Greg R. Alexander

A united states national reference for fetal growth

Objective To develop a current national fetal growth curve that can be used as a common reference point by researchers to facilitate investigations of the predictors and consequences of small and large for gestational age delivery. Methods Single live births to United States resident mothers in 1991 (n = 3,134,879) were used for the development of this curve, which was compared with four previously published fetal growth curves. Techniques were developed to address cases with implausible birth weightgestational age combinations and to smooth fetal growth curves across gestational age categories. Results In general, the previously published fetal growth curves underestimated the 1991 United States reference curve. This underestimation is most apparent during the latter weeks of gestation, approximately 33–38 weeks. Conclusion Our findings indicate that the prevalence of fetal growth restriction (FGR) will vary markedly, depending on the fetal growth curve used. Furthermore, many previously published fetal growth curves no longer provide an up-to-date reference for describing the distribution of birth weight by gestational age and for determining FGR that is consistent with the most recent live birth data for the entire United States.

Assessing the Role and Effectiveness of Prenatal Care: History, Challenges, and Directions for Future Research

Despite the widespread use of prenatal care, the evidence for its effectiveness remains equivocal and its primary purpose and effects continue to be a subject of debate. To provide some perspective on why the effectiveness and organization of prenatal care continue to be debated, the authors (a) briefly review the history of the development of prenatal care in the US; (b) attempt to conceptually define prenatal care in terms of its utilization, content, and quality; and, (c) highlight some of the research controversies and challenges facing investigators and advocates who seek to establish the value of prenatal care. In addition, the authors recommend directions for future research to address persistent questions regarding the function, structure, and significance of prenatal care in improving US perinatal outcomes.

The Role of Prenatal Care in Preventing Low Birth Weight

Prenatal care has long been endorsed as a means to identify mothers at risk of delivering a preterm or growth-retarded infant and to provide an array of available medical, nutritional, and educational interventions intended to reduce the determinants and incidence of low birth weight and other adverse pregnancy conditions and outcomes. Although the general notion that prenatal care is of value to both mother and child became widely accepted in this century, the empirical evidence supporting the association between prenatal care and reduced rates of low birth weight emerged slowly and has been equivocal. Much of the controversy over the effectiveness of prenatal care in preventing low birth weight stems from difficulties in defining what constitutes prenatal care and adequate prenatal care use. While the collective evidence regarding the efficacy of prenatal care to prevent low birth weight continues to be mixed, the literature indicates that the most likely known targets for prenatal interventions to prevent low birth weight rates are (1) psychosocial (aimed at smoking); (2) nutritional (aimed at low prepregnancy weight and inadequate weight gain); and (3) medical (aimed at general morbidity). System level approaches to impact the accessibility and the appropriateness of prenatal health care services to entire groups of women and population-wide health promotion, social service, and case management approaches may also offer potential benefits. However, data on the effectiveness of these services are lacking, and whether interventions focused on building cohesive, functional communities can do as much or more to improve low birth weight rates as individualized treatments has yet to be explored. The ultimate success of prenatal care in substantially reducing current low birth weight percentages in the United States may hinge on the development of a much broader and more unified conception of prenatal care than currently prevails. Recommendations for actions to maximize the impact of prenatal care on reducing low birth weight are proposed both for the public and for the biomedical, public health, and research communities.

The Impact of the Increasing Number of Multiple Births on the Rates of Preterm Birth and Low Birthweight: An International Study

OBJECTIVES We studied the effects of twins and triplets on perinatal health indicators in the overall population in the 1980s and 1990s in Canada, England and Wales, France, and the United States. METHODS Data were derived mostly from live birth registration. We used rates, relative risks, and population attributable risks for twins and triplets separately. RESULTS In each country, the increase in multiple births, and the increase in preterm delivery among multiple births, contributed almost equally to the rise in or stabilization of the overall rates of preterm delivery. Twins contributed a much larger proportion of the preterm deliveries and low-birthweight newborns than did triplets. CONCLUSIONS Twins have a major population-based impact on the trends of perinatal health indicators.

