Hair Relaxers & Keratin Straighteners
Everyday exposureBeauty & personal careAlso called · 9 names▾▴
hair relaxer, relaxer pregnancy, chemical straightener, hair straightening treatment, keratin treatment, keratin pregnancy, keratin smoothing service, formaldehyde hair treatment, methylene glycol hair treatment
Hair
General information · effects depend on amount, timing, and how it is used. About these sources
Research strength: 3 / 5
Blackmore-Prince et al. (1999) and Rosenberg et al. (2005) found no higher preterm-birth rate among people who used chemical relaxers. Pregnancy, Birth and Baby advises against formaldehyde-containing straightening treatments because formaldehyde is a known carcinogen. FDA says some heated smoothing products release formaldehyde into salon air, while ANSES recommends against glyoxylic-acid products after acute kidney injury reports. These cautions concern different services and are not trimester-specific. Ask your provider about your situation.
Last reviewed: 2026-10-18
Sources
- Blackmore-Prince et al. 1999: Chemical hair treatments and adverse pregnancy outcomeUS government
- Rosenberg et al. 2005: Hair-relaxer use and risk of preterm birthUS government
- Pregnancy, Birth and Baby: Hair care during pregnancy
- FDA: Hair Smoothing Products That Release Formaldehyde When HeatedUS government
- ANSES: Alert confirmed for glyoxylic acid in hair-straightening products
Research strength: 3 / 5
Blackmore-Prince et al. (1999) and Rosenberg et al. (2005) found no higher preterm-birth rate among people who used chemical relaxers. Pregnancy, Birth and Baby advises against formaldehyde-containing straightening treatments because formaldehyde is a known carcinogen. FDA says some heated smoothing products release formaldehyde into salon air, while ANSES recommends against glyoxylic-acid products after acute kidney injury reports. These cautions concern different services and are not trimester-specific. Ask your provider about your situation.
Last reviewed: 2026-10-18
Sources
- Blackmore-Prince et al. 1999: Chemical hair treatments and adverse pregnancy outcomeUS government
- Rosenberg et al. 2005: Hair-relaxer use and risk of preterm birthUS government
- Pregnancy, Birth and Baby: Hair care during pregnancy
- FDA: Hair Smoothing Products That Release Formaldehyde When HeatedUS government
- ANSES: Alert confirmed for glyoxylic acid in hair-straightening products
Research strength: 3 / 5
Blackmore-Prince et al. (1999) and Rosenberg et al. (2005) found no higher preterm-birth rate among people who used chemical relaxers. Pregnancy, Birth and Baby advises against formaldehyde-containing straightening treatments because formaldehyde is a known carcinogen. FDA says some heated smoothing products release formaldehyde into salon air, while ANSES recommends against glyoxylic-acid products after acute kidney injury reports. These cautions concern different services and are not trimester-specific. Ask your provider about your situation.
Last reviewed: 2026-10-18
Sources
- Blackmore-Prince et al. 1999: Chemical hair treatments and adverse pregnancy outcomeUS government
- Rosenberg et al. 2005: Hair-relaxer use and risk of preterm birthUS government
- Pregnancy, Birth and Baby: Hair care during pregnancy
- FDA: Hair Smoothing Products That Release Formaldehyde When HeatedUS government
- ANSES: Alert confirmed for glyoxylic acid in hair-straightening products
Hair Relaxers & Keratin Straighteners — Breastfeeding
Research strength: 1 / 5
The Breastfeeding Network includes hair straightening among treatments that can continue during breastfeeding, but it does not cite direct infant follow-up for these products. No product-specific milk-level or nursing-infant study was found for conventional relaxers or heated keratin services in this review. FDA's warning about heated formaldehyde products concerns air exposure, not infant effects through milk. Product-specific lactation evidence is insufficient. Ask your provider about your situation.
Last reviewed: 2026-10-18
Sources
- The Breastfeeding Network: Beauty Treatments and BreastfeedingBreastfeeding charity
- FDA: Hair Smoothing Products That Release Formaldehyde When HeatedUS government
Pregnancy
Two pregnancy studies examined personal use of conventional relaxers. Blackmore-Prince et al. (1999) studied 525 Black women and reported adjusted odds ratios of 0.7 (95% CI 0.4-1.1) for preterm birth and 0.6 (95% CI 0.4-1.1) for low birth weight. Rosenberg et al. (2005) analyzed 6,130 singleton births, including 497 preterm births; the adjusted odds ratio for preterm birth among ever-users was 1.0 (95% CI 0.6-1.8), with no higher estimates for starting young or frequent, long-duration use. Chua-Gocheco et al. (2008) independently summarized both studies but gave 5,944 for the Rosenberg population; the primary abstract reports 6,130, so the primary figure is used here. These studies examined preterm birth and/or low birth weight, not modern heated keratin services or every pregnancy outcome. No pregnancy-outcome study of heated keratin or glyoxylic-acid services was identified in this review.
