Storgaard, L, JP Bonde, E Ernst, M Spanô, CY Andersen, M Frydenberg and J Olsen. 2003. Does Smoking During Pregnancy Affect Sons’ Sperm Counts? Epidemiology 14:278–286.
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Mothers who smoke more than 10 cigarettes a day while pregnant may be cutting their son's sperm count, once he reaches adulthood, by almost half. This effect could be an important contributor to regional and temporal variation in sperm count.

What did they do? Storgaard et al. obtained smoking histories via questionnaires from the mothers of 265 men, a combination of twins and pairs of singleton brothers. They then analyzed the relationship between the mother's smoking pattern during pregnancy and their son's sperm concentration (aged 20-45 yrs). The men were questioned about confounding variables like current smoking, occupational history, month of birth, history of cryptorchidism, abstinence time, etc., and these were controlled for in the analysis. The scientists also took blood samples, which they analyzed for reproductive hormone levels.

What did they find? Compared to sons whose mothers didn't smoke, sons whose mothers smoked more than 10 cigarettes per day in pregnancy had, on average, a 48% reduction in sperm concentration, after adjusting for confounding variables. These men were 4.3 times more likely to have a sperm concentration beneath the WHO standard of 20 million per milliliter. Inhibin-B levels were suppressed and follicle-stimulating hormone levels up.

They found no effect on sperm volume or morphology, or on levels of testosterone, leutenizing hormone and sperm chromatin.

What does it mean? As in any epidemiological study, these results fall short of proving a causal relationship between intense smoking and lower sperm counts. But Storgaard et al. comment that if the relationship is causal, "the global smoking epidemic among women would correspond to a substantial decline in sperm concentration that may contribute to lower fecundity 20 to 30 years later."

Smoking may also contribute to geographic differences in sperm count, as cultures vary in the degree to which women smoke. Storgaard et al. cite data comparing smoking rates in Finland vs. Denmark: the former low, the latter high. As would be predicted from this, Finnish sperm counts are higher than Danish.

They also speculate about the mechanisms by which maternal smoking might impair sperm production in sons. Tobacco smoke is a complex chemical mixture, which includes not only chemicals created by burning of the tobacco itself, but also an array of synthetic chemicals, e.g., pesticides, that have been used in the process of growing tobacco and manufacturing cigarettes. Animal studies have shown that fetal exposure to tobacco smoke can reduce sperm numbers, and identified at least two components of tobacco smoke, nicotine and polycyclic aromatic hydrocarbons, which might be interfering with proper development of the relevant parts of the male reproductive tract.

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