Has Human Fertility Declined Over Time?

Epidemiology Branch, National Institute of Environmental Health Sciences, National Institute of Health, Department of Health and Human Services, Research Triangle Park, NC, USA.
Epidemiology (Impact Factor: 6.2). 08/2005; 16(4):494-9. DOI: 10.1097/01.ede.0000165391.65690.e1
Source: PubMed


Reports of decreased semen quality over time have raised concerns about possible reductions in human fertility. Studies of couple fertility have produced conflicting results. We evaluate how changes in the availability and use of effective contraception and induced abortion might bias the direct study of time trends in couple fertility.
We assess the potential for bias in the context of 2 common study designs: (1) a study of time-to-pregnancy that estimates fecundability (excluding unintended pregnancies) and (2) a study of infertility rates that categorizes couples as fertile or infertile (including couples with unintended pregnancies as fertile).
In time-to-pregnancy studies, bias alone could produce more than a 2-fold apparent increase in fecundability over recent decades. In studies of infertility rates, the bias works in the opposite direction: a 30% underestimation of infertility during earlier decades could produce an apparent decrease in fertility over time.
Over the past 5 decades, changes in social factors that affect the rate and fate of unintended pregnancies could substantially bias time trends in fertility. These biases may explain the conflicting reports in the literature. Except in rare settings in which the factors affecting reproductive choices have not changed, it is probably impossible to identify biologic changes in fertility over recent decades.

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    • "Female fecundity is defined as the biologic capacity for reproduction irrespective of pregnancy intentions (Buck Louis, 2011), and is speculated to be on the global decline though with limited empirical evidence (Lutz et al., 2003; te Velde et al., 2010). Some authors have suggested a role for endocrine disrupting chemicals such as persistent organochlorine pollutants (POPs) while other authors point to changes in lifestyle for recent birth cohorts (Sallmen et al., 2005). "
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    ABSTRACT: An evolving body of evidence suggests an adverse relation between persistent organochlorine pollutants (POPs) and menstruation, though prospective longitudinal measurement of menses is limited and served as the impetus for study. We prospectively assessed the relation between a mixture of persistent organochlorine compounds and menstrual cycle length and duration of bleeding in a cohort of women attempting to become pregnant. Eighty-three (83%) women contributing 447 cycles for analysis provided a blood specimen for the quantification of 76 polychlorinated biphenyls and seven organochlorine pesticides, and completed daily diaries on menstruation until a human chorionic gonadotropin confirmed pregnancy or 12 menstrual cycles without conception. Gas chromatography with electron capture detection was used to quantify concentrations (ng g(-1)serum); enzymatic methods were used to quantify serum lipids (mg dL(-1)). A linear regression model with a mixture distribution was used to identify chemicals grouped by purported biologic activity that significantly affected menstrual cycle length and duration of bleeding adjusting for age at menarche and enrollment, body mass index, and cigarette smoking. A significant 3-d increase in cycle length was observed for women in the highest tertile of estrogenic PCB congeners relative to the lowest tertile (β=3.20; 95% CI 0.36, 6.04). A significant reduction in bleeding (<1 d) was observed among women in the highest versus lowest tertile of aromatic fungicide exposure (γ=-0.15; 95% CI -0.29, -0.00). Select POPs were associated with changes in menstruation underscoring the importance of assessing chemical mixtures for female fecundity.
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    • "Geographical differences in time to pregnancy remain unexplained and do not appear to be linked to difference in semen quality. Changes in the availability and use of effective contraception and induced abortion bias the direct study of time trends such that it is probably impossible to identify biological changes in fertility over recent decades (Sallmén et al., 2005). "
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    Full-text · Article · Sep 2007 · Toxicological Sciences
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    • "With growing worldwide speculation that human fecundity is declining (Toppari et al., 1996), tools for monitoring a population's reproductive health are needed for the early identification of toxicants detrimental to human reproduction and development. Joffe (2003) and Olsen and Rachootin (2003) have suggested using time-to-pregnancy (TTP) as a proxy of fecundity for monitoring reproductive health, although other authors challenge the feasibility of this approach (Sallmen et al., 2005). Some authors have suggested using twinning rates to monitor the reproductive health of populations, in part, given its rarity and ease of identification (James, 1982). "
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    ABSTRACT: Mothers of multiples are alleged to be more fecund than mothers of singletons. Some authors have suggested monitoring twinning rates for assessing temporal changes in a population's reproductive health. Using a nested case-control design, we estimated the odds of a multiple birth in relation to fecundity in the US Collaborative Perinatal Project inclusive of 8546 pregnant women who reported a known time-to-pregnancy (TTP) upon enrolment in the cohort, 1959-1966. Case mothers comprised 81 women giving birth to twins/triplets; control mothers comprised 243 women giving birth to singletons matched to case mothers on maternal age at a ratio of 3:1. The odds ratio (OR) for a multiple birth within 6 months of trying adjusting for maternal age and prior pregnancies was estimated using logistic regression. Discrete time Cox regression analysis was also utilized to estimate the fecundability OR. Women with a TTP of <or=6 months were more likely to have a multiple birth than women reporting a TTP of >6 months [OR=1.95; 95% confidence interval (95% CI)=1.09-3.51]. Excluding pregnancies after 13+ months resulted in a loss of precision (OR=2.14; 95% CI=0.90-5.04). These data support higher fecundity among mothers of multiples than mothers of singletons.
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