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5 Most Common Causes of Infertility

Abnormal Sperm Count, Shape, Motility

Size does matter, but not where you think -- it's sperm size that's important when it comes to fertility. There is, in fact, a standard of perfection when it comes to sperm: oval, paddle-shaped head with a long tail.

Eggs aren't in search of crooked tails or pinheads -- in fact, they're not searching at all. It's the best sperm that finds the egg, and the curly-tailed dude just isn't going to cut it. (It's OK if a large percentage of a man's sperm aren't in tip-top shape -- you just want some of them to be.)

Sperm size and shape is referred to as sperm morphology, and it's one of three factors analyzed in a semen analysis. The other two are sperm motility, or movement, and sperm count.

Sperm have to be willing to get a move on. That last distance to the egg can only be crossed with some serious tail wiggling. More than 40 percent of a man's sperm have to be active swimmers for a good chance at fertility.

As far as numbers, sperm count is pretty straightforward -- you want to have more than 39 million sperm in an ejaculation for a chance at procreation [source: Mayo Clinic]. (That is not a typo. Million.)