How you conceive

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. These are the 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 1 to 2 million eggs in her ovaries (Rosenblatt 2007).

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 (Rosenblatt 2007), between your first period and menopause.

In Australia, the average age of menopause is 52 (FPV 2014). During each menstrual cycle, sometime after your period, one to three eggs start to reach maturity in one of your ovaries. The ripest egg is then released, a process known as ovulation.

The egg is quickly sucked up by the tulip-shaped opening of the nearest fallopian tube. There are two fallopian tubes, each about 10cm in length, which lead from the ovaries to the uterus.

Ovulation is usually about 14 days before your next period (ACOG 2015).

The exact time of ovulation depends on the length of your cycle. Several different hormones work together to control the length of your cycle, when your eggs ripen and the timing of ovulation. You can read more about these hormones in our article on the menstrual cycle.

The average egg lives for 12 to 24 hours after release (NCCWCH 2013, NHS 2014). It needs to be fertilised within this time by a sperm for a baby to be conceived. If your egg meets up with a healthy sperm on its way to the uterus, the process of creating a new life begins. If not, the egg ends its journey at the uterus and disintegrates.

If you haven't conceived, the ovary stops making oestrogen and progesterone. These are the two hormones that would help maintain a pregnancy. When the levels of these hormones drop, the thickened lining of your uterus is shed during your period. The remains of the unfertilised egg are shed at the same time.

Inside the man's body: how sperm are made

Women's bodies mature a single egg each month. Men's bodies, however, are almost constantly at work, producing millions of microscopic sperm. The sole purpose of each sperm is to swim towards and penetrate an egg.

From start to finish it takes two to three months to create a new sperm cell (NCCWCH 2013, Rebar 2008). The average sperm lives only a few weeks in a man's body, and at least 40 million are set free with each ejaculation. It means that men have to make sperm on a regular basis throughout their adult lives (Rebar 2008).

The same hormones that control ovulation in women stimulate the release of testosterone in men. Testosterone is 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 are sensitive to temperature. To produce healthy sperm efficiently they have to stay at a balmy 34 degrees C. This is about four degrees cooler than normal body temperature (Rebar 2008).

Once the sperm is created, it's stored in each testicle in the epididymis, a six-metre long coiled tube. Just before ejaculation the sperm is scooped up and mixed with semen (Lui 2007).

Despite the millions of sperm that are produced and released in each ejaculation, only one can fertilise each egg. The sex of your 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. 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 having sex

In addition to all the fun, your bodies are building up tension that you hope will end in orgasm. Having an orgasm 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.

A woman doesn't need to orgasm for conception to happen. Gentle uterine contractions can help the sperm along, but these happen without you having an orgasm (Levin 2002).

Many couples wonder if a particular sex position is best for conceiving. No one knows for sure. The most important thing about sex is that you're doing it often enough. For you to conceive, live sperm needs to be in your reproductive tract during ovulation.

Not all women ovulate during the middle of their cycle or at the same time in their cycle every month (Fehring et al 2006). To improve your chances of conception, aim to have sex every other day or so throughout your cycle (NCCWCH 2013).

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. While you and your partner enjoy a relaxing post-sex cuddle, a lot is going on 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 may be your cervical mucus, which can seem like an impenetrable net on your non-fertile days. When you're most fertile, however, it miraculously loosens up so the strongest swimmers can get through.

The sperm that survive still have a long road ahead. In all they need to travel about 18cm from the cervix through the uterus to the fallopian tubes. When you consider that they travel at a rate of roughly 2.5cm every 15 minutes, that's quite a trip. The fastest swimmers may find the egg in as little as 45 minutes. It can take the slowest up to 12 hours.

If the sperm don't find an egg in the fallopian tubes at the time of intercourse, they can survive inside you for up to seven days (NCCWCH 2013). This means that if you ovulate within this time window you could still conceive.

Your most fertile time is the five days leading up to ovulation and the day you ovulate (Setton et al 2016).

The mortality rate for sperm is very high and 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 that get near the egg, the race isn't over. Each one has to work frantically to penetrate the egg's outer shell and get inside before the others. The egg needs to be fertilised within 24 hours of its release.

When the hardiest sperm of the bunch makes it through, the egg changes instantly to prevent any others getting in. It's like a protective shield that clamps down over the egg at the exact moment the first sperm is safely inside.

Now a new life is born ...

During fertilisation, the genetic material in the sperm and egg combine to create a new cell that will rapidly start dividing. This bundle of new cells is known as the blastocyst. It continues travelling down the fallopian tube towards the uterus, a journey that can take another three days or so.

You're not actually pregnant until the blastocyst has attached itself to the wall of your uterus, where it will develop into an embryo and placenta.

Occasionally, the blastocyst will implant somewhere other than the uterus (usually in the fallopian tube). This is called an ectopic pregnancy, which is a medical emergency. The pregnancy won't survive outside the uterus and needs to be completely removed, to avoid damage to the fallopian tube.

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, a home pregnancy test will confirm it.

If you've got a little one on the way, congratulations, and welcome to the start of another incredible journey.

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