OBJECTIVE: To investigate the relationship between gestational weight gain and adverse pregnancy outcomes among women with normal prepregnancy body mass index.
METHODS: We conducted a population-based cohort study of women with normal prepregnancy body mass index who delivered full-term singletons using Missouri birth certificate data for 1999–2001. The cohort was divided into three groups (less than recommended [less than 25 lb], n=16,852; recommended [25–35 lb], n=37,292; more than recommended [more than 35 lb], n=40,552) based on Institute of Medicine gestational weight gain guidelines. Logistic regression was used to adjust for known confounders.
RESULTS: Compared with women gaining 25–35 lb, women gaining less than 25 lb during pregnancy had lower odds for preeclampsia (adjusted odds ratio [aOR] 0.56, 95% confidence interval [CI] 0.49–0.64), cephalopelvic disproportion (aOR 0.64, 95% CI 0.55–0.75), failed induction (aOR 0.68, 95% CI 0.59–0.78), cesarean delivery (aOR 0.82, 95% CI 0.78–0.87), and large for gestational age infants (aOR 0.40, 95% CI 0.37–0.44) and increased odds for small for gestational age infants (aOR 2.14, 95% CI 2.01–2.27). Likewise, women gaining more than 35 lb had lower odds for small for gestational age infants (aOR 0.48, 95% CI 0.45–0.50) and increased odds for preeclampsia (aOR 1.88, 95% CI 1.74–2.04), failed induction (aOR 1.51, 95% CI 1.39–1.64), cesarean delivery (aOR 1.35, 95% CI 1.29–1.40), and large for gestational age infants (aOR 2.43, 95% CI 2.30–2.56).
CONCLUSION: Our study shows that adherence to the current Institute of Medicine guidelines results in lower risks for adverse pregnancy, labor, and delivery outcomes when comparing all outcomes collectively.
LEVEL OF EVIDENCE: II
Women with a normal prepregnancy body mass index who adhere to gestational weight gain guidelines have a low risk of adverse pregnancy outcomes for full-term births.
From the Departments of 1Community Health and 2Obstetrics, Gynecology, and Women's Health, Saint Louis University, St. Louis, Missouri.
See related editorial on page 743.
Corresponding author: Thomas D. Myles, MD, Saint Louis University, Department of Obstetrics and Gynecology, 6420 Clayton Road, Suite 559, St. Louis, MO 63117; e-mail: email@example.com.
Financial Disclosure The authors have no potential conflicts of interest to disclose.
Gestational weight gain guidelines were established by the Institute of Medicine (IOM) in 1990.1 Based on these guidelines normal weight women (19.8–26.0 kg/m2) should gain 25–35 lb (11.4–15.9 kg) during their pregnancy. Several studies have shown that excessive gestational weight gain can lead to an increased risk of gestational diabetes,2 preeclampsia,3,4 failed induction,2 cephalopelvic disproportion or failure to progress,5,6 lacerations,2 instrumental delivery,3,7 preterm delivery,8 cesarean delivery,2–6,9,10 macrosomia,3,4,7,9–11 low 5-min Apgar score,11 postpartum infection,2 and weight retention.12–14 Other studies have shown that inadequate gestational weight gain increases the risk of fetal death,15 preterm labor,16 preterm delivery,8,15–17 and low birth weight infants.11,15,17
Some studies have examined the relationship between gestational weight gain and risk of adverse pregnancy outcomes by adjusting for prepregnancy weight and other potential confounders. Other studies have focused on pregnant women stratified by prepregnancy weight categories but only for pregnant women delivered at a single institution. The objective of our population-based study was to evaluate the relationship between gestational weight gain and adverse pregnancy outcomes among women with normal prepregnancy body mass index (BMI) who deliver full-term infants regardless of place of delivery in Missouri. We hypothesized that gestational weight gain outside the recommended guidelines would adversely affect the risk of specific pregnancy outcomes.
MATERIALS AND METHODS
A population-based cohort study was conducted using birth certificate data for all women residing in Missouri. The study population included all mothers with a normal prepregnancy BMI (19.8–26.0 kg/m2) who were 18 to 35 years of age at time of delivery and who delivered a full-term (37 weeks or more) singleton infant during the period January 1, 1999, to December 31, 2001. The prepregnancy BMI was based on maternal height and weight reported in the mother's obstetric record or, if missing, obtained from the mother during her postpartum hospital stay. Gestational age at time of delivery was estimated by the attending physician based on either the date of last menses or ultrasound measurements or both (whichever the clinician felt to be most accurate). Women aged younger than 18 years and older than 35 years were excluded because they were more likely to have adverse pregnancy outcomes unrelated to gestational weight gain.13,18–20 Non-Missouri residents were excluded to eliminate the influx of high-risk pregnancies from neighboring states, which may erroneously elevate adverse pregnancy outcomes for our geographically defined cohort. Preterm deliveries were excluded based on adverse outcomes potentially unrelated to gestational weight gain and because women delivering preterm have less time to gain the recommended amount of weight during their pregnancy.11 Multiple gestations were excluded to avoid the confounding effects of multiple births on gestational weight gain. When the study was completed, 2001 was the most recent year of available data.
