You are in: eMedicine Specialties > Obstetrics and Gynecology > General Obstetrics Estimation of Fetal WeightArticle Last Updated: Aug 21, 2007AUTHOR AND EDITOR INFORMATIONSection 1 of 11
 
Author: Gerard G Nahum, MD, FACOG, FACS, Adjunct Associate Professor of Obstetrics and Gynecology, Uniformed Services University of the Health Sciences; Director of Medical Affairs, Women's Healthcare, Bayer HealthCare Pharmaceuticals Gerard G Nahum is a member of the following medical societies: American College of Obstetricians and Gynecologists, American College of Surgeons, American Society for Reproductive Medicine, Association of Professors of Gynecology and Obstetrics, and Society of Laparoendoscopic Surgeons Editors: Andrea Witlin, DO, PhD, Former Assistant Professor, Department of Obstetrics and Gynecology, University of Texas Medical Branch; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; Richard S Legro, MD, Professor, Department of Obstetrics and Gynecology, Division of Reproductive Endocrinology, Pennsylvania State University College of Medicine; Consulting Staff, Milton S Hershey Medical Center; Frederick B Gaupp, MD, Consulting Staff, Department of Family Practice, Hancock Medical Center; Carl V Smith, MD, The Distinguished Chris J and Marie A Olson Chair of Obstetrics and Gynecology, Professor, Department of Obstetrics and Gynecology, University of Nebraska Medical Center Author and Editor Disclosure Synonyms and related keywords: birth weight prediction, estimation of birth weight, low birth weight, excessive fetal weight, fetal macrosomia, excessive birth weight, birth weight estimation, fetal complications, perinatal complications, preterm delivery, parturition, birth, pregnancy, labor, gestation, intrauterine growth restriction, intrauterine growth retardation, IUGR, shoulder dystocia, brachial plexus injuries, bony injuries, intrapartum asphyxia, birth canal injury, pelvic floor injury, postpartum hemorrhage, macrosomic fetus, ultrasonographic fetal biometry, obstetrical ultrasonography IMPORTANCE OF ANTENATAL FETAL WEIGHT ESTIMATIONSection 2 of 11
 
Both low birth weight and excessive fetal weight at delivery are associated with an increased risk of newborn complications during labor and the puerperium. The perinatal complications associated with low birth weight are attributable to preterm delivery, intrauterine growth restriction (IUGR), or both. For macrosomic fetuses (see Media file 1), potential complications associated with delivery include shoulder dystocia, brachial plexus injuries, bony injuries, and intrapartum asphyxia. Additional maternal risks associated with the delivery of a macrosomic fetus include birth canal and pelvic floor injuries and postpartum hemorrhage (see Table 1). The occurrence of cephalopelvic disproportion is more prevalent with increasing fetal size and contributes to an increased rate of both surgical vaginal delivery and cesarean delivery for macrosomic fetuses compared with fetuses of normal weight. Depending on many factors, the optimal birth-weight range to minimize the risk of fetal and maternal morbidity and mortality is 3000-4000 g. Limiting the potential complications associated with the birth of both small and excessively large fetuses requires an accurate estimation of fetal weight before delivery. This article reviews the factors that influence fetal growth and the methods that can be used to accurately estimate fetal weight. Table 1. Newborn and Maternal Complications Associated With a Birth Weight Greater than 4000 g
STANDARD FETAL GROWTH CURVESSection 3 of 11
Mean birth weight can be described as a function of gestational age. Several reports subdivide such results into those that apply to women of different races, to male versus female fetuses, and to primiparous versus multiparous status. Some have further stratified the data, creating specific curves and tables for women of different heights and weights. Standard fetal growth curves are useful for estimating the range of expected fetal weights at any particular gestational age. However, for the growth curves to be useful, the gestational age of the fetus must be properly established. Without adequate gestational dating, the data on standard fetal-growth curves cannot be interpreted successfully. The principal limitations of standard fetal-growth curves that are derived from population-based studies are the following: (1) They apply only to fetuses that are of normal size for their gestational age and not to those with