How Fertilization Happens

Egg and sperm have developed some pretty nifty tricks to make conception happen. Take a look.

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The miracle of conception involves a cast of millions (and you thought it was just you and your spouse in bed together!), but most of those millions are wasting their time. While you release only a single egg each month, your partner releases anywhere from 100 million to 300 million sperm in a single ejaculation, all with their eye on that single prize. But only one of those sperm will be crowned the victor in the frenzied fight to the fertilization finish, and it's a battle that's fraught with dangers lurking at every twist and turn of your reproductive tract (no one said it's easy being a sperm).

In fact, the odds are stacked against fertilization (and for most of your life you've probably been grateful for that). But when you're trying to make a baby, you're hoping that those odds swing in your favor, despite all the barriers that may come between your egg and all those sperm. Luckily, egg and sperm have developed some pretty nifty tricks to give themselves a fighting chance.

Fortify the troops. The liquid portion of semen not only provides the sperm with nourishment for the journey, it actually coagulates in a woman's vagina after ejaculation, forming a physical barrier that prevents the sperm from wandering very far in the wrong direction (which is a good thing — how many males do you know who are willing to ask for directions?). This protection disappears within half an hour, when the semen reliquefies. Any sperm that hasn't made it up through the cervix by then is clearly not worth saving (which is why it dribbles right out of you after you have sex). Of course, any sperm left behind for more than a few moments doesn't have much of a chance anyway; the vagina is an acidic place to hang out and quickly destroys any errant cells.

Call in the transport unit. The cervical canal is a much more welcoming environment, and sperm that make it there find themselves awash in a sea of cervical mucus. This is also a good thing, since that 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 ("all aboard!").

Gather steam. Even sperm that are intrepid enough to get this far aren't home free. A just-ejaculated sperm cell has to spend about an hour going through biochemical changes, picking up tail-thrashing speed as it makes its way into the uterus and fallopian tubes to find its target.

Check the coordinates. The biggest key to successful fertilization is timing. Sperm must reach their destination — your egg (which is slowly making its way down the fallopian tube from your ovary) — within the right time frame. If they get there too early, they risk dying before the egg shows up. Too late, and the egg will be gone and they'll have missed their shot, so to speak. They also need to pick their destination carefully — an egg is usually only present in one of your two fallopian tubes in any given month. Pick the wrong tube, and the sperm end up hanging out partying together with no guest of honor in sight.

Battle to the finish. Even the sperm who 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 reach the egg's nucleus, where the sperm will then release its own genetic contribution. And may the best sperm win!

Mission accomplished. As soon as the one lucky sperm cell succeeds in penetrating the egg, the egg immediately undergoes a chemical reaction that prevents other sperm cells from penetrating as well (to the victor belongs the spoils). Then the chromosomes carried by the sperm and egg come together, and the egg is officially fertilized. Within a matter of hours, the microscopic zygote divides, then divides again and again. About a week later, a ball of around 100 cells (called a blastocyst) reaches your uterus and settles down into the uterine lining. You have now reached implantation — the moment that fertilization gives way to pregnancy — when the sperm's journey gives way to your own incredible, life-transforming trip to parenthood.


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