Hot tubs early in pregnancy were linked to neural tube defects, and the sauna estimate was not statistically significant
This 1992 JAMA study followed 23,491 pregnant women in New England. The American College of Obstetricians and Gynecologists (ACOG) cites it when it links heat exposure to neural tube defects (birth defects of the brain and spinal cord).
The paper
Maternal Heat Exposure and Neural Tube Defects
Milunsky A. JAMA: The Journal of the American Medical Association, 1992;268(7):882-885.
See the study at a glance
- Design
- Prospective cohort study
- People
- 23,491 pregnant women
- Who they were
- Pregnant women having prenatal tests (alpha-fetoprotein screening or amniocentesis). Most came through private obstetric practices in New England, and the researchers had complete data on exposure and outcome for 97% of them.
- What they did
- Women reported early-pregnancy exposure to four heat sources: hot tubs, saunas, electric blankets and fever. The researchers adjusted the risk estimates for the mother's age, folic acid use, family history and other heat sources.
- How long
- Early pregnancy to birth
These numbers came out of it
1.8
After adjustment, first-trimester sauna use carried 1.8 times the risk of a neural tube defect. The plausible range (95% confidence interval) ran from 0.4 to 7.9 and takes in the possibility of no effect.
2.8
First-trimester hot tub use carried 2.8 times the risk after adjustment. Its plausible range ran from 1.2 to 6.5.
6.2
Women exposed to two types of heat had 6.2 times the risk. One type gave 1.9.
Adjusted relative risk with 95% confidence interval
Hot tub use carried the clearest risk among first-trimester heat sources
The estimates are adjusted for the mother's age, folic acid use, family history of neural tube defects and other heat sources.
The researchers did this
Researchers identified 23,491 women who were having alpha-fetoprotein screening or amniocentesis and asked them about heat exposure in early pregnancy.Source 1 · Cohort studyMaternal Heat Exposure and Neural Tube DefectsMilunsky A. JAMA: The Journal of the American Medical Association, 1992.Read the paper ↗PubMed ↗ Most came through private obstetric practices in New England. Complete data on exposure and outcome were available for 97% of them.
Before any adjustment, women with any heat exposure had 1.6 times the risk (95% CI 0.9–2.9).Source 1 · Cohort studyMaternal Heat Exposure and Neural Tube DefectsMilunsky A. JAMA: The Journal of the American Medical Association, 1992.Read the paper ↗PubMed ↗ After adjustment, hot tub use came out at 2.8 (1.2–6.5). The sauna estimate was 1.8, and its interval from 0.4 to 7.9 left room for no effect at all. Fever gave 1.8 (0.8–4.1).
Risk climbed with the number of heat sources. It went from 1.9 (0.9–3.7) after one type of exposure to 6.2 (2.2–17.2) after two.Source 1 · Cohort studyMaternal Heat Exposure and Neural Tube DefectsMilunsky A. JAMA: The Journal of the American Medical Association, 1992.Read the paper ↗PubMed ↗
The abstract doesn't say how many babies were affected or how hot and long the sessions were, and the full text is paywalled. ACOG's committee opinion on exercise in pregnancy cites this paper for its statement that heat from sources including saunas has been associated with neural tube defects.Source 2 · GuidancePhysical Activity and Exercise During Pregnancy and the Postpartum PeriodAmerican College of Obstetricians and Gynecologists, Committee on Obstetric Practice. American College of Obstetricians and Gynecologists, Committee on Obstetric Practice, 2020.Read the paper ↗
The study can't tell you these things
- Exposure was self-reported, and the abstract gives no data on temperature or session length.
- A sauna range running from 0.4 to 7.9 is too wide to show an effect in either direction.
- The study was observational, so it can't show that heat caused the defects.
- Most women came through private obstetric practices in New England.
People quote it like this
ACOG cites it when it links heat from hot tubs and saunas to neural tube defects.Source 2 · GuidancePhysical Activity and Exercise During Pregnancy and the Postpartum PeriodAmerican College of Obstetricians and Gynecologists, Committee on Obstetric Practice. American College of Obstetricians and Gynecologists, Committee on Obstetric Practice, 2020.Read the paper ↗ The paper's own sauna estimate wasn't statistically significant, and the clear result came from hot tubs.
We cited these sources
- 01Milunsky A. Maternal Heat Exposure and Neural Tube Defects. JAMA: The Journal of the American Medical Association. 1992;268(7):882-885. doi:10.1001/jama.1992.03490070064043 PMID 1640616
- 02American College of Obstetricians and Gynecologists, Committee on Obstetric Practice. Physical Activity and Exercise During Pregnancy and the Postpartum Period. American College of Obstetricians and Gynecologists, Committee on Obstetric Practice. 2020.