Selection bias in determining the age dependence of waiting time to pregnancy

Many studies have found decreased fecundability, that is, the ability to co nceive in a menstrual cycle, with increasing female age. To evaluate the ef fect of maternal age on waiting time to pregnancy, the authors reviewed hos pital charts of all pregnant women attending prophylactic antenatal care at Odense University Hospital, Denmark, during 1972-1987, Only the first preg nancy of each woman and only planned pregnancies were included (n = 14,754) , The fecundability odds ratio (FR) was calculated as the odds of a concept ion in a menstrual cycle among the older women divided by the odds among wo men aged 15-24 years. The FR for women aged 25-29 years was 1.12 (95% confi dence interval (CI): 1.04, 1.20), for women aged 30-34 years it was 1.15 (9 5% 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 wi th age is contrary to previous studies and may be explained by selection bi as, as sterile women were not included. In addition, some very fertile youn g women who became pregnant by accident before efficient birth control meth ods were available and therefore were excluded from time to pregnancy studi es may now use oral contraceptives until they plan a pregnancy later in lif e and are included.