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Pregnancy Risks Determined from Birth Certificate Data -- United States, 1989
The 1989 revision of the "U.S. Standard Certificate of Live Birth" includes new items of information about medical and lifestyle risk factors related to pregnancy, birth, and method of delivery (1,2). This report presents data about three of these new items: maternal weight gain during pregnancy, smoking during pregnancy, and method of delivery. For this analysis, to allow for more in-depth comparisons, data were limited to the two largest racial groups (black and white) in the United States.
During 1989, information for approximately 3.9 million live-birth certificates was recorded; however, because the District of Columbia, Rhode Island, Texas, and Virginia did not implement the revised certificates until March or April 1989, data on the new topics for the first three or four months of that year for those areas were not available. Overall, 17% of the birth certificates did not report maternal weight gain, 8% did not report data on smoking and 5% did not report on method of delivery (among areas reporting these items). Maternal Weight Gain
From 1974 through 1989, the guideline for weight gain during a normal pregnancy was 22-27 pounds (3); in 1990, the National Institute of Medicine set guidelines for maternal weight gain at 25-35 pounds for average-sized women (4). Analysis of 1989 birth-certificate data indicated that approximately 17% of white mothers and 27% of black mothers with gestations greater than or equal to 40 weeks gained less than or equal to 20 pounds; white mothers gained, on average, one half pound more than black mothers (Table 1). The risk for insufficient weight gain was highest among women aged greater than or equal to 35 years, those with less than a high school education, unmarried women, and women whose attendant at delivery was not a physician or a midwife. For each variable examined, black mothers were more likely to have low weight gain than white mothers (Table 1).
Among infants with gestations greater than or equal to 40 weeks, the percentage of low birthweight (LBW) (i.e., less than 2500 g (less than 5.5 lbs)) declined as maternal weight gain during pregnancy increased from less than 16 pounds to greater than or equal to 40 pounds (2.8%-0.8% for white infants and 6.8%-1.8% for black infants). The percentage of infants weighing at least 3500 g (7 lbs 12 oz) increased with maternal weight gain (for white infants, 44%-67%, and for black infants, 25%-50%) (Table 2). Smoking
Data from the 1989 birth certificates were used to identify maternal characteristics (e.g., educational attainment of mother and adequacy of prenatal care *) that predict variations in smoking practices. To ensure the validity of educational attainment as an analytic variable, this analysis was confined to mothers aged greater than or equal to 20 years.
During 1989, 19% of women who gave birth reported tobacco use during pregnancy **. However, smoking levels varied substantially by mother's educational attainment and adequacy of prenatal care. During 1989, 73% of births to women aged greater than or equal to 20 years were to mothers who had adequate care, 20% to those who had intermediate care, and 7% to those who had inadequate care. Mothers whose care was inadequate were twice as likely to have smoked as those who had adequate care (32% versus 16%).
The differences by adequacy of care persisted for mothers with greater than or equal to 12 years of education. Among comparably educated mothers, those who had inadequate prenatal care were considerably more likely to smoke than mothers who had adequate care (Table 3). However, among women with 9-11 years of education, 41%-46% were smokers, regardless of level of prenatal care. In addition, among mothers receiving adequate care, those with 9-11 years of education were eight times as likely to have smoked as were college graduates (41% versus 5%).
Among mothers aged greater than or equal to 20 years, reported prevalences of smoking varied by educational attainment and race. At most levels of care and education, white mothers were more likely to smoke than were black mothers. White mothers with 9-11 years of education were more likely than any other group to smoke (44%-48%), regardless of care. Among black mothers with 9-11 years of education, 36% smoked overall; however, smoking prevalences were substantially higher for those with inadequate care (46%) than for those with intermediate (35%) or adequate care (30%).
Regardless of race, adequacy of prenatal care, or mother's educational level, babies born to mothers who smoked were at substantially elevated risk for LBW (Table 3). In addition, babies born to black women were substantially more likely to have LBW than were babies born to white women. For example, even among women with low risk for having LBW infants (i.e., nonsmoking mothers who were collegegraduates and received adequate prenatal care), 8% of black infants and 4% of white infants had LBW. Method of Delivery
In 1989, 23% of all births and 16% of births to mothers who had no previous cesarean were by cesarean delivery (Table 4). Overall and primary cesarean rates differed markedly by the mother's residence, age, parity, and educational attainment. Rates were highest for mothers who resided in the South, were in the oldest years of childbearing, were having their first child, and had greater than or equal to 13 years of education. Rates were similar, however, by race.