clinically significant (and potentially pathologic) growth abnormalities. (2) The data from which they are derived are highly population dependent. (3) The standard deviation (SD) associated with the estimated mean birth weight for any particular gestational age is wide, typically exceeding 450-500 g. (4) The 95% confidence interval (CI) for fetal-weight estimates are >1600 g at term (ie, ±800 g [±1 lb 12 oz], or ±23%). (5) The gestational age of the fetus must be known with a high degree of accuracy to reliably use the growth curves. In general, standard fetal-growth curves can be expected to apply to large populations of pregnant women who have well-dated pregnancies, but the limits of their predictive accuracy make them less-than-ideal tools for estimating fetal weight in individual patients. In addition, fetal-growth curves are the most inaccurate at the extremes of fetal weight, which are the most clinically relevant situations in which they are used (eg, in fetuses that are either growth restricted or macrosomic). WHAT IS THE NORMAL RANGE FOR HUMAN BIRTH WEIGHT?Section 4 of 11
Deviations in fetal weight The diagnosis of clinically significant deviations in fetal weight is based on the presupposition that the reference range for fetal weight at each gestational age is established. Before a reference range for a human birth weight can be properly determined, the gestational age of the pregnancy must first be defined because fetal weight increases rapidly after the second trimester of pregnancy. Variations in fetal weight The normal gestational age for the spontaneous delivery of human pregnancies is well accepted as 280 days (40 wks) from the first day of the last normal menstrual period (266 d after ovulation). Because fewer than 3% of births occur at precisely 40 weeks' gestation and because the SD for term pregnancies is 1 week, the normal range of term birth weight is typically referenced to the mean birth weight for pregnancies delivered at 38-42 weeks' gestation (ie, mean term gestational age ±2 SDs). During this 4-week interval, the typical fetus gains approximately 12.7 ± 1.4 g/day, with a difference of ±0.3 g/day depending on the sex of the fetus. (Male fetuses gain weight more rapidly than female fetuses.) The average birth weight during this period varies substantially and depends on many factors, including maternal race, size, parity, pregnancy weight gain, glucose tolerance, hematocrit, and ambient elevation. In the United States, a study of 56,728 singleton births from 1975-1992 showed that the mean birth weight at 38-42 completed weeks' gestation was 3060-3520 g (range, 460 g [1 lb]). In Great Britain, a similar study of 41,718 newborns showed that the average was 3201-3753 g (range, 551 g [1 lb 3 oz]). In Singapore, a study of 11,026 neonates showed that the average was 2880-3290 g (range, 410 g [14 oz]). Because birth-weight data from population studies are often nonnormally distributed, the median birth weight at each gestational age is reported. In Canada, for live births recorded in 1986-1988, the median term birth weight at 38-42 weeks' gestation for 557,359 male singleton births was 3290-3800 g (range, 510 g [1 lb 2 oz]). In the United States, for 38,818 term male births in 1984-1991, the median birth weight was 3020-3572 g (range, 552 g [1 lb 3 oz]). In Sweden, for 32,087 term male births in 1956-1957, the median birth weight was 3300-3790 g (range, 490 g [1 lb 1 oz]). Birth weights of women from different racial groups Median term birth weights of neonates born to women from different racial groups differ substantially. Median birth weights were compared in 17,347 newborns of Caucasian and African American women of low socioeconomic status in the United States from 1959-1966. At 40 weeks' gestation, live-born Caucasian male singleton fetuses had a median weight of 3350 g compared with 3210 g for African American male neonates (difference of 140 g). A similar difference in median birth weight was also evident among female offspring. Caucasian female newborns at 40 weeks' gestation had a median birth weight of 3210 g, and African American female neonates had a median birth weight of 3100 g (difference of 110 g). Best method to determine the reference range for term birth weight Perhaps the best method for defining the reference range of term birth weight is to examine fetal weights at the 2 extremes of the range (ie, below the 5th-10th percentile and above the 90th-95th percentile). In the United States, a