Selection bias due to parity-conditioning in studies of time trends in fertility

Background: Studies of couple fertility over time have often examined study populations with broad age ranges at a cross-section of
time. An increase in fertility has been observed in studies that followed episodes of fertility events either prospectively among nulliparous women or retrospectively among parous women. Fertility has a
biological effect on parity. If defined at a cross-section of time, parity
will also be affected by year of birth, and thus becomes a collider.
Conditioning (stratifying, restricting, or adjusting) on a collider may
cause selection bias in the studied association.
Methods: A study with prospective follow-up was taken as the
model to assess the validity of fertility studies. We demonstrate the
potential for selection bias using causal graphs and nationwide birth
statistics and other demographic data. We tested the existence of parity-conditioning bias in data including both parous and nulliparous
women. We also used a simulation approach to assess the strength of
the bias in populations with prior at-risk cycles. Finally, we evaluated
the potential for selection bias due to conditioning on parity in various sampling frames.
Results: Analyses indicate that the observed increase in fertility over time can be entirely explained by selection bias due to
parity-conditioning.
Conclusion: Heterogeneity [...]