Wesselink AK, Wise LA, Hatch EE, Mikkelsen EM, Savitz DA, Kirwa K, Rothman KJ. A prospective cohort study of seasonal variation in spontaneous abortion. Epidemiology. 2022 May;33(3):441-8. doi: 10.1097/EDE.0000000000001474

BACKGROUND: Up to 30% of pregnancies end in spontaneous abortion, yet few risk factors have been identified. Examining seasonal patterns in risk of spontaneous abortion can generate new hypotheses regarding environmental and lifestyle determinants.

METHODS: We used data from Pregnancy Study Online (PRESTO), a preconception cohort study of pregnancy planners from the United States and Canada, to examine seasonal variation in spontaneous abortion risk. We enrolled 12,197 women during 2013-2020, 6,104 of whom reported a conception within 12 months of enrollment. On follow-up questionnaires, participants reported date of spontaneous abortion and weeks gestation at time of loss. We used periodic regression to estimate two aspects of seasonal occurrence: peak/low ratio-a measure of intensity of seasonal variation-and peak timing. We examined season at risk (from date of each gestational week) in relation to spontaneous abortion; in a secondary analysis, we examined season of conception in relation to spontaneous abortion. We controlled for seasonal patterns in attempt initiation via month the pregnancy attempt began.

RESULTS: Almost 20% of women experienced spontaneous abortion. Risk was highest in late August, with a peak/low ratio of 1.3 (95% confidence interval [CI]: 1.1, 1.6). This seasonal pattern was evident almost exclusively for spontaneous abortion at <8 weeks since last menstrual period date (peak/low ratio: 1.4, 95% CI: 1.2, 1.8), and associations were stronger among women living in the Southern and Midwestern United States.

CONCLUSIONS: Environmental or lifestyle factors more prevalent in late summer may be associated with increased risk of early spontaneous abortion.

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