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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). The ovaries are packed with eggs, which are made before you are even born. Every baby girl is born with up to 450,000 eggs in her ovaries. Many eggs begin dying off almost immediately and the rest steadily decrease in number as you get older. You'll probably release about 400 eggs, during your fertile years. This begins with your first period and ends when the menopause arrives, usually between the ages of 45 and 55.

Each month, usually some time during the middle of your menstrual cycle, between one and three eggs start to reach maturity in one of your ovaries. The ripest egg is 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 exact time of ovulation depends on the length of your cycle. In an average 28 day cycle, ovulation will most likely happen between the 12th and 15th days, counting day 1 as the first day of your last period. The length of your cycle, the ripening of your eggs and the timing of ovulation are controlled by several different hormones, which work together. See our article on your menstrual cycle for more about hormones.

The average egg lives and can be fertilised for about 12 to 24 hours after release, so it has to meet up with a sperm soon if a baby is to be conceived. If your egg does meet 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. When you have not conceived, the ovary stops making oestrogen and progesterone, the two hormones that would help maintain a pregnancy. Following the drop in the levels of these hormones, the thickened lining of your uterus is shed, along with the disintegrated egg, 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. The sole purpose in life of each sperm, 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. Since the average sperm lives only a few weeks in a man's body, and as many as 300 million are set free with each ejaculation, this sperm factory is kept pretty busy.

In men, the same hormones that control ovulation in women stimulate the release of testosterone; the hormone responsible for producing sperm. Sperm production starts in the testicles, the two glands contained 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 34 degrees C/ 94 degrees F; about four degrees cooler than normal body temperature. Once the sperm is created, it's stored in a 40-foot long coiled tube in the testicle, called the epididymis until it's scooped up and mixed with semen just before 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

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. The force of ejaculation gives the sperm a good head start on their way to the egg, but a woman's climax also aids conception. Some research shows that the wavelike contractions associated with the big O help pull the sperm further into the cervix. So really 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 entry from behind 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. Not all women ovulate during the middle of their cycle or at the same time in their cycle every month. So to improve your chances of conception, aim to make love at least every other day throughout your cycle.

While you relax, the sperm's job is just beginning

At this point you can't do much except cross your fingers and hope that you conceive. 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 travel to where it is needed for fertilisation to occur.

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.

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 12-24 hours; so there is still a chance of conception if you ovulate within this time window.

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 ...

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