When it comes to making a baby, the odds are stacked against any single sperm cell ever becoming a zygote through contact and penetration of a female egg. That's if a man has perfectly healthy sperm -- and if he has enough of them.
Let's look at the numbers: In the time it takes the human heart to produce one beat, a man's body produces 1,000 sperm cells [source: National Geographic]. On average, a man's ejaculate (about 2.75 milliliters of it) contains over a quarter-billion sperm [source: Lindemann]. Anything less than 20 million sperm per milliliter of semen is considered low, while 39 million or more per milliliter is considered optimal for fertilization [source: Mayo Clinic].
One study showed that more intimate sex causes men to produce higher quantities of sperm that are of higher quality [source: Campbell]. But many other factors can affect the number and health of sperm cells, including environmental causes, lifestyle choices and medical issues.
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Smoking, heavy drinking, drug use (including prescription drug use) or lack of exercise all contribute to diminished sperm count or poor sperm health. By maintaining a healthy lifestyle -- exercising, taking a daily multivitamin, eating nutritious meals with fruits and veggies -- you'll improve your odds of developing healthy sperm that are optimal for fertilization. Regardless of health or lifestyle, after age 50 there's generally a decline in sperm motility and quantity. And too much time in a hot bath or sauna can even affect sperm health, as the testes maintain a lower temperature that's preferable for the health of the sperm within.
Let's say that a man has perfectly healthy sperm cells. While a quarter-billion or so start off in search of the egg, fewer than 100 may ever get close to it.
But how long does it actually take for a sperm to penetrate an egg and form a zygote? What's the shortest time it would take? What's the longest? Keep your pants on -- we'll begin to uncover those answers in the next section.