Emily Oken

A Nearly Continuous Measure of Birth Weight for Gestational Age Using a United States National Reference

BackgroundFully understanding the determinants and sequelae of fetal growth requires a continuous measure of birth weight adjusted for gestational age. Published United States reference data, however, provide estimates only of the median and lowest and highest 5th and 10th percentiles for birth weight at each gestational age. The purpose of our analysis was to create more continuous reference measures of birth weight for gestational age for use in epidemiologic analyses.MethodsWe used data from the most recent nationwide United States Natality datasets to generate multiple reference percentiles of birth weight at each completed week of gestation from 22 through 44 weeks. Gestational age was determined from last menstrual period. We analyzed data from 6,690,717 singleton infants with recorded birth weight and sex born to United States resident mothers in 1999 and 2000.ResultsBirth weight rose with greater gestational age, with increasing slopes during the third trimester and a leveling off beyond 40 weeks. Boys had higher birth weights than girls, later born children higher weights than firstborns, and infants born to non-Hispanic white mothers higher birth weights than those born to non-Hispanic black mothers. These results correspond well with previously published estimates reporting limited percentiles.ConclusionsOur method provides comprehensive reference values of birth weight at 22 through 44 completed weeks of gestation, derived from broadly based nationwide data. Other approaches require assumptions of normality or of a functional relationship between gestational age and birth weight, which may not be appropriate. These data should prove useful for researchers investigating the predictors and outcomes of altered fetal growth.

Maternal smoking during pregnancy and child overweight: systematic review and meta-analysis

Objective:Perform a systematic review of studies reporting on the association between maternal prenatal cigarette smoking and child overweight.Design:Meta-analysis of observational studies.Data sources:Medline search and review of reference lists among studies published through June 2006.Review methods:Included studies reported an association between maternal smoking during pregnancy and risk of overweight among children at least 2 years of age. We did not include in the meta-analysis studies that provided only a continuous measure of adiposity, although those studies are discussed separately.Results:Based on results of 84 563 children reported in 14 observational studies, children whose mothers smoked during pregnancy were at elevated risk for overweight (pooled adjusted odds ratio (OR) 1.50, 95% CI: 1.36, 1.65) at ages 3–33 years, compared with children whose mothers did not smoke during pregnancy. The pooled estimate from unadjusted odds ratios (OR 1.52, 95% CI: 1.36, 1.69) was similar to the adjusted estimate, suggesting that sociodemographic and behavioral differences between smokers and nonsmokers did not explain the observed association. Although we observed evidence for publication bias, simulating a symmetric set of studies yielded a similar estimate (OR 1.40, 95% CI: 1.26, 1.55).Conclusions:Prenatal smoking exposure appears to increase rates of overweight in childhood. In parts of the world undergoing the epidemiologic transition, the continuing increase in smoking among young women could contribute to spiraling increases in rates of obesity-related health outcomes in the 21st century.

Maternal Fish Consumption, Hair Mercury, and Infant Cognition in a U.S. Cohort.

Fish and other seafood may contain organic mercury but also beneficial nutrients such as n-3 polyunsaturated fatty acids. We endeavored to study whether maternal fish consumption during pregnancy harms or benefits fetal brain development. We examined associations of maternal fish intake during pregnancy and maternal hair mercury at delivery with infant cognition among 135 mother–infant pairs in Project Viva, a prospective U.S. pregnancy and child cohort study. We assessed infant cognition by the percent novelty preference on visual recognition memory (VRM) testing at 6 months of age. Mothers consumed an average of 1.2 fish servings per week during the second trimester. Mean maternal hair mercury was 0.55 ppm, with 10% of samples > 1.2 ppm. Mean VRM score was 59.8 (range, 10.9–92.5). After adjusting for participant characteristics using linear regression, higher fish intake was associated with higher infant cognition. This association strengthened after adjustment for hair mercury level: For each additional weekly fish serving, offspring VRM score was 4.0 points higher [95% confidence interval (CI), 1.3 to 6.7]. However, an increase of 1 ppm in mercury was associated with a decrement in VRM score of 7.5 (95% CI, –13.7 to –1.2) points. VRM scores were highest among infants of women who consumed > 2 weekly fish servings but had mercury levels ≤1.2 ppm. Higher fish consumption in pregnancy was associated with better infant cognition, but higher mercury levels were associated with lower cognition. Women should continue to eat fish during pregnancy but choose varieties with lower mercury contamination.

