Estimation of Fetal Weight

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 excessively large fetuses, the potential complications associated with delivery include shoulder dystocia, brachial plexus injuries, bony injuries, and intrapartum asphyxia. The maternal risks associated with the delivery of an excessively large 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 both an increased rate of operative vaginal delivery and cesarean delivery for macrosomic fetuses compared with fetuses of normal weight. Depending on many factors, the optimal range for birthweight is thought to be 3000-4000 grams.

Limiting the potential complications associated with the birth of both small and excessively large fetuses requires that accurate estimation of fetal weight occur in advance of delivery. A review of the methods that can be used for the accurate estimation of fetal weight is the focus of this article.

Table 1. Newborn and Maternal Complications Associated with Birth Weight of Greater Than 4000 Grams*

Complication	Relative Risk	Attributable Risk %
Shoulder dystocia	2-38	2-18
Brachial plexus palsy	16-216	0.2-8
Bony injuries/fracture	1.4-97	0.2-6
Prolonged labor	2.2-3.2	2-7
Birth asphyxia/low Apgar scores	1.7-5.6	0.6-6
Forceps/vacuum extraction	1.5-3.6	8-14
Birth canal/perineal lacerations	1.6-5.1	3-7
Postpartum hemorrhage	1.6-5.2	2-5
Cephalopelvic disproportion	1.9-2.2	4-5
Cesarean delivery	1.2-2.9	4-14

*Data were compiled from 15 studies that investigated both the relative risk and the attributable risk of complications associated with the birth of macrosomic fetuses. The ranges reported reflect the differences among studies in the patient populations under investigation and differences in the criteria used for the diagnosis of each complication.
Relative and attributable risks are for fetuses weighing more than 4000 grams at delivery compared to controls weighing less than 4000 grams. The P values associated with each relative risk are less than .001 in all cases, except for birth canal/perineal lacerations, for which the P value is less than .05.

In general, these 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 for individual patients. The range of birth weights at any particular gestational age spans a wide array of values, with 95% confidence intervals of more than 1600 grams (3 lb 8 oz) at term. In addition, fetal growth curves are the most inaccurate at the extremes of fetal weight deviation (ie, women carrying fetuses that are either growth restricted or macrosomic).

Until a fetus is delivered, only those methods that can evaluate fetal size in utero are of any value in assessing into which of these 3 categories the fetus will fall. Depending on the precise nature of the patient population used for establishing the birth weight percentile ranks, these standards may be misleading if applied to other sets of gravidas. For instance, if standard birth weight curves for white women are applied inappropriately to black gravidas, a higher proportion of black women would appear to have birth weights below the 10th percentile compared to a matched group of white women.

Complications

The term low birth weight has been used to refer to different fetal weight ranges by different authors during different eras. Whereas substantially excessive neonatal morbidity and mortality was once associated with newborns weighing 2000-2500 grams, adverse neonatal outcomes attributable to low birth weight have been impacted successfully by the more modern neonatal care that has become available during the last quarter century. One classification scheme for the modern era that is based on fetal weight alone divides underweight newborns into 3 distinct categories. Using this schema, newborns can be categorized according to their risk for neonatal complications, as follows:

• Low birth weight (1501-2500 g)

• Very low birth weight (1001-1500 g)

• Extremely low birth weight (500-1000 g)

Subclassifications within these 3 weight groups are possible, according to the overall incidence of neonatal morbidity and mortality within each group and the gestational age within these different categories (especially within the very low birth weight and extremely low birth weight groups). Successfully classifying fetuses within each of these 3 broad categories with improved accuracy in advance of delivery can potentially aid in the prediction and possible avoidance of neonatal complications for underweight newborns.

Fetal macrosomia

The term fetal macrosomia denotes a fetal size that is too large. Ideally, this designation should be referenced to the mean of fetal and maternal dimensions within a given population, but, rather arbitrarily, it has been defined previously as a birth weight greater than 4000 grams, greater than 4100 grams, greater than 4500 grams, or greater than 4536 grams for all gravidas, depending on author and era. When fetal macrosomia is considered a birth weight greater than 4000 grams (8 lb 13 oz), it affects 2-15% of all gravidas, depending on the racial, ethnic, and socioeconomic composition of the population under study.

Gestational age at delivery

Gestational age at delivery is the most significant single determinant of newborn weight. Preterm delivery constitutes the single largest cause for low birth weight in newborns in the United States. Other potential causes for low birth weight can be attributed collectively to IUGR (previously termed intrauterine growth retardation). Causes include intrauterine infections, congenital syndromes, genetic abnormalities, and chronic uteroplacental insufficiency.

