In almost half the cases, man is the cause of a couple's infertility. In the last 4 decades, male fertility has been plummeting in developed countries, with sperm count decreasing by 1.5 % annually in the US and 3% in Europe and Australia. This, combined with the tendency of later pregnancies (that is after a woman's fertility peak has passed) in Western populations, results in increasingly more couples having difficulties in conceiving a baby.

One of the causes of sterility in males is the birth defect called cryptorchidism (undescended testicles), whose rate in newborn boys has been increasing steadily in the same period. Boys born with this condition are 60 % more likely to be infertile as adults than those who were born healthy. Currently, this is the main male-genital birth defect in the US, affecting roughly 3% of the newborn boys. In fact, poor testicular development in the womb may cause fertility issues even in boys who don't display visible birth genital defects.

Few risk factors have been known so far, but a 2007 Danish research led by Ida Damgaard has revealed an unexpected and major risk factor: regular alcohol drinking during pregnancy. The research carried in a period of 10-years monitored about 5,000 pregnant women in Denmark and Finland and their resulting offspring.

The pregnant women completed questionnaires about their intake of alcohol and caffeine, working conditions, diet, and medical history: "anything we could think of", said co-author Katharina Main, pediatrician at Rigshospitalet in Denmark.

128 of the newborn boys were cryptorchid. Boys delivered by mothers who drank at least five alcoholic drinks weekly during pregnancy were thrice more likely to have developed cryptorchidism than those whose mothers abstained. Pregnant woman drinking under five drinks a week had less significant rated of criptorchid boys but Main believes that alcohol harms even at lower levels. "I wouldn't say that four [drinks are] OK but five aren't", said Main.

In 75 % of the cryptorchid boys, the testes descended into the scrotum within 3 months after birth, while 25% required surgery for fixing the issue, but even in the cases when the testes descended on their own, they presented long-term abnormalities. "It isn't a sign that everything is OK . . . because when we look at the hormones, they still show an impairment of hormone production," said Main.

In 2006, the same team detected another risk factor for cryptorchidism: pesticides, including DDT, no longer in use but persisting in the environment.

Other known risk factors are premature births, low birth weight, gestational diabetes, and being a twin.

In Denmark, cryptorchidism rates are extremely high: the issue affects 9% of boys, compared to only 2.3 % of the boys born in the nearby Finland.

Alcohol could be the main culprit. In Denmark, public campaigns point to a decrease in alcohol consume in pregnant women, but they don't say anything about ceasing it completely, whereas Finland and most other countries ask for total abstinence during pregnancy.