Evidence on the Human Health Effects of Low Level Methylmercury Exposure

Background: Methylmercury (MeHg) is a known neuro-toxicant. Emerging evidence indicates it may have adverse effects on the neuro-logic and other body systems at common low levels of exposure. Impacts of MeHg exposure could vary by individual susceptibility or be confounded by bene-ficial nutrients in fish containing MeHg. Despite its global relevance, synthesis of the available literature on low-level MeHg exposure has been limited. Objectives: We undertook a synthesis of the current knowledge on the human health effects of low-level MeHg exposure to provide a basis for future research efforts, risk assessment, and exposure remediation policies worldwide. Data sources and extraction: We reviewed the published literature for original human epidemio-logic research articles that reported a direct biomarker of mercury exposure. To focus on high-quality studies and those specifically on low mercury exposure, we excluded case series, as well as studies of populations with unusually high fish consumption (e.g., the Seychelles), marine mammal consumption (e.g., the Faroe Islands, circumpolar, and other indigenous populations), or consumption of highly contaminated fish (e.g., gold-mining regions in the Amazon). Data synthesis: Recent evidence raises the possibility of effects of low-level MeHg exposure on fetal growth among susceptible subgroups and on infant growth in the first 2 years of life. Low-level effects of MeHg on neuro-logic outcomes may differ by age, sex, and timing of exposure. No clear pattern has been observed for cardio-vascular disease (CVD) risk across populations or for specific CVD end points. For the few studies evaluating immunologic effects associated with MeHg, results have been inconsistent. Conclusions: Studies targeted at identifying potential mechanisms of low-level MeHg effects and characterizing individual susceptibility, sexual dimorphism, and non-linearity in dose response would help guide future prevention, policy, and regulatory efforts surrounding MeHg exposure.

Maternal Fish Intake during Pregnancy, Blood Mercury Levels, and Child Cognition at Age 3 Years in a US Cohort

The balance of contaminant risk and nutritional benefit from maternal prenatal fish consumption for child cognitive development is not known. Using data from a prospective cohort study of 341 mother-child pairs in Massachusetts enrolled in 1999-2002, the authors studied associations of maternal second-trimester fish intake and erythrocyte mercury levels with childrens scores on the Peabody Picture Vocabulary Test (PPVT) and Wide Range Assessment of Visual Motor Abilities (WRAVMA) at age 3 years. Mean maternal total fish intake was 1.5 (standard deviation, 1.4) servings/week, and 40 (12%) mothers consumed >2 servings/week. Mean maternal mercury level was 3.8 (standard deviation, 3.8) ng/g. After adjustment using multivariable linear regression, higher fish intake was associated with better child cognitive test performance, and higher mercury levels with poorer test scores. Associations strengthened with inclusion of both fish and mercury: effect estimates for fish intake of >2 servings/week versus never were 2.2 (95% confidence interval (CI): -2.6, 7.0) for the PPVT and 6.4 (95% CI: 2.0, 10.8) for the WRAVMA; for mercury in the top decile, they were -4.5 (95% CI: -8.5, -0.4) for the PPVT and -4.6 (95% CI: -8.3, -0.9) for the WRAVMA. Fish consumption of < or =2 servings/week was not associated with a benefit. Dietary recommendations for pregnant women should incorporate the nutritional benefits as well as the risks of fish intake.

