Optimal interpregnancy interval in autism spectrum disorder: A multi-national study of a modifiable risk factor.
Waiting about three years between pregnancies is linked to the lowest autism rates in Nordic countries.
01Research in Context
What this study did
Pereira et al. (2021) looked at birth records from four Nordic countries. They asked: does the gap between pregnancies change the chance the second child has autism?
The team tracked thousands of brothers and sisters. They compared short gaps, long gaps, and medium gaps to see which group had more autism diagnoses.
What they found
The study showed a U-shaped pattern. Very short gaps (under one year) and very long gaps (over five years) both had higher autism rates.
The sweet spot sat around 35 months. Families who waited about three years had the lowest autism risk in their second child.
How this fits with other research
Bravo-Muñoz et al. (2025) pooled data on moms with diabetes or PCOS. They also found small jumps in autism odds, so Gavin’s birth-spacing clue joins a list of modest, changeable risks.
Schendel et al. (2022) used the same Danish registers but focused on genes and family mental-health history. Their work says biology and spacing probably add up instead of canceling out.
Miller et al. (2016) followed high-risk siblings to school age. Like Gavin, they show early family factors echo years later, backing the idea that timing matters.
Why it matters
You cannot change genes, but parents can plan birth spacing. Share the 35-month finding with families who ask about timing another baby. Stress that the risk shift is small and autism still stems from many causes. Use the info to guide questions during intake, not to alarm.
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02At a glance
03Original abstract
It is biologically plausible that risk of autism spectrum disorder (ASD) is elevated by both short and long interpregnancy intervals (IPI). We conducted a retrospective cohort study of singleton, non-nulliparous live births, 1998-2007 in Denmark, Finland, and Sweden (N = 925,523 births). Optimal IPI was defined as the IPI at which minimum risk was observed. Generalized additive models were used to estimate relative risks (RR) of ASD and 95% Confidence Intervals (CI). Population impact fractions (PIF) for ASD were estimated under scenarios for shifts in the IPI distribution. We observed that the association between ASD (N = 9302) and IPI was U-shaped for all countries. ASD risk was lowest (optimal IPI) at 35 months for all countries combined, and at 30, 33, and 39 months in Denmark, Finland, and Sweden, respectively. Fully adjusted RRs at IPIs of 6, 12, and 60 months were 1.41 (95% CI: 1.08, 1.85), 1.26 (95% CI: 1.02, 1.56), and 1.24 (95% CI: 0.98, 1.58) compared to an IPI of 35 months. Under the most conservative scenario PIFs ranged from 5% (95% CI: 1%-8%) in Denmark to 9% (95% CI: 6%-12%) in Sweden. The minimum ASD risk followed IPIs of 30-39 months across three countries. These results reflect both direct IPI effects and other, closely related social and biological pathways. If our results reflect biologically causal effects, increasing optimal IPIs and reducing their indications, such as unintended pregnancy and delayed age at first pregnancy has the potential to prevent a salient proportion of ASD cases. LAY SUMMARY: Waiting 35 months to conceive again after giving birth resulted in the least risk of autism. Shorter and longer intervals resulted in risks that were up to 50% and 85% higher, respectively. About 5% to 9% of autism cases might be avoided by optimizing birth spacing.
Autism research : official journal of the International Society for Autism Research, 2021 · doi:10.1002/aur.2599