Fecundity and twinning. A study within the Danish National Birth Cohort.
BACKGROUND
Decreased fecundity has been hypothesized as one of the causes of the widespread decline in twin rates in the 1960s and 1970s. The association between high fecundity and twinning is not, however, well documented.


METHODS
In the Danish National Birth Cohort, we identified 56 295 births (including 729 twin deliveries) with data on pregnancy planning and time to pregnancy (TTP) and no report of infertility treatment. We estimated the odds ratio (OR) of twinning as a function of TTP and having planned a pregnancy.


RESULTS
Increasing TTP correlated with decreased twinning rates. Compared with women who conceived within 2 months of trying, women who took longer than 1 year had an OR of 0.55 (95% confidence interval 0.39-0.77) of giving birth to twins. The association was slightly stronger for opposite sex twins. Women not planning a pregnancy had a lower rate of opposite sex twins than planners, but a comparable rate of same sex twins. We saw an excess of same sex pairs in women conceiving despite contraception.


CONCLUSIONS
A long TTP is associated with a reduced risk of twinning. The marked decline observed in the 1960s and 1970s may derive from a widespread decrease in fecundity.
Danish Epidemiology Science Centre, Department of Epidemiology and Social Medicine, University of Arhus, Vennelyst Boulevard 6, DK-8000 Arhus, Denmark. bassoo2@niehs.nih.gov
Olga
O
Basso
Kaare
K
Christensen
Jørn
J
Olsen
D000328:Adult; D015331:Cohort Studies; D016001:Confidence Intervals; D005260:Female; D005298:Fertility; D006801:Humans; D007231:Infant, Newborn; D008297:Male; D016017:Odds Ratio; D011247:Pregnancy; D012723:Sex; D013997:Time Factors; D014427:Twins