Overuse the Cellphone: Men who talked on cellphones for more than four hours a day produced only 75 percent as many sperm as those who did not use the devices, a 2006 Cleveland Clinic study found. They also had lower-quality sperm, including slower swimmers. Note: The researchers did not ask men about their diet or lifestyle, so the results of this study are not conclusive.
Get Chubby: In a Danish study of nearly 1,600 men, those who were overweight made significantly less sperm. In addition to having higher sperm counts, men in the normal-weight range—a body mass index (BMI) of 20 to 25—also had a lower percentage of abnormal sperm.
Expose Himself: A prospective dad needs to take care with chemicals, especially if he's a painter, printer, firefighter, agricultural worker or janitor. (A review of 10 years of studies linked these occupations to birth defects.) He should check if his workplace provides proper protection and ventilation.
Do Recreational Drugs: Smoking marijuana turns sperm into hyperswimmers that "burn out" before they're able to make their way to the egg. Cocaine may actually adhere to sperm without hampering its ability to fertilize an egg, increasing the chances that a child may be affected by the father's drug use.
Drink Too Much: Heavy drinking is linked with decreased sperm formation and function. While the precise link between moderate drinking and fertility in men is murky, a prospective father should play it safe by limiting alcoholic beverages to two a day, max.
Too Darn Hot: Sperm thrive at normal body temperature. Wearing tight briefs hikes the heat in the scrotum, possibly suppressing sperm production (though this isn't proven). Laptop computers may do the same: A study at the State University of New York at Stony Brook looked at 29 healthy men ages 21 to 35 and found that balancing computers on their laps for an hour increased scrotal temperature about 5* F.
For everything you need to know to maximize your chances of conceiving, visit our Getting Pregnant feature.