The decline in sperm count and increase in disorders of the male reproductive tract may be due to high concentrations of prenatal exposure to oestrogens.1–2 As the concentration of free oestrogens in plasma is much greater in twin pregnancies from the first weeks of gestation, and greater for dizygotic twins than monozygotic twins,3–4 we studied sperm count in twins and singleton brothers to see if twin brothers have lower sperm counts and if the lowest values are for dizygotic twins.
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 v 1.9).
Comment
Higher prenatal concentrations of oestrogen are not related to reduced sperm counts in adulthood. In particular, we did not find lower sperm counts in twin brothers: both the concentration and potency of oestrogens during pregnancy with twins are greater than for most environmental oestrogens.5
The low inhibin B concentration in monozygotic twins is surprising and could reflect slower multiplication of Sertoli's cells in fetal life. If so, high sperm counts indicate a compensatory mechanism which should be present in both monozygotic and dizygotic twins. Inhibin B is a new marker of testicular function, and deeper insight about its importance for male reproduction is needed.
Supplementary Material
Table.
Characteristics of semen and sex hormones. Values are medians (interquartile range) unless otherwise indicated
| Characteristic | Singletons (n=105) | Monozygotic twins (n=104) | Dizygotic twins (n=107) | P value*
|
||
|---|---|---|---|---|---|---|
| Singletons v monozygotic twins | Singletons v dizygotic twins | Singletons v all twins | ||||
| Sperm concentration (millions/ml) | 60.0 (26.0-99.0) | 71.5 (26.5-115.0) | 55.0 (25.0-103.0) | 0.65 | 0.78 | 0.42 |
| Sperm volume (ml) | 3.5 (2.8-4.5) | 3.4 (2.6-4.0) | 3.5 (2.5-4.5) | 0.76 | 0.33 | 0.67 |
| Sperm total count (millions) | 185 (96-367) | 237 (78-434) | 208 (84-328) | 0.83 | 0.86 | 0.47 |
| No (%) of men with sperm concentration <20 million/ml (% within each group) | 17 (16) | 15 (14) | 18 (16) | 0.72 | 0.90 | 0.90 |
| % of sperm with normal morphology† | 15 (10-21) | 12 (6-17) | 12 (7-18) | 0.03 | 0.70 | 0.16 |
| Sex hormone concentrations: | ||||||
| Testosterone (nmol/l) | 22.8 (17.2-26.4) | 19.8 (15.9-23.4) | 20.7 (16.7-24.1) | 0.16 | 0.99 | 0.34 |
| Follicle stimulating hormone (IU/l) | 3.9 (2.9-5.1) | 3.6 (2.1-6.2) | 4.4 (3.2-5.9) | 0.07 | 0.39 | 0.23 |
| Luteinising hormone (IU/l) | 2.3 (1.5-2.9) | 2.3 (1.6-3.0) | 2.4 (1.8-3.4) | 0.12 | 0.04 | 0.04 |
| Inhibin B (pg/ml) | 195 (155-255) | 155 (120-243) | 165 (131-210) | 0.004‡ | 0.43‡ | 0.07‡ |
Adjusted for duration of sexual abstinence, urogenital disorders, alcohol, age, smoking, season, and birth weight.
Morphology scored according to the World Health Organization's 1999 guidelines.
Also adjusted for sampling time.
Acknowledgments
We thank Gorm Dancher, research technician Kirsten Lunding, and technicians at the laboratory at the Institute of Anatomy, Aarhus University.
Footnotes
Funding: Danish Research Council (Jnr 9802562), Danish Ministry of Health: Research Centre for Environmental Health Found (Jnr 383-4-1999), Danish Health Insurance Fund (Jnr 11/207-98 and 2000B514), and Danish Epidemiology Science Centre, University of Aarhus, Denmark.
Competing interests: None declared.
Figure A shows how the study groups were established and table A gives characteristics of the participants.
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