Effect of prenatal exposure to oestrogen on quality of semen: comparison of twins and singleton brothers

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Abstract
Participants, methods, and results From the population based Danish twin registry, we selected 250 monozygotic and 250 dizygotic pairs of twin brothers and from the Danish civil registration 500 pairs of singleton brothers. All the men were 20-45 years old and born in Denmark. Of the 2000 men, 778 (38.9%) agreed to participate (40% of the singletons (396), 39% of the dizygotic twins (197), and 37% of the monozygotic twins(185)). Because of our estimates of suitable sample size, we stopped enrolment when we had collected semen samples from 105 singleton brothers, 104 monozygotic twins, and 107 dizygotic twins. The men produced semen by masturbation and we analysed it in less than 1.5 hours in a mobile laboratory at the participants' home (n=113) or at a stationary laboratory (n=203). The sperm concentration was counted in an improved Neubauer haemacytometer (Marienfeld, Lauda-Königshofen); sperm morphology was classified according to 1999 World Health Organization criteria. Sperm counts and sex hormone concentrations were positively skewed, and we transformed them to their cubic root to normalise their distributions. Morphology measurements were logit transformed. All potential confounders were included in the multiple linear regression. The crude median sperm count was 19% higher among monozygotic twins and 9% lower among dizygotic twins than among singletons (table). The groups did not differ significantly with respect to any of the measures of semen quantity and quality. Inhibin B concentrations were significantly higher for singletons than for monozygotic twins. We did not expect selection bias due to differential fertility to explain our findings: we found no difference between men who agreed to participate and those who did not in terms of the number of children they had (1.8 v1.9).