1994-1996 U.S. Singleton Birth Weight Percentiles for Gestational Age by Race, Hispanic Origin, and Gender

Objectives: Establishing and comparing race, ethnic, and gender-specific birth weight percentiles for gestational age is requisite for investigating the determinants of variations in fetal growth. In this study, we calculate percentiles of birth weight for gestational age for the total 1994–1996 U.S. population and contrast these percentiles by racial/ethnic and gender groups. Methods: Single live births to U.S. resident mothers were selected from the 1994–1996 U.S. Natality Files. After exclusions, 5,973,440 non-Hispanic Whites, 1,393,908 non-Hispanic African Americans, 1,683,333 Hispanics, 80,187 Native Americans, and 510,021 other racial/ethnic groups were used to calculate distribution percentiles of birth weight for each gestational age for which there were at least 50 cases to calculate the 50th percentile and 100 cases to calculate the 10th percentile. Results: Fetal growth patterns among the four U.S. racial/ethnic groups varied markedly and, across the gestational age range, there was considerable oscillation in the relative ranking of any one groups birth weight percentile value in comparison to the others. Males had relatively higher birth weight percentile values than females. The proportion of infants with a birth weight value less than 1994–1996 U.S. populations 10th percentile value of birth weight for their corresponding gestational age was 7.87 for non-Hispanic Whites, 15.43 for non-Hispanic African Americans, 9.30 for Hispanics, and 8.81 for Native Americans. Conclusions: While the factors underlying trends and population subgroup differences in fetal growth are unclear, nutrition, smoking habits, health status, and maternal morbidity are possible precursors for part of the variations in patterns of fetal growth. As prenatal care has been touted as a means to reduce the risk of fetal growth restriction at term, assuring the availability and accessibility of comprehensive prenatal care services is viewed as an essential corollary in the effort to improve fetal growth patterns in the United States.

Preventing low birth weight: is prenatal care the answer?

Objectives: To review the evidence of effectiveness of prenatal care for preventing low birth weight (LBW). Methods: We reviewed original research, systematic reviews, meta-analyses and commentaries for evidence of effectiveness of the three core components of prenatal care - risk assessment, health promotion and medical and psychosocial interventions - for preventing the two constituents of LBW: preterm birth and intrauterine growth restriction (IUGR). Results: Clinical risk assessment will fail to identify the majority of pregnancies at risk for preterm delivery or IUGR. While biophysical and biochemical modalities appear promising, their cost-effectiveness has not been demonstrated, nor can their routine use be recommended in the absence of effective interventions. Smoking cessation programs appear to be modestly effective. There is insufficient evidence to conclude a benefit for nutrition interventions, work counseling or preterm birth education. Only antenatal corticosteroid therapy has demonstrated a clear benefit in the tertiary prevention of preterm delivery. Interventions for which there is insufficient evidence to conclude a benefit include bed rest, hydration, sedation, cerclage, progesterone supplementation, antibiotic treatment, tocolysis without concomitant use of corticosteroids, thyrotropin-releasing hormone, psychosocial support and home visitation. Additionally, there is a paucity of evidence supporting the effectiveness of prenatal interventions, such as low-dose aspirin, bed rest, maternal hyperoxygenation, plasma volume expansion and antenatal fetal assessment, in preventing IUGR or its associated morbidity and mortality. Conclusions: Neither preterm birth nor IUGR can be effectively prevented by prenatal care in its present form. Preventing LBW will require reconceptualization of prenatal care as part of a longitudinally and contextually integrated strategy to promote optimal development of womens reproductive health not only during pregnancy, but over the life course.

Childbearing beyond maternal age 50 and fetal outcomes in the United States.

OBJECTIVE To estimate whether achieving pregnancy beyond maternal age of 50 years compromises fetal well-being and survival. METHODS This was a retrospective study on all deliveries in the United States from 1997 to 1999. Four maternal age groups of 20–29 (young), 30–39 (mature), 40–49 (very mature), and 50 or more years (older) were constructed to assess risk gradients for fetal morbidity and mortality. RESULTS A total of 539 deliveries among older mothers (aged 50 and above) were documented (four per 100,000). Among singleton gestations, the risks for low birth weight, preterm, and very preterm were tripled among older mothers, whereas the occurrence of very low birth weight, small size for gestational age, and fetal mortality were approximately doubled compared with those for young mothers. Older mothers also had greater risks for fetal morbidity and mortality than their immediate younger counterparts (40–49 year olds) except for very low birth weight. Among multiple gestations, the differences in risk between older and young mothers were lower than those noted among singletons. Still, compared with young mothers, older mothers had significantly higher risks of low birth weight, very low birth weight, very preterm, and small size for gestational age. Older mothers also had higher risk estimates for multiples than 40–49-year-old gravidas in terms of all fetal morbidity and mortality indices. CONCLUSION Pregnancy beyond age 50 was associated with increased risks for the fetus. Our findings suggest that this age group is a distinct obstetric high-risk entity that requires special counseling before and after conception.