At least half the infants were delivered by cesarean when eclampsia (52%), abruptio placenta (57%), fetal distress (63%), dysfunctional labor (64%), cord prolapse (68%), placenta previa (82%), malpresentation (84%), or cephalopelvic disproportion (98%) were diagnosed (1). The risk for cesarean increased with maternal weight gain during pregnancy, from approximately 21% for mothers who gained less than 31 pounds, to 29% for mothers who gained greater than or equal to 41 pounds. Cesarean rates were highest for babies weighing less than 2500 g or greater than or equal to 4000 g, for babies born prematurely, for twins, and for babies with certain abnormal conditions (e.g., hyaline membrane disease, meconium aspiration syndrome, or need for assisted ventilation).
Less than 20% of infants born to mothers who had a previous cesarean were delivered vaginally (VBAC). VBAC rates were generally lowest for women comprising the groups with highest overall and primary cesarean rates.
Reported by: Div of Vital Statistics, National Center for Health Statistics, CDC.
Editorial Note: The findings in this report regarding maternal weight gain are consistent with a previous study, based on the 1980 National Natality Survey, that indicated substantial disparities in weight gain by race and sociodemographic status and that women with inadequate weight gain during pregnancy generally have lower weight live-born infants and a higher risk for a fetal death (5). Insufficient weight gain among high-risk women remains a public health concern because of its association with poor birth outcomes.
The large differences in maternal weight gain by race are consistent with the medical advice on weight gain reported by mothers (i.e., black women are more likely to report being advised to gain less weight than currently recommended, regardless of their age, education, or marital status) (6). Therefore, physicians and other health-care providers who advise on maternal weight gain during pregnancy should provide the recent guidelines of 28-40 pounds for women with low weight-for-height, 25-35 pounds for average weight-for-height women, and 15-25 pounds for women with high weight-for-height, for full-term pregnancies (4).
Results on smoking by women during pregnancy from this analysis are consistent with results from the 1988 National Maternal and Infant Health Survey, conducted by CDC's National Center for Health Statistics (NCHS). Smoking by women during pregnancy is associated with a variety of poor pregnancy outcomes, including higher rates of miscarriage, LBW, intrauterine growth retardation, and preterm birth (7). In turn, LBW and preterm birth are major predictors of infant mortality and infant and childhood morbidity. Therefore, intense intervention efforts to curtail smoking should be directed toward the group of pregnant women having the highest prevalence of smoking (i.e., mothers with less than a high school education).
The racial disparity in the incidence of LBW persisted even when births to women of the same education, prenatal care, and smoking status were compared. Therefore, determination of reasons for this difference will require further assessment of other lifestyle, nutritional, and environmental factors.
Since 1965, the National Hospital Discharge Survey (NHDS), conducted by NCHS, has provided national data on cesarean delivery rates. In this report, the cesarean delivery rates based on data from live-birth certificates are consistent with rates derived from the 1989 NHDS (1,8). The addition of the question regarding method of delivery to the 1989 revised "U.S. Standard Certificate of Live Birth" has provided public-health practitioners and health researchers with a basis for determining cesarean rates for a wider range of maternal demographic and health characteristics, as well as for smaller geographic areas and by health characteristics of the infant.
Since the early 1980s, the American College of Obstetricians and Gynecologists has encouraged an expanded use of VBAC to reduce the overall cesarean rate (9). Information now available from birth certificates will permit health professionals to more closely monitor changes in cesarean and VBAC rates and compare their rates to international, state, or small-area rates.
The findings in this report have at least two limitations. First, reporting may not be as complete as it could be in subsequent years because 1989 was the first year of reporting with this revised birth certificate and three states and the District of Columbia did not begin reporting new data immediately. Second, for data on maternal weight gain, because prepregnancy weight and height are not recorded on the birth certificate, it was not possible to determine what proportion of women achieved the recommended levels for their weight-for-height; no other sources exist on maternal weight gain from health-care providers for comparison. Despite these limitations, the new data from the revised live birth certificates will facilitate tracking of results of public health initiatives designed to improve maternal health and pregnancy outcome.
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