Is the natural twinning rate still declining?

To the Editor: An unexplained reduction of one third in twinning rates was observed in Western countries from the 1930s through the 1970s followed by a sharp rise from the 1980s onward due mainly to infertility treatment. There is concern about a general decline in fecundability, and it is unknown whether the “natural twinning rate” (which is a marker of fecundability) is still declining beneath the increase of twinning due to infertility treatment. We used a follow-up study of more than 60,000 pregnant women, who reported on fertility treatment during pregnancy, to show that the decline in natural twinning rates has apparently stopped. The Danish National Birth Cohort is a nationwide study that enrolled women early in pregnancy. Approximately 60% of all eligible women were informed about the study, depending on the cooperation of general practitioners, and approximately 60% of the informed women agreed to participate. The women were typically enrolled in the study at the first visit to the general practitioner (usually in the first trimester). At this time, the participants provided a blood sample and were interviewed about health-related issues, including potential fetal risk factors and lifestyle. Around the beginning of the third trimester, the participants again provided a blood sample and were again interviewed; topics included waiting time to pregnancy and infertility treatment. We used the National Birth Registry to identify 61,995 singleton and twin births from the 28th gestational week occurring between 1998 and 2001 of women participating in the Danish National Birth Cohort. We excluded 49 pregnancies from the analyses because they were either triplets or stillbirths with no information on twinning status. Among the remaining 61,946 pregnancies, 1311 (2.1%) resulted in twins; this percentage is comparable to that (2.0%) for the Danish population in the same time period, suggesting no selection into the cohort depending on whether the pregnancy was a twin pregnancy. Women taking 6 months or longer to conceive were asked whether they had received infertility treatment; among the 3873 women answering positively to this question, 589 (15.2%) had twins. The remaining 58,073 pregnancies, with no infertility treatment reported, resulted in 722 pairs of twins, which corresponds to a natural twinning rate of 1.24% (95% confidence interval 1.15–1.33%). As expected, the natural twinning rate increased with maternal age (from 1.0% in mothers younger than 25 years of age to 1.5% among mothers age 35 an older), with the most marked increase being for opposite-sexed twins who are all dizygotic. Twinning rates (Fig. 1) and maternal age distribution for births in Denmark in 1930–2001 were obtained through Statistics Denmark. From 1970 to 1980, when the overall twinning rate was historically low, the mean maternal age was very stable, around 26.6 (within a 0.2-year standard deviation), whereas the mean maternal age in the more recent Danish National Birth Cohort was 29.9 4.3 years. Age standardization of the natural twinning rate in the Danish National Birth Cohort to the maternal age distribution of 1980 changed the estimate of twinning rates only slightly from 1.24% (1.15–1.33%) to 1.15% (1.06–1.23%), which is markedly higher than the rate of 0.97% (0.95–0.99%) in the period 1970–1980. This follow-up study provides evidence for the end of the decline in natural twinning and may even suggest an increase in natural twinning rates. The rate of monozygotic twinning is fairly constant over time and place, whereas the frequency of natural dizygotic twinning depends on maternal age, parity, ethnicity, and body mass index. However, none of these factors appears to account for the observed, marked changes in natural twinning rates, which remain unexplained.