Maternal race

Another major determinant of fetal weight is maternal race. Black and Asian women have smaller fetuses compared to white women when appropriately matched for gestational age. Not surprisingly, white gravidas show a significantly higher prevalence of fetal macrosomia compared with black and Asian gravidas, and nonwhite gravidas have a significantly higher prevalence of small-for-gestational-age newborns compared to white women.

Other maternal and pregnancy-specific determinants

After gestational age and maternal race, 6 other major maternal and pregnancy-specific determinants of birth weight are relevant (see Table 3), which include the following:

• Maternal height

• Obesity

• Pregnancy weight gain

• Age

• Parity

• Fetal sex

Taken together, these measurable demographic factors can help explain more than one third of the variance in term birth weight. By comparison, paternal factors are only minimally important in determining fetal weight. Paternal height is the only routinely measured paternal demographic variable that has significant influence on fetal weight, but it accounts independently for less than 2% of the variance. Fetal sex is associated significantly with birth weight; female fetuses are known to be smaller than male fetuses when matched for gestational age. Although fetal sex is a significant predictor of fetal weight, it accounts independently for less than 2% of the variance.

Table 3. Routinely Measurable Parental and Pregnancy-Specific Demographic Factors That Influence Fetal Weight

Parental or Pregnancy-Specific Factors	First Order Correlation With Birth Weight
Gestational age at delivery	0.29-0.41
Maternal weight at 26 wk	0.30-0.39
Maternal height	0.22-0.26
Maternal pregnancy weight gain	0.15-0.17
Maternal body mass index	0.17
Maternal age	0.02-0.14
Parity	0.01-0.19
Paternal height	0.14-0.23
Fetal sex	0.13-0.19

Diabetes mellitus

Uncontrolled maternal diabetes mellitus is a condition commonly associated with excessive fetal weight. Glucose is the primary substrate used by fetuses for growth. When maternal glucose levels are excessive, abnormally high rates of fetal growth can be expected. Even in women without frank diabetes mellitus, elevated glucose screening test values in pregnancy predispose to increasing birth weight. Because of the stringent glucose criteria now used to monitor and treat women with frank diabetes during pregnancy, the group of women now most at risk for fetal macrosomia are those who are unmonitored and untreated who have abnormal 1-hour glucose screening test results during pregnancy and subsequently have normal 3-hour glucose tolerance tests with a single abnormal value indicative of only mild glucose intolerance.

Other maternal illnesses and complications of pregnancy

Several maternal illnesses and complications of pregnancy are associated with decreased birth weight. The most common associated illnesses are chronic maternal hypertension and preeclampsia. Some intrauterine infections (eg, viral, parasitic, bacterial) are associated with small-for-gestational-age fetuses. In addition, several major environmental factors can have an adverse effect on fetal size, with the 2 chief among these being high altitude and cigarette smoking.

These prospectively measurable variables can explain 36% of the variance in term birth weight and can help predict birth weight accurately to within 267 grams (7.6% of individual birth weights). In addition, 75% of newborn weights can be estimated properly to within 10% of actual birth weight using this technique.

Equation 1, the equation generated for this purpose, is as follows:

Birth weight (g) = gestational age (d) X [9.36 + 0.262 X fetal sex + 0.000237 X maternal height (cm) X maternal weight at 26 wk (kg) + 4.81 X maternal weight gain rate (kg/d) X (parity + 1)], where fetal sex is equal to +1 for males, -1 for females, and 0 for unknown sex and gestational age is equal to days since onset of last normal menses, which equals the conception age (d) + 14.

• The trade-off between sensitivity and specificity for the prediction of fetal macrosomia greater than 4000 grams is shown in Image 1.

• The trade-off between the positive and negative predictive value for the prediction of birth weight greater than 4000 grams is shown in Image 2.

• The overall accuracy of the equation for the prediction of fetal macrosomia is shown in a receiver operating characteristic curve in Image 3. The area under this curve is 0.82.

• Using a cutoff value of 3775 grams, the sensitivity for the prediction of fetal macrosomia greater than 4000 grams is 54%, the specificity is 92%, the positive predictive value is 52%, and the negative predictive value is 93%.

These results are consistent with those reported previously using a similar quantitative maternal characteristics approach.

