of myths about how to conceive a boy or girl, and some are backed by a bit of scientific evidence, but on the whole, a child's sex is determined randomly.


What happens while you're making love


The big moment has arrived and you're actually having sex. In addition to all the fun, your bodies are building up tension that you hope will end in orgasm. That wonderful, pleasurable release also has an important biological function. In men, orgasm propels sperm-rich semen into the vagina and up towards the cervix at roughly 10 miles per hour. This gives the sperm a good 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 big O help pull the sperm farther into the cervix. So 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. The most important thing about sex is that you're both having a good time and you're doing it frequently enough for live sperm to be 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.


While you relax, the sperms' job is just beginning


If conception is going to happen, it will be in the first few hours after sex. 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 bottom) 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 you're most fertile and it miraculously loosens up so a few of the strongest swimmers can get through. 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.

The mortality rate for sperm is so high that only a few dozen 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. When 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 fertilised within about 24 hours as the genetic material from the sperm combines with the genetic material in the egg to create a new cell that will rapidly start dividing. 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. (You can have an ectopic pregnancy if the embryo implants somewhere other than the uterus, usually in the Fallopian tube. An ectopic pregnancy is not viable, and the embryo has to be surgically removed to prevent rupture and damage to the Fallopian tube.) That final leg of the trip can take another three days or so, but it will be a couple of weeks until you miss a period and suspect that you're going to have a baby.

Once you have missed your period -- or noticed one of the other signs of pregnancy -- you can use a home pregnancy test to find out for sure if you've got a little one on the way. If so, congratulations, and welcome to the start of another incredible journey.

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