Breastfeeding
The Breastfeeding Network includes hair straightening among treatments that can continue during breastfeeding. Its page does not cite direct infant follow-up for relaxers or keratin services. No product-specific milk-level or nursing-infant study was found in this review. FDA's formaldehyde warning describes inhalation during heated services and does not measure transfer into milk or infant outcomes.
MotherToBaby describes conventional relaxers as using chemicals such as ammonium thioglycolate or, in older preparations, sodium hydroxide. FDA says some heated smoothing products contain formaldehyde, formalin, or methylene glycol and can release formaldehyde gas when heated. Glyoxylic-acid straightening products are a separate, often heat-activated service; the kidney findings about them do not apply to every conventional relaxer.
What was tested
Blackmore-Prince et al. used a case-control design among 525 Black women and examined chemical straightener or curl-product use during pregnancy or within three months before conception, with preterm birth and low birth weight as outcomes. Rosenberg et al. compared 497 preterm births with 5,633 longer-gestation singleton births in a study of 6,130 births. Neither study identified the current product formula or tested heated keratin services. The 2025 glyoxylic-acid case series selected symptomatic patients after exposure, while the 2026 exposure study measured two marketed products and modeled exposures rather than clinical outcomes.
Kidney findings for heated glyoxylic-acid straightening products
- Source A says
- ANSES concluded glyoxylic acid was very likely responsible for acute kidney injury and recommended against products containing it pending regulatory measures. A 2025 case series reported acute kidney injury in 12 of 13 symptomatic women exposed to these products. (ANSES: Alert confirmed for glyoxylic acid in hair-straightening products, Rabinovich et al. 2025, Clinical Toxicology)
- Source B says
- A 2026 exposure study of two marketed glyoxylic-acid products reported low modeled exposures and concluded that its measured conditions did not indicate health concerns for consumers and stylists. (Hewitt et al. 2026, Toxicology and Applied Pharmacology)
- Why they differ
- The sources studied different questions. ANSES reviewed clinical reports and toxicology, and the case series describes symptomatic patients. Hewitt et al. measured exposure from two products and modeled kidney exposure rather than following clinical outcomes. Wella and Kao employees and consultants were among the study authors, and both companies sell products containing glyoxylic acid.
- What the evidence shows overall
- The case series signals possible kidney injury but cannot estimate how often it occurs among all users. The exposure study applies to the two products and treatment conditions measured; it does not settle the clinical reports or apply to every formula. These findings concern heated glyoxylic-acid services, not sodium-hydroxide or ammonium-thioglycolate relaxers.
Adult uterine cancer findings
- Source A says
- In the Sister Study, frequent straightening-product use, more than four times in the previous year, was associated with higher uterine cancer incidence (HR 2.55, 95% CI 1.46-4.45). (Chang et al. 2022, Journal of the National Cancer Institute)
- Source B says
- In the Black Women's Health Study, the overall heavy-use estimate was not statistically significant (HR 1.18, 95% CI 0.81-1.71), while estimates were higher among postmenopausal participants; results among premenopausal participants were null. (Bertrand et al. 2023, Environmental Research)
- Why they differ
- The studies used different populations and exposure definitions. The Sister Study measured recent use in the prior year; the Black Women's Health Study used long-term frequency and duration. Neither identified product brands or formulas.
- Other sources
- A 2025 systematic review found that the evidence did not support hair relaxers as a risk factor for breast, ovarian, or uterine cancer in Black women, while noting that the included studies did not characterize the products' chemicals. (Hernandez et al. 2025, Journal of Applied Toxicology)
- What the evidence shows overall
- These are adult observational findings, not proof that a product caused cancer and not evidence about pregnancy or infants. The studies used different populations and did not identify ingredients, so the findings should remain distinct rather than being combined into a single product-specific estimate.
- FDA identifies formaldehyde, formalin, and methylene glycol as ingredient names to check on retail product labels. (FDA)
- FDA suggests asking salon professionals whether a smoothing product contains formaldehyde-related ingredients and reviewing a product label or Safety Data Sheet when available. (FDA)
- Pregnancy, Birth and Baby says to speak with a doctor if someone is unsure about a chemical hair treatment during pregnancy. (Pregnancy, Birth and Baby)
- Health Canada limits formaldehyde to 0.01% in hair straighteners intended for forced convection or heating that releases formaldehyde vapours; this condition does not apply to all hair products or jurisdictions. (Health Canada)
- ANSES recommends against using hair-straightening products containing glyoxylic acid while regulatory measures are pending. (ANSES)
General research, not specific to pregnancy.