comprehensive study of 3,134,879 live births in 1991 showed that, at 38-42 weeks' gestation, the 5th percentile of birth weight was 2543-2764 g, the 10th percentile was 2714-2935 g, the 90th percentile was 3867-4098 g, and the 95th percentile was 4027-4213 g. Several studies from the last half of the 20th century demonstrated consistent results, showing that the 10th percentile of birth weight over the range of gestational ages listed above was 2430-3152 g, whereas the 90th percentile was 3600-4360 g. The most consistent feature of all these studies was the wide range of birth weights in the 5th-95th percentile. This range is equivalent to defining the empirical 90% CI for normal birth weight and, in the case of the most recent large-scale American study from 1991, this range is >1400 g (3 lb 1 oz). By using an 80% CI as an alternate measure, this range narrows to approximately 1100 g (2 lb 7 oz). These findings suggest that the reference range of birth weight at term might be defined as 3450 ± 700 g (2750-4150 g, or 6 lb 1 oz to 9 lb 2 oz). Table 2 shows specific birth weights associated with the different percentile ranks from 11 large studies. Table 2. Term Birth-Weight Percentiles for Singleton Live Births at 40 Weeks' Gestation
*Ultrasonographically dated. Perhaps the best method for establishing the reference range of term birth weight is to define the point at which newborns begin to substantially vary from the mean with respect to their prevalence of perinatal complications and perinatal death. Even within neonatal groupings that are well matched for gestational age, poor perinatal outcomes occur most frequently in fetuses who are born with weights at the extreme ends of the range (ie, <10th percentile and >90th percentile for each gestational age). By using this approach to establish a criterion, the reference range of term birth weight can be defined somewhat narrowly as about 3250-4250, or 3750 ± 500 g (7 lb 3 oz to 9 lb 6 oz). Data from a British cohort study of 3599 neonates born in 1946 suggested that increasing term birth weight is positively correlated with cognitive ability in later life. This result persisted even after neonates of low birth weight (<2500 g) were excluded from analysis, so that all of the remaining neonates weighed 2500-5000 g. DEFINITIONS OF DEVIATIONS IN FETAL GROWTHSection 5 of 11
Categories of fetal weight Fetal weight may be characterized as falling into 1 of 3 categories: (1) reference range (generally defined as the 10th-90th percentile for gestational age), (2) small for gestational age (<10th percentile), or (3) large for gestational age (>90th percentile). Until a fetus is delivered, only methods that help in evaluating fetal size in utero are of any value in assigning the fetus to these categories. Depending on the patient population that was evaluated to establish the percentiles, the standards may be misleading if they are applied to other sets of pregnant women. For instance, if standard birth-weight curves for Caucasian women are inappropriately applied to African American women, the proportion of newborns who appear to have birth weights below the 10th percentile is higher in African American neonates than in an appropriately matched group of Caucasian neonates. Complications Various authors in different eras have used the term low birth weight to refer to different ranges of fetal weight. Although excessive neonatal morbidity and mortality was once associated with newborns weighing 2000-2500 g, advances in neonatal care during the last quarter century have improved adverse neonatal outcomes attributable to low birth weight. One modern classification scheme for underweight newborns is based on fetal weight alone. By using this schema, newborns can be categorized by weight to assess their risk for neonatal complications, as follows: (1) low birth weight (1501-2500 g), (2) very low birth weight (1001-1500 g), or (3) extremely low birth weight (500-1000 g). Subclassifications in these weight groups are possible according to the incidence of neonatal morbidity and mortality in each group and the gestational age of the newborns (especially those with very low or extremely low birth weight). Successfully and accurately classifying fetuses in each of these three broad categories before delivery can help in predicting and possibly avoiding neonatal complications in underweight newborns. Fetal macrosomia The term fetal macrosomia denotes a fetus that is too large (see Media file 1). In ideal