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  • Title: Tobacco smoking and twinning.
    Author: Olsen J, Bønnelykke B, Nielsen J.
    Journal: Acta Med Scand; 1988; 224(5):491-4. PubMed ID: 3264449.
    Abstract:
    In a case-control study an association was found between mothers' smoking habits and the frequency of dizygotic twinning. As cases were included all twins born in Denmark in 1984 and 1985. A random sample of 1.5% of mothers to singletons, born in the same period were selected as controls. Only live-borns, conceived after no hormonal treatment, were included in the study. The finding may be due to the well-known anti-estrogen effect of smoking. A rapid increase in female smoking has coincided with an end to an earlier decline of twinning rates. Further, estrogen levels during the luteal phase are lower in women who smoke than in those who do not smoke. Therefore, scientists suspect that a change in the hormonal balance between estrogen, follicle stimulating, and luteinizing hormones in female smokers might increase the probability of double ovulation. To test the hypothesis that dizygotic (DZ) twin mothers smoke more frequently than mothers of singletons and monozygotic (MZ) mothers, researchers have conducted a case control study in Denmark using as cases all women who gave birth to live born twins in 1984-1985. The randomly chosen controls included 1.5% of all women who gave birth to live born singletons in the same period. There were 250 MZ twins, 390 DZ twins, and 210 could not be classified. In general, researchers found no association between smoking habits and twinning. Yet smokers did have an increased frequency of DZ twins. This increased frequency only occurred, however, in mothers who smoke 10 cigarettes/day (10-14 cigarettes, OR=1.4; 15-19, OR=1.3; and 20+, OR=1.7). Since only live borns were included in this study and smoking increases the risk of fetal death, the present findings may be underestimate.
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