Maternal Fish Consumption, Mercury Levels, and Risk of Preterm Delivery

Fei Xue; Claudia Holzman; Mohammad Hossein Rahbar; Kay Trosko; Lawrence Fischer

Disclosures

Environ Health Perspect. 2007;115(1):42-47. 

In This Article

Materials and Methods

Population

The POUCH Study recruited women from 52 participating prenatal clinics located in five Michigan communities, each of which include urban, suburban, and rural areas (Holzman et al. 2001) (Figure 1). Women were enrolled in the 15th to 27th week of pregnancy, with approximately 70% before the 24th week. Eligibility criteria included screening for maternal serum alpha-fetoprotein (MSAFP) levels between 15 and 22 weeks of pregnancy, > 14 years of age, competency in English, singleton pregnancy with no known congenital or chromosomal anomalies at the time of recruitment, and no prepregnancy diabetes mellitus. A total of 1,226 women were enrolled in the POUCH cohort during the first part of the study (8 September 1998 through 31 July 2001). All were included in these analyses, except for five because of loss to follow-up and an additional 197 because hair samples were not available (69 declined hair sampling and 129 had hair that was < 7.6 cm long or in a woven hairstyle). Mid-pregnancy hair mercury levels were assessed in the remaining 1,024 cohort women. The study protocol was approved by human subjects review boards at participating institutions. Before enrollment, all participants provided written consent.

Figure 1.

Location of the five Michigan communities in this study.

Due to Health Insurance Portability and Accountability Act (HIPAA) regulations, it was not possible to determine an exact response rate or compare characteristics of participants with those of women who declined enrollment in the POUCH study. However, we were able to compare POUCH study data with data recorded on birth certificates of women who delivered in the five study communities in 2000. Ethnic-specific analyses (white non-Hispanic, African American), weighted by the proportion of women enrolled from each community, showed that the POUCH sample was very similar to community mothers on most factors measured-age, parity, education levels, and the proportions of women with Medicaid insurance, preterm delivery, previous stillbirth, previous preterm infant, and previous low birth weight infant. The one exception was the percentage of African Americans > 30 years of age, which was lower in POUCH (14%) than in community birth certificates (21%).

Gestational Age

Gestational age at delivery was determined by date of first day of the last menstrual period (LMP), or a gestational age estimate from an early ultrasound (≤ 25 weeks), the latter used when the two estimates disagreed by > 2 weeks. Early ultrasound data were available for 93% of women.

Maternal Characteristics and Fish Consumption

Information on maternal characteristics including age, ethnicity, education, Medicaid insurance status, and smoking was collected through in-person interviews and self-administered questionnaires at enrollment. As part of the interview, women were asked "During this pregnancy how often have you eaten any of the following fish: shellfish, canned fish, other fish you purchased at a store or restaurant [referred to as bought fish], sport-caught fish in Michigan waters, and some other fish?" For each fish category, respondents were asked about their number of meals per day, week, month, or previous 6 months. Six months was used as an option for infrequent consumers of fish and was thought to capture the period from conception to study interview for most women in the study. The data were then scaled so that all fish consumption -- fish categories and total -- could be expressed as meals per 6 months, thereby describing levels of fish intake in approximately the first 6 months of pregnancy. Total fish consumption was calculated by summing consumption of all categories of fish and shellfish.

Mercury Levels in Hair

At enrollment, approximately ≥ 100 strands of hair were cut close to the scalp from the posterior vertex region of women with hair at least 7.6 cm in length. A segment of hair closest to the scalp, approximating exposure during pregnancy, was assessed for total mercury levels. The length of segment used varied by gestational week at enrollment, assuming average hair growth of approximately 1.3 cm per month (Saitoh et al. 1967). Before analysis, hair was washed with acetone and water to remove mercury deposited from external sources. Cold vapor atomic absorption spectrometry (CVAAS; model M6000; Cetac Technology, Omaha, NE) was used to quantify total mercury levels in hair. It is estimated that around 70-90% of mercury in hair is methylmercury (Chen et al. 2002). Because of this and other factors, researchers consider total mercury levels in hair to be a useful biomarker of exposure to methylmercury (Berglund et al. 2005; Chen et al. 2002).

Analytic Strategy

We used generalized linear models (GLM) to assess the relationships between mercury levels in hair, maternal characteristics, and fish consumption. We used multicovariate logistic regression to evaluate the association between maternal mercury levels and risk of preterm delivery (< 37 weeks' gestation), moderate preterm delivery (35-36 weeks' gestation), and very preterm delivery (< 35 weeks' gestation). Threshold levels for mercury effects were tested at quintile cutoffs and at the 90th percentile. To meet the underlying assumptions of the GLM, mercury levels were transformed to natural log scale (micrograms per gram) for analyses and then transformed back to mercury levels (micrograms per gram) for display in tables. All analyses were conducted using SAS 9.0 software (SAS Institute Inc., Cary, NC). On examining self-reports of total fish consumption, we found three women who had consumed > 300 fish meals in the 6 months corresponding to the first half of pregnancy. These outliers could not be verified and were removed from the analyses.

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