Do gravidity and age affect pregnancy outcome?

Fetal loss has generally been found to vary with gravidity, previous experience of fetal loss, and maternal age, but the literature is divided on the reasons for these associations. In this paper we examine pregnancy histories obtained retrospectively from a nationally representative one-in-one-thousand sample of women in Australia aged 20 to 59 years. The relations of fetal loss ratios with both gravidity and previous outcome are consistent with heterogeneity of risk over the study population and a stopping rule, whereby high-risk women undertake more pregnancies than low-risk women to achieve the same number of live births. Evidence is presented that elevated loss ratios in the teens indicate not higher risk but a selection for short gestation intervals, while loss ratios beyond the mid-thirties do not point unequivocally to a substantial increase in risk at the older reproductive ages.