A man's body is almost constantly at work producing millions of microscopic sperm, whose sole purpose is to penetrate an egg. While women are born with all of the eggs they'll ever need, men aren't born with ready-made sperm. They have to be produced on a regular basis, and from start to finish it takes 64 to 72 days for new sperm cells to develop.
The average sperm lives only a few weeks in a man's body, and about 250 million are released with each ejaculation. That means new sperm are always in production.
Sperm begin developing in the testicles, the two glands in the scrotal sac beneath the penis. (See illustration above.) The testicles hang outside a man's body because they're quite sensitive to temperature.
To produce healthy sperm, testicles have to stay around 94 degrees Fahrenheit – about four degrees cooler than normal body temperature. The sperm are stored in a part of the testicle called the epididymis before mixing with semen just prior to ejaculation.
Despite the millions of sperm produced and released with each ejaculation, only one can fertilize an egg – this is the case even for identical twins. The sex of the resulting embryo depends on which type of sperm burrows into the egg first. Sperm with a Y chromosome make a boy baby, and sperm with an X chromosome make a girl.
Plenty of myths about how to choose a baby's sex have been circulating for centuries. Some are backed by a bit of scientific evidence, but a child's sex is pretty much randomly determined.
Does having an orgasm help baby-making?
Besides being pleasurable, that wonderful sensation known as an orgasm also has an important biological function.
In men, having an orgasm propels sperm-rich semen into the vagina and up against the cervix, helping them reach the fallopian tubes minutes later. This gives sperm a head start on their way to the egg.
A woman's orgasm also might help conception: Some studies suggest that the wavelike contractions associated with the female orgasm pull sperm farther into the cervix (but other research says there's no real evidence this is true).
Still, having an orgasm couldn't hurt – and just might help – your chances of getting pregnant.
Many couples also wonder whether a particular sexual position is best for baby-making. You may have heard that certain positions are the best because they allow for deeper penetration, but there is no evidence that sex position has any effect on pregnancy rates.
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 to have live sperm 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.
Which sperm gets to the egg first?
At this point, you can't do much except cross your fingers and hope. You may have heard that it helps if the woman stays on her back afterward with a pillow elevating her bottom so gravity can help the sperm get to the waiting egg, but there is no evidence this helps achieve pregnancy.
While you and your partner are cuddling, a great deal of activity is taking place inside your body. Those millions of sperm have begun their quest to find the 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 be impenetrable, except on the days when you're most fertile. Then it miraculously thins enough for a few of the strongest sperm to get through.
But that's not all – the sperm that survive still have a long road ahead. In all, they need to travel about 7 inches from the cervix through the uterus to the fallopian tubes.
If there isn't an egg in one of the fallopian tubes after ejaculation, the sperm can live in the woman's reproductive tract for up to five days. Only a few dozen sperm ever make it to the egg. The rest get trapped, head 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 still have to penetrate the egg's outer shell and get inside before the others.
And as soon as the hardiest one 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 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, if the embryo implants somewhere other than the uterus, such as the fallopian tube, an ectopic pregnancy results. An ectopic pregnancy is not viable, and you'll either need to take medication to stop it from growing or have surgery to prevent your fallopian tube from rupturing.
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 another sign of pregnancy, you can use a home pregnancy test to find out for sure if you have a little one on the way.
