Are hair relaxers safe during pregnancy?

What does the evidence say about hair relaxers during pregnancy?

1. Hair relaxers use chemicals like ammonium thioglycolate or, in older formulas, sodium hydroxide, and when used as directed, only a small amount is absorbed through unbroken, healthy scalp skin. 1
2. Some hair-straightening products contain formaldehyde or release it when heated, a specific concern MotherToBaby calls out separately from ordinary hair dye. 1
3. In a cohort of nearly 34,000 women, those who used hair straighteners more than four times in the past year had more than double the hazard of developing uterine cancer by age 70 compared with women who never used them, about a 2.4 percentage-point absolute increase. 2
4. In a preconception cohort of over 11,000 people trying to conceive, current hair-relaxer users had about 19 percent lower odds of conceiving in a given cycle than people who'd never used relaxers, with the biggest drop among those with 10 or more years of use. 3
5. In professional salons, airborne formaldehyde during straightening procedures can spike to about three times the average shift level, with some salons exceeding occupational exposure limits, which is why ventilation matters even for a single at-home application. 4

Is hair relaxers safe in each trimester?

• First trimester. This is the most sensitive window for organ development, so it's a reasonable time to skip formaldehyde-releasing straightening formulas specifically and stick to a well-ventilated space if you go ahead.
• Second trimester. MotherToBaby's reassurance is about occasional, directed use on healthy, unbroken scalp skin. If your scalp is irritated, freshly scratched, or sunburned, absorption is likely higher, so it's worth waiting for that skin to settle down first.
• Third trimester. The uterine cancer and fecundability signals in the research are tied to years of frequent use, not a single late-pregnancy touch-up, so an occasional relaxer application now is a different exposure question than a lifelong routine.