Mothers waiting for longer than one year to conceive their first child gave birth to babies with a higher risk of neonatal death compared with children conceived sooner. We restricted the analysis to primiparae (73.5% of whom reported no previous pregnancies) because death of a previous baby may influence both the decision to conceive again and its outcome.
Infertility treatment was self reported and was only asked of women taking longer than six months to conceive, but there was little difference in risk between treated and untreated, although the causes of death may differ between the two groups.
Only about 35% of eligible women participated in the cohort,5
and this could cause bias if participants with a long time to pregnancy were at a different level of risk compared to the non-participants. Furthermore, we could not distinguish the length of infertility beyond one year, which limits our ability to identify a dose-response, if it exists.
We collected information on time to pregnancy and confounders before delivery, reducing the potential for other types of bias. The mother's job title may be a poor proxy for social class, but the adjustment appeared to have little effect on our estimates.
A long time to pregnancy per se is not commonly considered a marker of increased risk, and untreated women with a history of infertility may seek (or receive) inadequate prenatal care.
Even though neonatal death was a rare event in this population, it is a serious outcome and any potential risk marker should be considered. Our finding needs, however, to be corroborated elsewhere before it can be stated that a long time to pregnancy increases the risk of neonatal death.
If infertility itself is associated with adverse outcomes, an appropriate comparison group should be used when assessing effect of infertility treatment on pregnancy outcomes, lest adverse effects of treatment be overestimated.
What is already known on this topic
Infertility treatment is correlated to adverse pregnancy outcomes, and evidence indicates that subfecundity per se is also associated with adverse pregnancy outcomes
What this study adds
Subfecundity may be associated with an increased risk of neonatal death and should be included as a risk indicator in neonatal care