The study population was divided into three groups based on total gestational weight gain and current IOM recommendations.1 The three groups included women who gained within (25–35 lb), less than (less than 25 lb), and more than (more than 35 lb) the recommended amount of weight during their pregnancy.
Based on the results of other studies focusing on gestational weight gain,2–7,9–11,15,16 we included the following pregnancy outcomes that are defined below and routinely collected on all birth certificates recorded in Missouri: preeclampsia, fetal distress, cephalopelvic disproportion (CPD), failed induction, instrumental delivery, and cesarean delivery. All outcomes were assessed as dichotomous covariates. Preeclampsia was defined as the development of pregnancy-associated hypertension and proteinuria after the 20th week of gestation. This included women with blood pressure of 140/90 mm Hg or greater, or an increase in blood pressure of 30 mm Hg or more systolic or 15 mm Hg or more diastolic on two measurements taken 6 hours apart. Because of the small number of women who developed eclampsia, this group was combined with preeclamptic women and hereafter referred to as “preeclampsia.” Fetal distress was defined as signs indicative of fetal hypoxia, which included persistent abnormal fetal heart rate patterns, low scalp pH, significant meconium staining of amniotic fluid and low umbilical cord pH. Cephalopelvic disproportion was defined as the condition when the size, presentation, or position of the fetal head to the maternal pelvis prevented cervical dilation or descent of the fetal head. Failed induction was defined as a birth that required a cesarean delivery despite induction of labor. Instrumental delivery was measured as delivery using forceps or vacuum extraction. Small for gestational age (SGA) status and large for gestational age (LGA) status were computed to identify all newborns with birth weights below the 10th percentile and above the 90th percentile, respectively, for gestational age and race or ethnicity, with the United States population serving as the reference for fetal growth,21 because these two outcomes were not routinely collected on the birth certificate.
Factors associated with gestational weight gain in prior studies were evaluated as potential confounders in our study.3,6,7,9,10,13,14,17,22 All information was obtained from birth certificates recorded in Missouri. Maternal demographic and lifestyle covariates were age (18–24, 25–30, 31–35 years), race or ethnicity (non-Hispanic white, non-Hispanic African American, other races or ethnicities), education (less than 12, 12, 13–16, more than 16 years), income, and tobacco and alcohol use. Income status was assessed using Medicaid enrollment status as a proxy, because income was not captured on the birth certificate. Medical and obstetric covariates included maternal height (less than 60, 60–64, 65–69, more than 69 inches), prior pregnancy, inadequate prenatal care use, child's gender, and birth year. Prenatal care use was assessed using the algorithm developed by Kotelchuck,23 and inadequate prenatal care use was defined as less than 80% of expected visits based on gestational age at time of delivery.
The percentage of individuals in the study population with specific demographic, lifestyle, medical, and obstetric characteristics was computed for each gestational weight gain group. A χ2 test was used to assess differences among the three groups. Statistical significance was established with a P<.05. The frequency of each adverse pregnancy outcome was expressed as the percent of total study population with specific outcome. The outcome-specific percent was reported for each of the three gestational weight gain groups and for 10-lb weight gain increments. Women who gained the recommended amount of weight during their pregnancy served as the reference group. In the multivariable analysis we controlled for the following covariates: age, race, previous pregnancy, smoking status, alcohol status, Medicaid status, maternal education level, maternal height, Kotelchuck index of prenatal care use, child's sex, and child's birth year. Logistic regression analysis was used to adjust the association between gestational weight gain and specific outcomes, and 95% confidence intervals (CI) were computed to evaluate the precision of each odds ratio (OR). All analyses were performed using SPSS 12.0 software (SPSS Inc., Chicago, IL). This research was reviewed by the Saint Louis University institutional review board and was classified as exempt from the United States Department of Health and Human Services regulations for the protection of human subjects.
A total of 94,696 eligible women were included in our study population. More than 60% of women gained outside the IOM guidelines: 17.8% gained less than 25 lb, whereas 42.8% gained more than 35 lb during their pregnancy. The majority of women who were outside the recommended gestational weight gain groups were younger, non-Hispanic African American, less educated, on Medicaid, and tobacco users (Table 1).
The risk (or percentages) for each adverse pregnancy outcome was statistically significantly different (P<.01) among the three gestational weight gain groups (Table 2). The risks for preeclampsia, LGA infant, CPD, failed induction, instrumental delivery, and cesarean delivery increased with increasing gestational weight gain. The risk for SGA infant decreased with increasing gestational weight gain. The adjusted odds ratios showed similar patterns for preeclampsia, SGA infant, LGA infant, CPD, instrumental delivery, and cesarean delivery.
Figures 1–3 plot the risk for each adverse pregnancy outcome by 10-lb increments in gestational weight gain. Women who gained 25–34 lb during their pregnancy had lower risks for most outcomes when balancing the risk for SGA status and other adverse pregnancy outcomes. Women who gained 15–24 lb had the lowest risks for most outcomes, but increased their risk of having an SGA infant from 9.6% to 14.3%. Women who gained more than 34 lb had higher risks for all outcomes, although their risk of having an SGA infant decreased from 9.6% to 6.6%.
