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Has human fertility declined over time?: why we may never know
Markku Sallmén1, Clarice R Weinberg, Donna Day Baird
1Epidemiology Branch, National Institute of Environmental Health Sciences, National Institute of Health, Department of Health and Human Services, Research Triangle Park, NC, USA. Markku.Sallmen@ttl.fi
Epidemiology (Cambridge, Mass.)
|June 14, 2005
Summary
Changes in contraception and abortion use can significantly skew studies on human fertility trends. These social factors, not biological changes, likely explain conflicting research findings on fertility over time.
Area of Science:
- Reproductive Health
- Demography
- Epidemiology
Background:
- Declining semen quality raises concerns about human fertility.
- Conflicting study results on couple fertility trends exist.
- Investigating the impact of contraception and abortion on fertility study bias is crucial.
Purpose of the Study:
- To evaluate how changes in contraception and induced abortion availability and use may bias direct studies of couple fertility time trends.
Main Methods:
- Assessed bias in time-to-pregnancy (fecundability) studies, excluding unintended pregnancies.
- Assessed bias in infertility rate studies, including unintended pregnancies as fertile.
Main Results:
- Time-to-pregnancy studies may show over a 2-fold apparent increase in fecundability due to bias.
- Infertility rate studies might show an apparent decrease in fertility due to bias (e.g., 30% underestimation).
Conclusions:
- Social factors influencing unintended pregnancies can substantially bias fertility time trends.
- These biases likely explain conflicting reports in fertility research.
- Identifying true biological fertility changes is difficult without stable reproductive choice factors.