American Journal of Epidemiology, Vol 152, Issue 6 565-572, Copyright © 2000 by Oxford University Press
TK Jensen, T Scheike, N Keiding, I Schaumburg and P Grandjean
Many studies have found decreased fecundability, that is, the ability to
conceive in a menstrual cycle, with increasing female age. To evaluate the
effect of maternal age on waiting time to pregnancy, the authors reviewed
hospital charts of all pregnant women attending prophylactic antenatal care
at Odense University Hospital, Denmark, during 1972-1987. Only the first
pregnancy of each woman and only planned pregnancies were included (n =
14,754). The fecundability odds ratio (FR) was calculated as the odds of a
conception in a menstrual cycle among the older women divided by the odds
among women aged 15-24 years. The FR for women aged 25-29 years was 1.12
(95% confidence interval (CI): 1.04, 1.20), for women aged 30-34 years it
was 1.15 (95% CI: 1.01, 1.30), and for women above 34 years the FR was 2.44
(95% CI: 1.84, 3.22) after adjustment for confounders. The increased
fecundability with age is contrary to previous studies and may be explained
by selection bias, as sterile women were not included. In addition, some
very fertile young women who became pregnant by accident before efficient
birth control methods were available and therefore were excluded from time
to pregnancy studies may now use oral contraceptives until they plan a
pregnancy later in life and are included.
ARTICLES
Selection bias in determining the age dependence of waiting time to pregnancy
Department of Environmental Medicine, Institute of Public Health, University of Southern Denmark, Odense. t.koldjensen@ic.ac.uk
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