Short Sleep Duration in Infancy and Risk of Childhood Overweight

OBJECTIVE To examine the extent to which infant sleep duration is associated with overweight at age 3 years. DESIGN Longitudinal survey. SETTING Multisite group practice in Massachusetts. PARTICIPANTS Nine hundred fifteen children in Project Viva, a prospective cohort. Main Exposure At childrens ages 6 months, 1 year, and 2 years, mothers reported the number of hours their children slept in a 24-hour period, from which we calculated a weighted average of daily sleep. MAIN OUTCOME MEASURES We used multivariate regression analyses to predict the independent effects of sleep duration (< 12 h/d vs > or = 12 h/d) on body mass index (BMI) (calculated as the weight in kilograms divided by the height in meters squared) z score, the sum of subscapular and triceps skinfold thicknesses, and overweight (BMI for age and sex > or = 95th percentile) at age 3 years. RESULTS The childrens mean (SD) duration of daily sleep was 12.3 (1.1) hours. At age 3 years, 83 children (9%) were overweight; the mean (SD) BMI z score and sum of subscapular and triceps skinfold thicknesses were 0.44 (1.03) and 16.66 (4.06) mm, respectively. After adjusting for maternal education, income, prepregnancy BMI, marital status, smoking history, and breastfeeding duration and childs race/ethnicity, birth weight, 6-month weight-for-length z score, daily television viewing, and daily participation in active play, we found that infant sleep of less than 12 h/d was associated with a higher BMI z score (beta, 0.16; 95% confidence interval, 0.02-0.29), higher sum of subscapular and triceps skinfold thicknesses (beta, 0.79 mm; 95% confidence interval, 0.18-1.40), and increased odds of overweight (odds ratio, 2.04; 95% confidence interval, 1.07-3.91). CONCLUSION Daily sleep duration of less than 12 hours during infancy appears to be a risk factor for overweight and adiposity in preschool-aged children.

Weight Status in the First 6 Months of Life and Obesity at 3 Years of Age

OBJECTIVE. The goal was to examine the associations of weight-for-length at birth and at 6 months with obesity at 3 years of age. METHODS. We studied 559 children in Project Viva, an ongoing, prospective, cohort study of pregnant women and their children. We measured length and weight at birth, 6 months, and 3 years. Our main exposures were weight-for-length z score at birth adjusted for gestational age and weight-for-length z score at 6 months adjusted for weight-for-length z score at birth. We used multivariate regression analyses to predict the independent effects of birth weight-for-length z score and, separately, 6-month weight-for-length z score on BMI z score, the sum of subscapular and triceps skinfold thicknesses, and obesity (BMI for age and gender of ≥95th percentile) at age 3. RESULTS. Mean weights at birth, 6 months, and 3 years were 3.55, 8.15, and 15.67 kg, respectively. Corresponding lengths were 49.9, 66.9, and 97.4 cm. At 3 years, 48 children (9%) were obese. After adjustment for confounding variables and birth weight-for-length z score, each increment in 6-month weight-for-length z score was associated with higher BMI z scores, higher sums of subscapular and triceps skinfold thicknesses, and increased odds of obesity at age 3. The predicted obesity prevalence among children in the highest quartiles of both birth and 6-month weight-for-length z scores was 40%, compared with 1% for children in the lowest quartiles of both. Whereas birth weight-for-length z scores were associated with higher BMI z scores, the magnitude of effect was smaller than that of weight-for-length z scores at 6 months. CONCLUSIONS. More-rapid increases in weight for length in the first 6 months of life were associated with sharply increased risk of obesity at 3 years of age. Changes in weight status in infancy may influence risk of later obesity more than weight status at birth.