Women who gained less than 25 lb were more likely to deliver an SGA child (adjusted OR [aOR] 2.14, 95% CI 2.01–2.27) than women who gained 25–35 lb during their pregnancy (Table 2). These women were less likely to develop preeclampsia (aOR 0.56, 95% CI 0.49–0.64) and less likely to deliver an LGA child (aOR 0.40, 95% CI 0.37–0.44). A protective effect was also found for CPD (aOR 0.64, 95% CI 0.55–0.75), failed induction (aOR 0.68, 95% CI 0.59–0.78), and cesarean delivery (aOR 0.82, 95% CI 0.78–0.87).
Women who gained more than 35 lb were more likely to develop preeclampsia (aOR 1.88, 95% CI 1.74–2.04) and deliver an LGA infant (aOR 2.43, 95% CI 2.30–2.56) than women who gained 25–35 lb during their pregnancy. Women who gained more gestational weight were 1.11 (95% CI 1.05–1.19) times more likely to have fetal distress and 1.58 (95% CI 1.44–1.75) times more likely to experience CPD than women who gained the recommended amount of weight. These women were at an increased risk for failed induction (aOR 1.51, 95% CI 1.39–1.64) and cesarean delivery (aOR 1.35, 95% CI 1.29–1.40). A protective effect was found for delivering an SGA infant (aOR 0.48, 95% CI 0.45–0.50).
Our study shows that adherence to the current IOM guidelines results in lower risks for adverse pregnancy, labor, and delivery outcomes when comparing all outcomes collectively. Women in our study population who gained less gestational weight than the IOM recommendations were more likely to deliver an SGA infant, whereas those who gained more weight than recommended were more likely to have preeclampsia, LGA infant, fetal distress, CPD, failed induction, and cesarean delivery. When balancing the risk for SGA status and other adverse pregnancy outcomes, the ideal gestational weight gain for our population seems to be 25–34 lb, which falls within the current IOM guidelines. Our results do not support the recommendations from another study that reported optimal pregnancy outcomes for average size women with gestational weight gain of 31–40 lb.24 Our results also cannot be used to advise women to gain less weight than currently recommended because our category of women gaining less than 25 lb during their pregnancy does not include a lower limit for which the lowest risks for preeclampsia, LGA infant, fetal distress, CPD, failed induction, and cesarean delivery outweigh the higher risk of having an SGA infant.
Based on IOM guidelines, about 40% of normal weight women in our study population gained the recommended amount of weight (25–35 lb) during their pregnancy, whereas 18% and 43% gained less or more weight than recommended. Caulfield and colleagues22 reported similar percentages of African-American women gaining outside the recommended range (38% compared with 34%), but a higher percentage of white women overgaining (43%) than undergaining (25%). Our study provides additional evidence that more than one half of all pregnant women are not abiding by the current IOM guidelines and may be increasing their risk of adverse pregnancy outcomes.
The major strength of our study was the availability of pregnancy-related data for a population-based cohort study of all Missouri residents regardless of their hospital of delivery. Our study population included all women who delivered full-term, singleton infants during the three most current years (1999–2001). This large study population (N=94,696) provided precise estimates of adverse pregnancy outcomes that allowed us sufficient statistical power (more than 80%) to show statistically significant odds ratios of 1.5 or greater for women whose gestational weight gain was outside the IOM guidelines.
Like all observational studies, our study has specific limitations. Selection bias can result from inaccurate reporting of height and weight for estimating prepregnancy BMI, which was used to identify all normal-weight women for our study population. Although self-reported height and weight may be underreported and overreported, other studies have shown self-reported weight to be highly correlated25 and within 3 lb26 of the actual measured weight. Information bias can affect study results from inaccurate collection of data when determining total gestational weight gain and assessing primary outcomes. Gestational weight gain was obtained from the mother's obstetric record, but by recall if this source was unavailable. We are unaware of any studies that have examined the reliability of gestational weight gain as reported on the birth certificate. However, an earlier study comparing birth certificate with hospital discharge data in Missouri showed minimal underreporting of women with preeclampsia (85%) and CPD (89%) and minimal overreporting for those with eclampsia (110%) and fetal distress or meconium (132%) (Schramm WS. Data quality: new certificates. Proceedings of the AVRHS/VSCP project directors meeting. San Francisco, California; 1991). We were able to assess and adjust our results for confounding bias, but only for the covariates listed on the birth certificate. We acknowledge that our results may be influenced by other important confounders, including dietary and physical activity habits of the women in our study population.
Our findings have several clinical and public health implications. Our study results support the current IOM gestational weight gain guidelines for women with normal prepregnancy body BMI and who deliver a full-term singleton infant. Our study results also indicate that nearly one half of all normal weight women in Missouri are gaining too much gestational weight, which may increase their risk of some adverse pregnancy outcomes. Clinicians should discuss the short- and long-term risks with their patients and monitor more closely gestational weight gain throughout the pregnancy.
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