Shorter cycles: trihalomethanes and menstruation. (Science Selections).

Prompted by increased concerns about the health effects of chlorination by-products, a team of researchers led by Gayle C. Windham of the California Department of Health Services studied tap water consumption in a sample of Northern California women, and found that higher chlorination by-product concentrations in water were associated with a decrease in menstrual cycle length [EHP 111:935-941]. The Women's Reproductive Health Study is the first study to examine menstrual cycle function relative to tap water concentrations of trihalomethanes (THMs), the chlorination by-products most commonly measured in quarterly monitoring by water utility companies. The study also examined reproductive health effects from a number of other exposures, such as smoking and chemical solvent exposure.

THMs have been found in previous studies to be associated with adverse pregnancy outcomes including miscarriage and low birth weight. To find out if THMs are associated with other reproductive end points such as menstrual function, as well as to pinpoint possible mechanisms, the team evaluated data from the Women's Reproductive Health Study in a sample consisting of 403 married women aged 18-39 in Northern California.

After reporting lifestyle information and daily water drinking habits, the women collected and froze first-morning urine samples for the duration of 2-9 menstrual cycles and kept a daily diary to record, among other items, timing of menstrual bleeding (menses). The research team analyzed daily urine samples for estrogen and progesterone metabolites using enzyme-linked immunoassay. This gave them clues about each woman's ovulatory status; the day of ovulation was estimated using an accepted algorithm that selects the day after the peak of the estrogen-to-progesterone ratio. They analyzed cycle length variability and mean length of menstrual cycle, follicular and luteal phases, and menses.

The team also estimated total THM (TTHM) concentrations in tap water based upon an average of measurements taken by each of 10 water utility companies in the study area. The researchers gathered quarterly monitoring data of THM concentrations collected at various points in each utility's distribution system. They calculated concentrations for each of 4 individual THM compounds--bromoform, chloroform, chlorodibromomethane, and bromodichloromethane. They added individual THM compounds to calculate TTHMs and used those data to estimate TTHM concentrations in each woman's water supply during each menstrual cycle. They also added just the 3 brominated compounds for a separate estimate.

The researchers examined numerous potential confounders, including smoking, alcohol and caffeine consumption, pregnancy history, body mass index, age, race, education, income, and employment. They found very little evidence for confounding of TTHM effects on mean cycle length. Only 3% of the participants had average TTHM concentrations above the U.S. Environmental Protection Agency's annual maximum contaminant level of 80 micrograms per liter ([micro]g/L). For this study, high exposure was defined as 60 [micro]g/L or higher per day.

Overall, the investigators found a decrease in cycle length of 0.18 days per 10-[micro]g/L increase in TTHMs, with the most highly exposed cycles 1 day shorter after adjustment for variables. The decrease stemmed from shorter follicular phase length; little difference was seen in luteal phase or menses duration. High concentrations of the 3 brominated by-products together significantly decreased cycle length--by 1.2 days at the highest exposure level. Chloroform concentrations were not associated with a significant decrease in cycle length.

The shortened follicular phase presumably indicates a change in ovarian function, including earlier ovulation. This potentially could affect maturation of the egg, thickening of the uterine wall, and timing of when a woman is more or less likely to conceive--that is, the likelihood of getting pregnant. The team recommends future studies to confirm this possibility.