Couples wanting to conceive a boy should not smoke, scientists say. New research reported by Japanese and Danish scientists in The Lancet medical journal on Friday showed that couples who smoked around the time their child was conceived were less likely to have a son.

“If the father smokes more than 20 cigarettes a day and the mother does not smoke the sex ratio is significantly decreased with fewer boys than girls,” Professor Anne Grete Byskov, of University Hospital of Copenhagen, said in an interview.

If both parents smoke the chances of producing a male child are even lower.