Chemical hair straighteners (“relaxers”) are used by millions of North Americans, particularly women of color. Hair relaxers may contain endocrine-disrupting compounds, which can harm fertility. We evaluated the association between hair relaxer use and fecundability among 11,274 participants from Pregnancy Study Online (PRESTO), a North American preconception cohort study. During 2014-2022, participants completed a baseline questionnaire on which they reported their history of relaxer use and completed follow-up questionnaires every 8 weeks for 12 months or until pregnancy, whichever came first. We used multivariable-adjusted proportional probabilities regression models to estimate fecundability ratios (FR) and 95% confidence intervals (CI). Relative to never use, fecundability was lower among current (FR=0.81, 95% CI: 0.64-1.03) and former (FR=0.89, 95% CI: 0.81-0.98) users of hair relaxers. FRs for first use of hair relaxers at age <10, 10-19, and ≥20 years were 0.73 (95% CI: 0.55-0.96), 0.93 (95% CI: 0.83-1.04), and 0.85 (95% CI: 0.74-0.98), respectively. Fecundability was lowest among those with longer durations of use (≥10 years vs. never: FR=0.71, 95% CI: 0.54-0.91) and more frequent use (≥5 times/year vs. never: FR=0.82, 95% CI: 0.60-1.11), but associations were non-monotonic. In this preconception cohort study, use of chemical hair straighteners was associated with slightly reduced fecundability.
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