Fluoride exposure among pregnant women in California

Water fluoridation has been cited as one of the dental field’s most successful interventions to improve dental health. Since 1945, when the United States implemented community water fluoridation, rates of cavities have declined. Also, providing fluoridated tap water to most residents, regardless of income, education, or socioeconomic status, can reduce disparities in dental health.

Fluoride can help prevent cavities by improving the tooth’s ability to withstand acids produced by bacteria in the mouth. When fluoride is ingested, however, it can affect the cells that are responsible for enamel formation, which results in white or brown patches appearing on the teeth, referred to as a mottled appearance. This is called enamel fluorosis, and national surveys have shown that the incidence of enamel fluorosis has been steadily rising in the U.S. population. We know that fluoride affects these enamel-forming cells, so it is plausible that fluoride can affect other cells in the body too.

Recent studies suggest that higher levels of fluoride exposure may have potentially neurotoxic effects among children. The presence of fluoride in pregnant women and exposure to babies in-utero have not been examined in the United States. Therefore, we set out to answer the following questions using participant samples that had already been collected and stored.

  • Do the levels of fluoride from community water supplies reflect fluoride levels in the plasma and urine of pregnant women?
  • Do the levels of fluoride in amniotic fluid during mid-gestation reflect maternal fluoride exposure?

From 2014–2016 we collected plasma, urine, and amniotic fluid samples from women in their second trimester of pregnancy who were seeking care at Zuckerberg San Francisco General Hospital. Our population of women included 48 participants from around Northern California and the Central Valley. We selected a diverse spectrum of participants to better reflect California’s demographics. They were 42% White, 25% Black, 19% Latina, and 15% Asian or Pacific Islander. The average water fluoride concentration was 0.5 mg/L, which is below the current Centers for Disease Control and Prevention (CDC) recommendation of 0.7 mg/L.

We used mothers’ addresses to link them to their community water supply and gathered publicly available data to identify the water fluoride levels during the year the women’s samples were collected. We measured fluoride in plasma, urine, and amniotic fluid.

We found that the amount of fluoride in community water correlated with fluoride in bodily fluids even after adjusting for maternal age, race/ethnicity, maternal BMI, smoking status, and gestational age at the time of sample collection. Every pregnant woman in our study had some exposure to fluoride, and we found that fluoride passed through the placenta into the amniotic fluid which indicates fetal exposure.

dana figure
Correlations between fluoride concentrations in water, maternal urine, maternal urine adjusted for specific gravity, maternal serum, and amniotic fluid. Each box represents the correlation in fluoride concentration between the sample type on the right and the sample type along the diagonal. Green indicates the strongest correlation. A correlation is a numerical value which represents the similarity between two sets of numbers. Higher correlation means that the two sets of numbers are more similar, such that as one number tends to increase, the other number also tends to increase. A correlation of 1 means that the numbers are the same. A correlation of 0 means that the numbers are statistically independent, or that they are completely unrelated.

What does this mean?

Our findings reveal two key points. First, the level of fluoride that mothers are exposed to during pregnancy from their community water supplies is reflected in their own urine fluoride levels. These results support the use of urinary fluoride as a biomarker for systemic exposure which is helpful for future research. Using urinary fluoride as a biomarker allows researchers to correlate the amount of fluoride a pregnant person is exposed to with the amount of fluoride found in maternal plasma and amniotic fluid.

Second, our finding that fetuses are exposed to fluoride when it passes through the placenta into amniotic fluid raises concern and the need for more research. If fluoride exposure in-utero does hinder neurodevelopment, then exposure to fluoride during pregnancy would likely be problematic.

More research needs to be done to determine what this means for recommended levels in water. Public health guidelines may need to be revised to consider the trade-offs between the benefits of community water fluoridation for cavities and the potential drawbacks for neurodevelopment. Other approaches may need to be implemented to realize the benefits of using fluoride early in childhood for dental carries versus widespread exposure during pregnancy. We hope our findings spur the research community to conduct more studies that can potentially inform these new guidelines.

About the author

danaDana Goin has been a postdoctoral scholar with the UCSF Program on Reproductive Health and the Environment since June 2019. She finished a PhD in Epidemiology from the University of California, Berkeley in May 2019. Her work focuses on the social and environmental determinants of maternal and infant health.

Co-authors: Dawud Abduweli Uyghurturk, E. Angeles Martinez-Mier, Tracey J. Woodruff, and Pamela K. Den Besten.