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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    Chemosphere 12/2011; 85(11):1742-8. DOI:10.1016/j.chemosphere.2011.09.027 · 3.34 Impact Factor
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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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    Toxicological Sciences 09/2007; 98(2):332-47. DOI:10.1093/toxsci/kfm008 · 3.85 Impact Factor
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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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