While you release only a single egg each month, your partner releases millions of sperm in a single ejaculation — all with an eye on that single prize. But only one of those sperm will be crowned the victor, and the odds are stacked against fertilization.

Luckily, egg and sperm have developed some pretty cool tricks to give themselves a fighting chance. Read on to find out how fertilization actually works.

What is fertilization?

fertilization and implantation medical illustration

Fertilization is when a man’s sperm combines with a woman's egg to form a single cell. It’s one of the first steps toward pregnancy, but there are more hurdles to cross.

Where does fertilization occur?

Some people think fertilization happens in the uterus, since that’s where the baby develops. However, fertilization actually occurs in the fallopian tubes.

Each sperm has a single goal: to meet up with the egg. To reach the target, though, a sperm cell has to go on a lengthy and strenuous journey.

First, it must make its way from the vagina to the cervix, and then it has to swim through the uterus to the fallopian tubes. Once there, if the sperm is the lucky one, it will penetrate the egg and fertilize it. The newly fertilized egg (now called a zygote) will make its way from the fallopian tubes to the uterus. There, it will hopefully implant in the uterine wall and grow into a baby.

Most of the time, implantation occurs in the uterus as it should. However, sometimes the fertilized egg implants and grows somewhere outside of the uterus. This is called an ectopic pregnancy.

Usually, an ectopic pregnancy occurs in a fallopian tube (which is why you may have heard of it being called a tubal pregnancy), but it may also happen in an ovary, the cervix or even the abdomen.

Since none of those areas has the right tissue or the space to accommodate a growing pregnancy, an ectopic pregnancy is not a viable pregnancy.

When does fertilization occur? 

For fertilization to happen, the timing and conditions must be just right. The fertilization process can only occur during a fairly small window within just a few days of ovulation (a few days before up until a day after you ovulate).

Even if a sperm does make its way to the fallopian tube, that doesn’t mean it will definitely be able to fertilize the egg. Aside from various fertility-related problems that can hamper the process, the timing may be off, meaning the sperm arrived too early or too late to meet up with the egg. Or the sperm may have entered the wrong fallopian tube (an egg is usually present in only one of the fallopian tubes in any given month).

How long does it take for a sperm to fertilize an egg?

Contrary to what many people believe, fertilization doesn’t occur immediately after sex. The fastest sperm can reach the egg in as little as an hour after, but the entire fertilization process can take several hours. After ejaculation, the sperm are gearing up for that long journey.

For about a half an hour after sex, the semen coagulates in a woman's vagina, forming a physical barrier that prevents the sperm from wandering very far in the wrong direction. This protection disappears within 30 minutes, when the semen reliquefies.

Any sperm that haven't made it up through the cervix by then are eliminated from the running. The vagina is very acidic and quickly destroys any errant cells, including sperm.

The sperm that make it to the next step — the cervical canal — spend time going through biochemical changes, picking up the tail-thrashing speed that’s required for them to swim their way through the uterus and fallopian tubes to find their target. 

The cervical canal is a much more welcoming environment, where the mucus is specially designed to transport sperm efficiently when you're most fertile.

As you approach ovulation, your suddenly copious mucus becomes stretchy, clear and thin (that's one of the reasons why observing it is such an effective method of determining your ovulation time). The changes happen on a microscopic level as well, as strings of molecules line up like train tracks so that sperm can hop on and ride to their destination.

The sperm that do reach the egg still have their work cut out for them. The race is on to be the first one to plow through the hard outer layer of the egg.

And there's plenty of competition. Hundreds of sperm will surround the egg during the frantic battle to the finish, all trying to penetrate the egg's membrane to reach the cytoplasm, where the sperm will then release its own genetic contribution. 

As soon as one lucky sperm cell succeeds in penetrating the egg, the egg immediately undergoes a chemical reaction that prevents other sperm cells from following suit. Then the chromosomes carried by the sperm and egg come together, and the egg is officially fertilized.

Can you feel when an egg gets fertilized?

You won’t feel when an egg gets fertilized. You also won’t feel pregnant after two or three days. But some women can feel implantation, the process in which the fertilized egg travels down the fallopian tube and buries itself deep within the wall of the uterus.

This happens about eight to nine days after fertilization, though it can happen earlier or later, depending. Symptoms of implantation may include abdominal cramps and light bleeding. Whether you feel anything or not, implantation is the moment that fertilization gives way to pregnancy!

How long can sperm live inside you to get pregnant?

Sperm can survive inside the female reproductive tract for about 72 hours. And, in some cases, the sperm can live for up to five days. But the lifespan of an egg is much shorter. The egg lives for only 12 to 24 hours after ovulation.

If sperm aren't hanging around by time the egg makes it to the fallopian tube (or if they don't reach the destination shortly after the egg arrives), the body reabsorbs the egg, and the window of opportunity for conception that month closes.

As you've no doubt learned by now, the road to fertilization is a bumpy one, with plenty of twists and turns. It takes a lot of work, the right conditions and perfect timing for the egg and sperm to meet up and produce that baby you're hoping for. But once they do, you've embarked on your own amazing journey: the journey of parenthood.