What causes Interbirth Interval length?

What causes Interbirth Interval length?

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Interbirth Interval length among humans is a point of interesting research. Discovering the underlying reasons about what is optimal Interbirth Interval length and what affects different Interbirth Interval lengths in various societies. The plausible reasons for enhanced Interbirth Interval length are infant night waking, which leads to tiring of mother in the form of maternal sleep disturbance, social support from family, and postpartum depression.

The study is focused on the mother-offspring conflict theory. The introduction of siblings can decrease the mother’s investment of her limited resources in any one infant. Since the infant has 100% of its own genes but shares only 50% with its siblings, the infant’s fitness helps more from investment compared to investment in any siblings. For the mother’s fitness, it is, on the other hand, better to spread the investment equally between all current and potential future offspring.


The study is conducted by a team of researchers from Åbo Akademi University, Finland. The study surveyed 729 mothers, aged 22 to 60 years old. The mean age of the participant is 41.3 years. Only mothers with two children were considered in the study. Of the participating mothers, 433 (59.4%) had two children and 296 (40.6%) had three or more. The mean for the Interbirth Interval between the first and second child was 39.9 months and for the Interbirth Interval between the second and third child 46.4 months.

To study the impact of Infant night waking, maternal sleep disturbance, postpartum depression, questionnaires like “how many times on average did your child wake up per night?”, “How many times on average did your child wake up per night?” were asked to the respondents. With the results, statistical models were developed.

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Study findings

The study observes that infant night waking was associated with more maternal sleep disturbance and that maternal sleep disturbance was associated with more postpartum depression symptoms. It also correlates those mothers with more sleeping problems had slightly longer interbirth intervals. Also, on the contrary, the study finds that first-time mothers who suffer from more severe sleep problems, but not from symptoms of postpartum depression, have a shorter interbirth interval between their first and second child. The plausible reason behind such observation is those mothers who are sleep deprived but receive social support do not develop postpartum depression symptoms, and this protects against long Interbirth intervals. It is also more likely that these mothers have a supportive partner, helping the conception of an added child.

Infant night waking is an adaptive strategy to tire the mother, making her less likely to conceive with an added child, thus lengthening the interbirth intervals. Interestingly, the results show that postpartum depression symptoms could themselves be adaptive from the offspring’s fitness’ point of view, by being associated with longer Interbirth intervals. In other words, if infant night waking was indeed an adaptive strategy to extend the Interbirth Interval by exhausting the mother, it seems like maternal sleeping difficulties alone are not sufficient to result in longer Interbirth Intervals.

The study can be found here.

David Simpson