How babies are made

How do women's eggs develop?

For women, a potential pregnancy begins in the ovaries, those two almond-shaped glands attached to either side of the uterus. (See illustration below.)

Ovaries come fully stocked: Newborn baby girls are born with about 2 million eggs that have developed in utero – that's more than a lifetime's supply. The eggs begin dying off almost immediately, and no more are ever produced.

At the time of her first period, a young woman has approximately a couple hundred thousand eggs. An accelerated decline begins at age 37 or 38, with about a thousand eggs left at menopause (usually between age 45 and 55).

During the middle of the menstrual cycle, most likely between days 6 and 14 for women with a 28-day cycle, follicle-stimulating hormone, produced by the pituitary gland, causes follicles in one of a woman's ovaries to begin to mature. At about day 14, an increase in luteinizing hormone, also made by the pituitary, causes the ovary to release its egg. The egg then travels through the fallopian tube to the uterus.

This release, called ovulation, starts the conception clock ticking. The egg lives only about 24 hours after ovulation, so it has to be fertilized soon for conception to happen. Your "fertile window" is the five days leading up to ovulation, as well as the day of ovulation and the day after. That's about seven days total.

So how do you make a baby, exactly? 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, the egg ends its journey at the uterus, where it either dissolves or is absorbed by the body. When pregnancy doesn't occur, the ovary eventually stops making estrogen and progesterone (hormones that help maintain a pregnancy), and the thickened lining of the uterus is shed during your period.

How is sperm produced?

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, beginning at the onset of puberty when a man begins to produce millions of sperm cells each day.

The average sperm lives only a few weeks in a man's body, and up to 500 million are released with each ejaculation. That means new sperm are always in production, ready for making a baby.

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 about 1 to 2 degrees Celsius cooler than normal body temperature – that's around 95 degrees Fahrenheit. 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. (When two separate eggs are fertilized by two separate sperm in the same pregnancy, a mother has fraternal 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 gender is pretty much randomly determined.

Will having an orgasm help you get pregnant?

Besides being pleasurable, that 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, which could help you get pregnant fast.

Some researchers theorize a woman's orgasm could help with conception – that the wavelike contractions associated with the female orgasm pull sperm farther into the cervix. But there's no real evidence that having an orgasm will help you get pregnant.

Still, having an orgasm couldn't hurt – and just might help – your chances of getting pregnant.

Does lying on your back after sex help you get pregnant?

Many couples also wonder whether a particular sexual position is best to make a baby. You may have heard that certain positions are ideal because they allow for deeper penetration, but there is no evidence that sex position has any effect on pregnancy rates.

So, 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 have sex every 1 to 2 days during the middle of your cycle.

At this point, you can't do much except cross your fingers and hope. You may have also 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's no evidence this helps achieve pregnancy either.

What happens to the sperm and egg after you have sex?

After sex, a great deal of activity is taking place inside your body. Those millions of sperm have begun their quest to find the egg to make a baby, 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. They need to travel 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.

It's important to know that regular cannabis use by your partner can contribute to lower sperm count as well as the ability of the sperm to travel.

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. At the end of this period, the embryo becomes a mass of very organized cells called a blastocyst. You're not actually pregnant until the blastocyst travels the rest of the way down the fallopian tube and attaches itself to the wall of your uterus. This is known as implantation and usually happens five to six days after fertilization.

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.