Adult studies have examined fibroids, menstrual symptoms, and cancers among people who use hair straighteners and relaxers. Results vary by study, product type, and outcome, and most studies did not identify the chemicals in the products. These findings are not pregnancy or infant outcomes.
- The 2022 Sister Study followed 33,947 women with a uterus; frequent straightening-product use, more than four times in the prior year, was associated with uterine cancer incidence (HR 2.55, 95% CI 1.46-4.45). (Chang et al. 2022, Journal of the National Cancer Institute)
- In a cohort of 44,798 Black women, overall heavy relaxer use was not statistically significant for uterine cancer (HR 1.18, 95% CI 0.81-1.71); higher estimates appeared in postmenopausal subgroups, while results among premenopausal women were null. (Bertrand et al. 2023, Environmental Research)
- A 2025 systematic review of eight studies concluded that the overall evidence did not support hair relaxers as a risk factor for breast, ovarian, or uterine cancer in Black women. It found one study with a statistically significant association with fibroids, and no included study measured the chemicals in the products. (Hernandez et al. 2025, Journal of Applied Toxicology)
- A 2025 Sister Study analysis of 4,162 Black women found small or imprecise associations between straightener use at ages 10-13 and fibroids; several confidence intervals included no association. (Ogunsina et al. 2025, Environmental Health Perspectives)
- A 2026 prospective ultrasound study of 1,693 Black women found a higher fibroid-incidence estimate for recent use of temporary relaxers than for permanent relaxers, but the estimate was based on relatively few cases and the study did not collect ingredient or brand data. (Wise et al. 2026, American Journal of Epidemiology)
- A 2026 cohort of 14,366 participants found current relaxer use was associated with a prevalence ratio of 1.16 (95% CI 1.00-1.33) for abnormal uterine bleeding and 1.30 (95% CI 0.94-1.80) for dysmenorrhea. The study was observational. (Geller et al. 2026, Journal of Women's Health)
- A 2026 cohort study of 46,287 women reported possible associations with thyroid cancer, non-Hodgkin lymphoma, and pancreatic cancer, but found little evidence of a dose response and called for further research. (Bailey et al. 2026, Journal of the National Cancer Institute)
- A 2025 case series reported acute kidney injury in 12 of 13 symptomatic women after exposure to glyoxylic-acid straightening products. A case series cannot estimate how often this outcome occurs among all users. (Rabinovich et al. 2025, Clinical Toxicology)
Experiences from moms and non-medical voices. Not medical evidence.
- Blackmore-Prince et al. 1999, American Journal of EpidemiologyUS governmentPrimary
- Rosenberg et al. 2005, Ethnicity & DiseaseUS governmentPrimary
- Chua-Gocheco et al. 2008, Canadian Family PhysicianUS governmentPrimaryIndependent check
- MotherToBaby: Hair TreatmentsTeratology serviceSecondary
- Pregnancy, Birth and Baby: Hair care during pregnancySecondary
- FDA: Hair Smoothing Products That Release Formaldehyde When HeatedUS governmentSecondaryPage dated 2024-10-15
- Health Canada: Safety of Cosmetic IngredientsGovernment regulatorSecondaryIndependent check
- The Breastfeeding Network: Beauty Treatments and BreastfeedingBreastfeeding charitySecondary
- ANSES: Warning on the risks of hair straightening products containing glyoxylic acidSecondaryPage dated 2024-10-16
- ANSES: Alert confirmed for glyoxylic acid in hair-straightening productsSecondaryPage dated 2025-01-23
- Rabinovich et al. 2025, Clinical ToxicologyUS governmentPrimaryIndependent check
- Hewitt et al. 2026, Toxicology and Applied PharmacologyUS governmentPrimary
- Chang et al. 2022, Journal of the National Cancer InstituteUS governmentPrimary
- Bertrand et al. 2023, Environmental ResearchUS governmentPrimary
- Hernandez et al. 2025, Journal of Applied ToxicologyUS governmentPrimary
- Ogunsina et al. 2025, Environmental Health PerspectivesUS governmentPrimary
- Wise et al. 2026, American Journal of EpidemiologyUS governmentPrimary
- Geller et al. 2026, Journal of Women's HealthUS governmentPrimary
- Bailey et al. 2026, Journal of the National Cancer InstituteUS governmentPrimary
- Weaver et al. 2026, Current Environmental Health ReportsUS governmentPrimary
Last reviewed 2026-10-18
Independent check 2026-10-18
Not yet clinician-reviewed
Ask your provider about your situation.