Decline in fish consumption among pregnant women after a national mercury advisory

OBJECTIVE A well-publicized January 2001 federal advisory recommended that pregnant women limit consumption of certain fish because of concerns about mercury contamination. We endeavored to estimate the extent to which pregnant women changed fish consumption habits after dissemination of this national advisory. METHODS We performed interrupted time series analysis of data from a cohort of pregnant women (2235 who completed at least one dietary questionnaire) visiting obstetric offices in a multispecialty group practice in eastern Massachusetts, surveyed before the advisory from April 1999 through December 2000 and after the advisory from April 2001 through February 2002. Main outcome measures were consumption of total fish and of four fish types: canned tuna, dark meat fish, shellfish, and white meat fish. Subjects reported fish consumption on semiquantitative food frequency questionnaires administered at each trimester of pregnancy. RESULTS We observed diminished consumption of dark meat fish, canned tuna, and white meat fish after the national mercury advisory. These decreases resulted in a reduction in total fish consumption of approximately 1.4 servings per month (95% confidence interval 0.7, 2.0) from December 2000 to April 2001, with ongoing declines through the end of the study period. There was no change in shellfish intake. CONCLUSION After dissemination of federal recommendations, pregnant women in this cohort reported reduced consumption of fish, including tuna, dark meat fish, and white meat fish. Because these fish may confer nutritional benefits to mother and infant, public health implications of these changes remain unclear.

Timing of Solid Food Introduction and Risk of Obesity in Preschool-Aged Children

OBJECTIVE: To examine the association between timing of introduction of solid foods during infancy and obesity at 3 years of age. METHODS: We studied 847 children in Project Viva, a prospective pre-birth cohort study. The primary outcome was obesity at 3 years of age (BMI for age and gender ≥95th percentile). The primary exposure was the timing of introduction of solid foods, categorized as <4, 4 to 5, and ≥6 months. We ran separate logistic regression models for infants who were breastfed for at least 4 months (“breastfed”) and infants who were never breastfed or stopped breastfeeding before the age of four months (“formula-fed”), adjusting for child and maternal characteristics, which included change in weight-for-age z score from 0 to 4 months–a marker of early infant growth. RESULTS: In the first 4 months of life, 568 infants (67%) were breastfed and 279 (32%) were formula-fed. At age 3 years, 75 children (9%) were obese. Among breastfed infants, the timing of solid food introduction was not associated with odds of obesity (odds ratio: 1.1 [95% confidence interval: 0.3–4.4]). Among formula-fed infants, introduction of solid foods before 4 months was associated with a sixfold increase in odds of obesity at age 3 years; the association was not explained by rapid early growth (odds ratio after adjustment: 6.3 [95% confidence interval: 2.3–6.9]). CONCLUSIONS: Among formula-fed infants or infants weaned before the age of 4 months, introduction of solid foods before the age of 4 months was associated with increased odds of obesity at age 3 years.

Associations of Gestational Weight Gain With Short- and Longer-term Maternal and Child Health Outcomes

The authors investigated the rate of gestational weight gain associated with the lowest combined risk of 5 short- and longer-term maternal and child health outcomes for 2,012 mother-child pairs recruited in 1999-2002 into Project Viva, a prebirth cohort study in Massachusetts. Within each maternal prepregnancy body mass index (BMI, kg/m(2)) stratum, they performed a logistic regression analysis predicting all 5 outcomes, from which they determined the rate of gain at which average predicted prevalence of the adverse outcomes was the lowest. The mean rate of total gestational weight gain was 0.39 kg/week (standard deviation, 0.14). The prevalence of small for gestational age was 6%, large for gestational age was 14%, preterm delivery was 7%, substantial postpartum weight retention was 16%, and child obesity was 10%. The lowest predicted outcome prevalence occurred with a 0.28-kg/week gain for women whose BMI was 18.5-24.9, a 0.03-kg/week loss for a BMI of 25.0-29.9, and a 0.19-kg/week loss for a BMI of >or=30.0 kg/m(2)--the lowest observed weight changes in overweight and obese women. For normal-weight and overweight women, lowest-risk gains varied modestly with adjustment for maternal characteristics and with different outcome weightings. For obese women, the lowest-risk weight change was weight loss in all models. Recommendations for gestational weight gain for obese women should be revised.