39 million sperm, according to the Mayo Clinic. Some causes of a low sperm count: Infection: Chlamydia, gonorrhea and other infections can cause scarring in the testes that prevents passage of sperm. Hormone imbalances: Low testosterone, the result of abnormalities of certain glands, may cause infertility. Don't worry, though. A woman can get pregnant even if her partner's sperm underperform. Conception can occur with a low sperm count, but it often takes longer, says Dr. Norian. Treating low sperm count with medication (if the problem is hormonal) often helps boost the odds. 4. You can be too thin You've decided to take the pregnancy plunge. Fearing the weight gain that comes with it, you hit the treadmill like a college track star before you conceive, and achieve a weight worthy of a magazine cover. Big mistake. Lack of fat can slow production of the hormones needed for ovulation (which includes estrogen), says Dr. Norian. Very obese women should lose 5% of their body weight, he adds. Even that small amount can start ovulation. 6. Over-exercising Yes, exercise is good for you. But running 40 miles a week? That won't help you if you're trying to get pregnant. Women who over-exercise risk losing body fat that helps produce estrogen, which spurs ovulation. They may experience infrequent or light periods, or their periods stop altogether. Exercising 4-5 days per week for 30 minutes, maintaining a heart rate of 120-130 beats per minute, is appropriate, says Dr. Norian. Men who exercise too much