Men who drink just two cups of coffee a day could double their partner’s chance of falling pregnant.
A new study of 500 couples trying for a baby suggests caffeine in the week before a couple has sex might increase the chance of conception.
The research presented at the American Society for Reproductive Medicine annual conference in Denver found caffeine intake in men doubled the chance of pregnancy.
However, women who drank alcohol before and after ovulation reduced the likelihood of pregnancy.
Dr Sunni Mumford, lead author of study from the US National Institutes of Health, said: “We were somewhat surprised by the results though the research on male caffeine intake and its effects on fertility is pretty mixed.
“These results highlight the importance of lifestyle factors in both male and female partners during sensitive windows of reproduction to influence fecundability, and the need for appropriate preconception guidance for couples seeking pregnancy.”
Last year, a review of 28 papers on caffeine intake and semen published in the Journal of Nutrition found it may effect male fertility by possibly damaging the sperm DNA.
But Prof Sheena Lewis, an expert in reproduction expert at Queen’s University in Belfast said there was a possible mechanism that could support the latest findings involving two chemicals in the body called ATP, or adenosine triphosphate and GTP, or guanosine triphosphate.
“Caffeine prevents these chemicals from breaking down so more energy is available to cells including sperm so they can swim faster or longer,” she said.
“It is good news because lots of infertility is caused by sperm that are poor swimmers.”
However, other experts remained cautious and advised men trying to have a baby not to rush out and buy highly caffeinated drinks.
Prof Charles Kingsland is clinical director of Care Fertility and the founder of the Britain’s largest NHS IVF unit.
He said: “This study shows what you eat and drink is very important before you wish to get pregnant.
“Unequivocally, smoking and excess alcohol before conceiving is bad for developing healthy babies. These substances are essentially poisons.
“Caffeine is more contentious.”
Dr Channa Jayasena, consultant in reproductive endocrinology at Imperial College London, said: “Previous studies have found that high caffeine intake is bad for your sperm count.
“So it’s really surprising that male coffee drinkers were more likely to get their partner pregnant.
“I would advise expectant dads not to increase their caffeine intake but to wait until we have more evidence.”
Leading UK fertility expert Prof Allan Pacey, professor of andrology at Sheffield University, said: “This is an interesting observation but we should recognise that it’s just that.
“I can think of no sensible biological mechanism by which a man drinking tea or coffee in the week before his partner ovulates can increase the chance of her getting pregnant.”
The study also confirmed previous research linking alcohol intake with reduced pregnancy rates.
Women who consumed one alcoholic drink every other day were 26 per cent less likely to conceive.