The timing between one pregnancy and the next may affect the risk of pregnancy complications. Both short and long interpregnancy intervals have been associated with adverse outcome, but the bulk of adverse effects have been associated with short intervals [1-3]. It is important to determine whether the interpregnancy interval is a significant independent biological risk factor for adverse pregnancy events because women have some control over the spacing of their pregnancies and thus could potentially reduce their risk of such outcomes. Avoidance of short intervals can be achieved through use of contraception, but avoidance of long interpregnancy intervals is more problematic since a desired pregnancy may be precluded by subfertility, availability of a partner, economic issues, or illness.
Determining whether any particular factor has an important influence on pregnancy is complex. Many variables are known to influence pregnancy outcome and a single factor such as interpregnancy interval cannot be studied without accounting for myriad potential confounders. There are many reasons for the associations described in this topic, and causality should not be assumed. Some examples:
- If the interval between births is being measured and the second birth is preterm, the interbirth interval will be shorter than for a term birth and associated with higher neonatal mortality due to prematurity .
- After a stillbirth or neonatal death, women often wish to conceive again with minimal delay. If the causative factors for the fetal or neonatal death recur, an association between short interpregnancy interval and perinatal mortality would be observed.
- Breastfeeding both improves infant survival and lengthens the interval between pregnancies due to lactational amenorrhea, thus confounding the relationship between longer interpregnancy interval and improved pregnancy outcome. However, breastfeeding may also deplete the mother.
- Socioeconomic factors and maternal health issues may affect both timing of pregnancy and outcome of pregnancy. In some cases, closer birth spacing is the unintended result of lack of correct and consistent use of contraception, and in others, it is an intentional response to later initiation of childbearing .
- The type of pregnancy event needs to be considered. Miscarriages and stillbirth may deplete the mother less than a live born term infant who is breastfed.
This topic will review available data and guidelines on interpregnancy interval, as well as controversies regarding the significance of interpregnancy interval in the determination of obstetrical outcomes.