How are babies made?

We all know the basics of making a baby. You have sex, then the sperm travels a long way to meet an egg. And boom—you have a baby. Right? Well, it’s a bit more complicated than that.

Knowing the mechanics of how you get pregnant can be helpful when it comes to understanding your fertility. So here is exactly what happens.

It starts with ovulation

Without ovulation, you will not be able to conceive a child naturally. Ovulation is the process that releases a mature egg from a woman's ovary into one of the fallopian tubes so it can be fertilized. Ovulation is prompted by a series of hormonal changes.

A couple of days before ovulation, one follicle in the ovary becomes the dominant follicle and it carries the oocyte (an immature egg cell) for that month, explains Sara Twogood, an OB/GYN and assistant professor of clinical obstetrics and gynaecology at the USC Keck School of Medicine in Los Angeles. The dominant follicle isn’t identified until a couple of days before ovulation then matures until ovulation, she adds.

You will experience a spike in luteinizing hormone, the egg cell undergoes changes, then 36 hours after the LH surge, the oocyte will be released from the ovary, which is what we call ovulation.

Since each woman’s menstrual cycle differs in length, the exact time of ovulation varies. Most women ovulate about 14 days before their period. But things like stress or illness can affect your hormones and in turn influence when you ovulate.

This is why many women use ovulation tests to predict when they’re ovulating. Tracking your basal body temperature or examining your cervical mucus can also clue you in to when it’s prime baby-making time.

Then you need sperm

An egg by itself isn’t going to produce a baby. It needs to have a rendezvous with a man’s sperm.

It’s estimated that men make 1,500 sperm every second of every day! You don’t need to be a mathematician to know that’s a lot of sperm.

But it takes 72 days for a single sperm to mature. Each starts as a germ cell that, when nourished by "nurse cells" in the testicle, become sperm. When it finally grows a tail, it can swim out into the epididymis (the duct behind the testes). Here, the sperm will brush up on its swimming skills and hang out until it leaves the body through ejaculation.

When men ejaculate, they can release anywhere from 40 million to one billion sperm! While that number seems high, very few make it to the egg.

The vagina is a very acidic and deadly environment for these little guys (many die within minutes of arriving)—and that’s just the beginning of a long and tedious journey for them. The sperm must swim through the cervix to get to the uterus and eventually find their way to the fallopian tube. There is no GPS, so by this point many sperm have swum in the wrong direction or have just run out of energy.

Once the sperm finds the fallopian tube, it’s all about timing.

If the egg meets the sperm

If the sperm is lucky, a woman would have just ovulated and an egg will be on its way soon. But this waiting game is one with an expiration date. Sperm can stay alive for 72 hours, perhaps a bit more, in the female reproductive system, says Twogood. But after ovulation, an egg only lives for 24 hours at most. So it really is a race against time for the sperm and the egg to meet. And your odds of getting pregnant each month are 25 to 35 percent if you are under 35 years of age.

When a sperm does finally meet the egg, it’s time to get down to business. “There is an activation that helps the sperm penetrate the outer protective layer of the egg, and enzymes are released that remove the extraneous parts of the sperm so the chromosomes can join with those of the egg,” says Twogood.

Once the sperm and egg connect, the zona pellucida (the external surface of the egg) undergoes a reaction, which makes it impossible for other sperm to attach and penetrate the egg, explains Twogood.

In a perfect world, the sperm makes it to the fallopian tube, waits for the egg and fertilizes it inside the uterus. However, in rare cases the fertilized egg implants somewhere other than the uterus. When this happens, an ectopic pregnancy can form. This typically happens inside the fallopian tube and is not a viable pregnancy. If not dealt with immediately, it can be life-threatening. According to the Mayo Clinic, the first sign of an ectopic pregnancy is pelvic pain. You should also watch out for light vaginal bleeding, an urge to have a bowel movement and shoulder pain. If your tube ruptures, the pelvic pain will get worse and you may experience light-headedness and even faint.

If the egg is never fertilized, the lining of the uterus is shed and you get your period. Contrary to what some people believe, sperm doesn’t come out when you get your period—they die off and get absorbed by the body.

A baby is made

Each sperm carries a different chromosome—an X or Y. While a man can’t control which sperm will make it to the egg, he will ultimately decide the sex of the baby because all unfertilized eggs carry an X chromosome. If the egg meets with a sperm that is also carrying an X chromosome, little Julie will be here in nine months. If the egg meets with a sperm carrying a Y chromosome, get ready for a little Jake.

Can you increase your odds of getting pregnant?

You may wonder if there are any positions that are better than others for increasing your chances of conceiving. Ejaculation inside the vagina is necessary to make a baby, but it doesn’t actually matter which position makes that happen, explains Twogood. Some people think lying on your back for several minutes after getting busy will act like a speed pass for the sperm to get to its destination. That’s not true, says Twogood.

It’s also not true that a woman needs to have an orgasm to get pregnant. "It's theorized that the contractions of the cervix and the uterus may help facilitate the movement of the sperm," says Twogood. But there has never been any research to back up this belief. So, in the end, do what feels right for you and your partner. With the right timing, you could have a baby in nine months!