A fast sperm cell could have reached its goal while you're still curled up in bed together.

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From Sex to Conception

There are close to equal numbers of sperm that contain either a male chromosome or a female chromosome. This determines the gender of the baby at the very point of egg fertilization. It will also play a role in the race to reach the egg. While male sperm aren't as hardy, they have a speed advantage. Though slower than their male counterparts, the advantage in longevity goes to female sperm.

Sperm ejaculated outside of a female's body have a lifespan that tops out at a few hours and may be as brief as a few minutes [source: Harms]. But if they survive long enough to reach the fallopian tube, sperm cells can relax and enjoy a safe environment that's conducive to their continued survival. Some actually bind to the fallopian tube itself, as a means of receiving direct nourishment [source: National Geographic].

When an egg enters the fallopian tube from the other end, it produces a scent that drives the sperm cells crazy. They become very excited and begin a process called capacitation, during which they shed certain proteins and become very excited. Both the shedding of proteins and the boost in mobility aid the sperm cell in its quest to penetrate the egg.

When a sperm cell finally makes contact with the egg, the head of the sperm releases enzymes that help it penetrate the egg's exterior. Once a sperm cell does, the two cells -- the sperm and the egg -- form a single-celled unit called a zygote, and the change prevents any other sperm cells from getting through.

The fastest sperm can get to a fallopian tube is about 30 minutes, meaning that the quickest conception could occur following sex is in the half-hour range [source: WebMD]. This means that, following sex, the egg could be fertilized before you've gotten up to get a drink of water.

Conception can occur as many as five days after sex or possibly longer, as strong, healthy sperm can survive for about that many days (and perhaps even longer) in the supportive environment of the fallopian tubes as they wait for an egg to be released, if one isn't already present [source: Harms].

You can click on the next page for lots more information on fertility and conception in far less time than it takes to create a zygote.