ArticlesTime trends in biological fertility in Britain
Introduction
Evidence for a secular decline in sperm concentration1 in some places,2, 3, 4 including Britain,4 but not in others5, 6, 7, 8, 9 has raised the question whether biological fertility has declined. Although many factors other than sperm density affect couple fertility, a high sperm concentration in Finland6 is paralleled by the observation that Finnish couples take less time to conceive, on average, than do British couples.10 There are also concerns that the health of the male reproductive system has deteriorated in other ways—eg, there has been a widespread increase in the incidence of testicular cancer.11
Time to pregnancy (TTP), measured in months or cycles, is a sensitive measure of the biological fertility of a couple. A growing number of published studies has examined the effect on TTP of age and smoking habit of both parents,12 educational level,13 recent use of oral contraception,14 timing of intercourse,15 maternal caffeine consumption,16, 17 and occupational use of agents such as solvents18 and pesticides.19 Except at the extremes of maternal age, any effects are small and do not explain the striking between-couple heterogeneity in TTP, which is similar in all assessed populations (including historical ones). Biological fertility is related to sperm concentration up to 40 × 106/mL, and to sperm morphology.20
Validation studies of TTP recall have shown that women's self-reports are unbiased as a measure of the true value, as assessed by a concurrent (follow-up) method.21 At a group level, the TTP distribution thus derived corresponds very closely to that obtained prospectively.22, 23, 24 Furthermore, recall is stable: the degree of correspondence is as good for pregnancies that occurred 14–20 years before data collection as for those occurring within 14 years.24
Occupational studies have shown that men are also able to give values of TTP which result in closely comparable distributions, and with similar data quality, as judged by the degree of digit preference.25 In the UK 1958 Birth Cohort—a nationally representative study that includes men and women—male respondents' values gave not only the same distribution, but also the same analytical results as those of the female respondents,13 albeit with a slightly higher item non-response rate. To have two separate groups is advantageous, since they can be examined for consistency. A retrospective cohort study was therefore done to assess time trends in biological fertility, by undertaking a cross-sectional survey that covered a broad age range of both sexes.
The primary hypothesis (hypothesis 1) was that there has been an increasing trend in TTP (declining fertility) over calendar time, as a period effect relating to the year when the pregnancy occurred. For these analyses, TTP was measured from the starting time—ie, when unprotected intercourse began before the first pregnancy. This time could have been when contraception was stopped, or the date of marriage. Because some of the descriptive epidemiology on declining male reproductive health tends to suggest a birth-cohort effect relating to the man's year of birth, consistent with an onset in early life,4, 26 subsidiary hypotheses were also examined: that a trend corresponded to the male partner's year of birth (hypothesis 2), and for completeness, that a trend was associated with the female partner's year of birth (hypothesis 3).
Section snippets
Data collection
Every month, the Office for National Statistics carries out a cross-sectional survey representative of the resident population of Great Britain. Households are randomly selected, and within each, respondents are selected, again at random. Data are collected by trained interviewers, and routine quality checks are carried out. About 1900 interviews are done per month, with a response rate of more than 70%. Each month, modules on one or more specific topics are included, and sociodemographic data
Results
The response rate to the survey was 72%. The TTP section was completed by most respondents: the refusal rate was less than 5% in each 5-year age/sex group, and showed no trend with age. 2010 respondents reported that they had had or fathered at least one pregnancy. Of these, 71 were omitted because a birth-control failure was reported to have occurred, and four were excluded because their starting time was uncertain. 156 and 12, respectively, were excluded because there had been a miscarriage
Discussion
This study found a clear rise in couple fertility in recent decades. The tendency towards quicker conception was found throughout the TTP distribution, including the low values, and therefore cannot be explained by any behaviour change occurring after some months of waiting. It was also seen among TTP values above 12 months (table 2), and therefore cannot be due to a trend towards earlier recognition of pregnancy. The trend was consistent between male and female respondents.
Whether response
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