title: Has human fertility declined over time?: why we may never know.
abstract: BACKGROUND
  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.
  
  
  METHODS
  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).
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Epidemiology 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
authors.forename: Markku
authors.initials: M
authors.lastname: Sallmén
authors.forename: Clarice R
authors.initials: CR
authors.lastname: Weinberg
authors.forename: Donna Day
authors.initials: DD
authors.lastname: Baird
authors.forename: Marja-Liisa
authors.initials: ML
authors.lastname: Lindbohm
authors.forename: Allen J
authors.initials: AJ
authors.lastname: Wilcox
mesh_terms: D000028:Abortion, Induced; D015982:Bias; D003198:Computer Simulation; D003267:Contraception / Q000009:adverse effects / Q000706:statistics & numerical data; D003268:Contraception Behavior / Q000639:trends; D004812:Epidemiologic Methods; D005260:Female; D005298:Fertility / Q000502:physiology; D006801:Humans; D007246:Infertility / Q000453:epidemiology* / Q000503:physiopathology; D008297:Male; D008954:Models, Biological; D011247:Pregnancy; D012044:Regression Analysis; D012107:Research Design; D013997:Time Factors