usage, this designation should be referenced to the mean fetal and maternal dimensions in a given population; however, it has been arbitrarily and variously defined as a birth weight >4000 g (8 lb 13 oz), >4100 g (9 lb), >4500 g, or >4536 g (10 lb) for all pregnant women, depending on the author and era. When fetal macrosomia is considered a birth weight of >4000 g, it affects 2-15% of all pregnant women, depending on the racial, ethnic, and socioeconomic composition of the population under study. What we would like to know before delivery Two issues concerning the size of a fetus before delivery are of interest to clinicians: the absolute size and relative size of the fetus. The first issue is absolute fetal size (ie, actual fetal dimensions and/or fetal weight). This information is important in estimating the risk of perinatal morbidity and mortality, but it is difficult to accurately assess before delivery. The second is the relative size of the fetus (relative fetal dimensions and/or fetal weight compared with data from some reference group). This is important to assess whether there is either IUGR or fetal macrosomia (both of which are independent predictors of perinatal morbidity and mortality). These determinations are difficult to accurately perform before delivery and depend on accurate estimations of absolute size and/or weight. These measures are difficult because they depend on choosing an appropriate reference group for comparison. FACTORS CONTRIBUTING TO DIFFERENCES IN FETAL WEIGHTSection 6 of 11
Many factors, both endogenous and extrinsic, can influence fetal weight. These encompass maternal factors (eg, race, stature, genetics), paternal factors (eg, paternal height), environmental influences (eg, altitude, availability of adequate nutrition), physiologic factors (eg, altered glucose metabolism, hemoglobin concentration, microvascular integrity), pathologic factors (eg, hypertension, uterine malformations), and complications of pregnancy (eg, gestational diabetes mellitus, preeclampsia). Gestational Age at DeliveryGestational age at delivery is the most significant determinant of newborn weight. Preterm delivery is the leading cause of low birth weight newborns in the United States. Other potential causes for low birth weight can collectively be attributed to IUGR (previously termed intrauterine growth retardation). Causes of IUGR include intrauterine infections, congenital syndromes, genetic abnormalities, and chronic uteroplacental insufficiency. In the case of postdate gestations, the rate of fetal macrosomia is 17-29%, as compared with 2-15% for the general population delivering term pregnancies. The rate of fetal macrosomia increases steadily with advancing gestational age beyond 37 weeks of pregnancy (see Table 3). Consistent with this observation, 99% of fetuses that weigh >4000 g are delivered at >37 weeks' gestation.
Maternal RaceA systematic difference is observed in the mean birth weight of babies born to mothers of different races and ethnicities. Depending on the mother's race, mean birth weights differ by as much as 141-395 g at term. Although some of this difference may be attributed to systematic differences in the mean gestational age of fetuses at delivery, to systematic racial differences in maternal characteristics (eg, maternal height and weight), and to differences in the prevalence of various diseases and complications of pregnancy that occur in different populations, an additional disparity in birth weight is attributable to racial differences per se. The precise cause of this effect is unknown, but it may be related to genetic or metabolic differences in fetal-growth potential for women of different races. Because of this effect, properly characterizing the race of mothers is important to accurately predict birth weights. As an example, African American and Asian women have fetuses that are smaller than those of Caucasian women when appropriately matched for gestational age. If a single birth-weight standard is used, Caucasian pregnant women have a significantly higher prevalence of fetal macrosomia compared with that of African American and Asian pregnant women, and women from these latter groups have a significantly higher prevalence of small-for-gestational-age newborns compared with that of Caucasian women. Other Parental, Environmental, and Pregnancy-Specific DeterminantsAfter gestational age and maternal race, major parental, environmental, and pregnancy-specific determinants of bir |