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: Each second, a man's body produces at least 1,500 sperm cells [source: Dell'Amore]. On average, a man's ejaculate (about 2.75 milliliters, or 0.09 fluid ounces, 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.
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 — can improve a man's 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.