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.
