SAN ANTONIO ? Men who want to be fathers should lay off the pot-smoking and french fries.

Two small studies presented to a fertility conference suggest that marijuana use and excessive body fat may make it harder for a man’s sperm to complete their upstream swim to fertilize an egg.

A study of 22 college students who were heavy marijuana users suggested their sperm moved too fast too soon and lost oomph before they could reach their destination — the egg.

“Sperm in semen are supposed to sit quietly and wait for their chance to get out of the semen,” said Dr. Lani Burkman, a researcher at the University of Buffalo. “The sperm of marijuana smokers when sitting in the semen were very, very active.”

The men studied smoked marijuana about twice a day.

Burkman said the effect on the sperm came from marijuana’s ingredients. She said earlier laboratory work indicated sperm generally couldn’t keep up an accelerated pace for more than six hours or so, while it may take 24 hours or longer to get to the uterus.

If the sperm swim vigorously early on, “they will not have this vigorous motility by the time they are getting somewhere close to the egg and will not be able to penetrate the egg,” Burkman said.

The research also found that the marijuana smokers had only about half as many sperm per volume of semen as the control group of men who did not smoke marijuana.

Burkman said it could take four to six months of marijuana abstinence for the sperm characteristics to return to normal.

Dr. Janice Bailey, a fertility researcher at Laval University in Quebec, said the conclusions of the marijuana study reinforced the point that men could make lifestyle choices to improve their chances of becoming a dad.

Body fat, too, can be bad for making babies, according to another study presented to the American Society for Reproductive Medicine meeting this week.

Researchers at a fertility clinic in Atlanta examined 30 sperm samples and found that excess weight can lead to a lower sperm count and a higher number of sperm with damaged DNA.

William Roudebush, one of the researchers, said the effect was most pronounced for obese men, those with a body mass index greater than 30.

Dr. Larry Lipshultz, a male fertility specialist at Baylor University’s College of Medicine in Houston, agreed with Roudebush that one explanation is that all that body fat on very heavy men creates “a lot of extra localized heat” in the groin area that can be detrimental to sperm.