Levels of excess infant deaths attributable to maternal smoking during pregnancy in the United States.

Objectives: The objectives of the study were: 1) To determine the risk of infant mortality associated with prenatal cigarette smoking; 2) To assess whether the relationship, if existent, was dose-dependent; 3) To explore the morbidity pathway that explains the effect of tobacco smoke on infant mortality, and 4) to compute excess infant deaths attributable to maternal smoking in the United States. Methods: Retrospective cohort study on 3,004,616 singleton live births that occurred in 1997 in the United States using the US national linked birth/infant death data. Excess infant deaths due to maternal smoking were computed using the population-attributable risk (PAR). Results: Overall, 13.2% of pregnant women who delivered live births in 1997 smoked during pregnancy. The rate of infant mortality was 40% higher in this group as compared to nonsmoking gravidas (P < 0.0001). This risk increased with the amount of cigarettes consumed prenatally in a dose-dependent fashion (p for trend <0.0001). Small-for-gestational age rather than preterm birth is the main mechanism through which smoking causes excess infant mortality. We estimated that about 5% of infant deaths in the United States were attributable to maternal smoking while pregnant, with variations by race/ethnicity. The proportion of infant deaths attributable to maternal smoking was highest among American Indians at 13%, almost three times the national average. If pregnant smokers were to halt tobacco use a total of 986 infant deaths would be averted annually. Conclusions: Smoking during pregnancy accounts for a sizeable number of infant deaths in the United States. This highlights the need for infusion of more resources into existing smoking cessation campaigns in order to achieve higher quit rates, and substantially diminish current levels of smoking-associated infant deaths.

Extreme Obesity and Risk of Stillbirth Among Black and White Gravidas

OBJECTIVE: To estimate the risk for stillbirth among three generally accepted obesity subtypes based on severity. METHODS: We used the Missouri maternally linked cohort data containing births from 1978 to 1997. Using prepregnancy weight and height, mothers were classified on the basis of calculated body mass index (BMI) above 30 into three subsets: class I (30–34.9), class II (35–39.9), and extreme obesity (greater than or equal to 40). Using normal-weight, white women (18.5–24.9) as a reference, we applied Cox proportional hazard regression models to estimate risks for stillbirth. RESULTS: The prevalence of obesity in pregnant women was 9.5% (12.8% among blacks and 8.9% among whites). Overall, obese mothers were about 40% more likely to experience stillbirth compared with nonobese gravidas (adjusted hazard ratio 1.4; 95% confidence interval [CI] 1.3–1.5). The risk for stillbirth increased in a dose-dependent fashion with increase in BMI: class I (adjusted hazard ratio 1.3; 95% CI 1.2–1.4); class II (adjusted hazard ratio 1.4; 95% CI 1.3–1.6) and extreme obesity (adjusted hazard ratio 1.9; 95% CI 1.6–2.1; P for trend <.01). Obese black mothers experienced more stillbirths than their white counterparts (adjusted hazard ratio 1.9; 95% CI 1.7–2.1 compared with adjusted hazard ratio 1.4; 95% CI 1.3–1.5). The black disadvantage in stillbirth widened with increase in BMI, with the greatest difference observed among extremely obese black mothers (adjusted hazard ratio 2.3; 95% CI 1.8–2.9). CONCLUSION: Obesity is a risk factor for stillbirth, particularly among extremely obese, black mothers. Strategies to reduce black–white disparities in birth outcomes should consider targeting obese, black women. LEVEL OF EVIDENCE: II

Racial Differences in Prenatal Care Use in the United States: Are Disparities Decreasing?

OBJECTIVES We examined trends and racial disparities (White, African American) in trimester of prenatal care initiation and adequacy of prenatal care utilization for US women and specific high-risk subgroups, e.g., unmarried, young, or less-educated mothers. METHODS Data from 1981-1998 US natality files on singleton live births to US resident mothers were examined. RESULTS Overall, early and adequate use of care improved for both racial groups, and racial disparities in prenatal care use have been markedly reduced, except for some young mothers. CONCLUSIONS While improvements are evident, it is doubtful that the Healthy People 2000 objective for prenatal care will soon be attained for African Americans or Whites. Further efforts are needed to understand influences on and to address barriers to prenatal care.