Time trends in biological fertility in Britain.

BACKGROUND

There is evidence of a decline in semen quality in some countries, including Britain, in recent decades. This retrospective cohort study examined the hypothesis that biological fertility had also declined. The trend in couple fertility was assessed by means of time to pregnancy (TTP--a sensitive and validated measure of fertility.

METHODS

A representative sample of the British population aged 16-59 years was surveyed. TTP was obtained for all births conceived after unprotected intercourse that began during 1961-93, excluding contraceptive failures. The sample size was 1540.

FINDINGS

In contrast to the original hypothesis, this study found that fertility has increased; the rising trend was accompanied by slight dips during 1976-80 and 1986-90. These results were consistent between male and female respondents, and undiminished by adjustment for possible confounding factors. A stronger and more consistent relation was found with the year when unprotected intercourse started (a period effect) than with the year of birth of either partner (a birth cohort effect).

INTERPRETATION

The findings could not be explained by trends in age at first birth, increased treatment of subfertility, or changes in oral contraceptive use. If a decline in male fertility has occurred, it has been more than compensated for by a countervailing increase in couple fertility.