Which of the methods for predicting fetal weight is the most accurate

The accuracy of the different methods of predicting fetal weight depends on the gestational age and range of birth weights under study. Again, for this purpose, dividing fetuses into 3 birth weight categories of less than 2500 grams, 2500-4000 grams, and greater than 4000 grams is useful. The relative accuracy of clinical palpation versus obstetric sonographic fetal biometry for these 3 birth weight ranges is shown in Table 11. For the clinically significant birth weight ranges of less than 2500 grams and greater than 4000 grams, the accuracy of sonographic fetal biometry appears to be superior to clinical palpation for predicting the occurrence of low birth weight fetuses weighing less than 2500 grams, whereas the 2 techniques appear to be comparable in predictive accuracy for fetuses weighing 2500 grams or more.

Table 11. Accuracy of Clinical Palpation Versus Sonographic Fetal Biometry for Predicting Actual Birth Weight of Less Than 2500 Grams, 2500-4000 Grams, and Greater Than 4000 Grams*

	Clinical Palpation		Ultrasonic Fetal Biometry
Birth Weight	Abs % Error	% Within 10%	Abs % Error	% Within 10%
<2500 g	13.7-19.8%	40-49%	10.5-11%	56-63%
2500-4000 g	7.2-10.4%	60-75%	7-10.5%	58-71%
>4000 g	9.1-9.5%	53-61%	8.1-9.5%	59-62%

*Adapted from Chauhan et al and Sherman et al
Abs % error - Mean absolute percentage error of fetal weight predictions
% within 10% - Percentage of fetuses with weight accurately predicted to within 10% of actual birth weight

A recent study directly comparing the 4 different methods of fetal weight prediction in 44 normal term pregnancies found no difference between the accuracy of the clinical methods (eg, clinical palpation, birth weight prediction equation, maternal self-estimation of fetal weight) and ultrasonic fetal biometric techniques for predicting term birth weight. Eight different ultrasonic fetal biometric algorithms were assessed for this purpose. The mean birth weight for newborns in this study was 3445 458 grams, with a birth weight range of 2485-4790 grams. These results are summarized in Table 12. No systematic advantage was found with the ultrasonic technique for predicting term birth weight over the clinical methods.

Seven other recent studies directly compared the accuracy of clinical palpation to ultrasonographic fetal biometry using the same gravidas (see Table 5), and 3 compared clinical palpation to parous patients' self-estimates of fetal weight after 37 completed weeks' gestation.

One study compared clinical palpation to both ultrasonographic fetal biometry and parous patients' self-estimations of fetal weight. All of the methods have significant predictive errors in birth weight estimations for term fetuses that range from 290-560 grams, and no consistent or clear superiority of ultrasonographic fetal biometry over the other techniques of fetal weight estimation was found.

Table 12. Comparison of Results for Different Methods of Predicting Term Birth Weight*

	Correlation Coefficient with Actual Birth Weight	Mean Absolute Error	Mean Absolute % Error	% Within 15% of Actual Birth Weight
Clinical methods
Birth weight prediction equation	0.55	312 g	9.8%	86%
Leopold maneuvers	0.60	336 g	9.9%	83%
Maternal self-estimated fetal weight	0.45	402 g	11.5%	67%
Ultrasonic methods
Hadlock ultrasound equation 1	0.61	292 g	8.4%	88%
Combs ultrasound equation	0.60	285 g	8.3%	82%
Hadlock ultrasound equation 3	0.60	325 g	9.4%	83%
Hadlock ultrasound eq. 2	0.58	328 g	9.4%	78%
Campbell ultrasound equation	0.42	368 g	10.3%	79%
Warsof ultrasound equation 2	0.63	370 g	10.3%	61%
Warsof ultrasound equation 1	0.40	359 g	10.9%	72%
Shepard ultrasound equation	0.52	402 g	11.4%	63%

*Study of 44 term patients by the author (Nahum et al, 1999)

Option for suppression of labor in women carrying undersized fetuses

In general, the case can be made to attempt labor suppression in women carrying preterm fetuses weighing less than 2000-2500 grams. As stressed previously, most low weight fetuses are associated with preterm gestations. However, any recommendation in this circumstance regarding tocolysis presupposes the following: (1) no immediate fetal or maternal indications mitigate toward the timely delivery of the undersized fetus and (2) the undersized fetus will continue to grow along an acceptable growth curve if the gestation is allowed to continue. In many cases, both of these assumptions are invalid. For instance, many women who deliver preterm neonates are allowed to do so because of compelling fetal or maternal medical conditions that warrant timely delivery (eg, intrauterine infection, severe uteroplacental insufficiency, severe preeclampsia). If fetal infection or IUGR is present, the preterm delivery of an underweight fetus may be indicated.

The increased risk of perinatal complications associated with the delivery of an underweight fetus in these circumstances may be outweighed entirely by the increased risk of morbidity and mortality for both the fetus and mother with allowing the pregnancy to continue. Additionally, in some circumstances, the inadequate velocity of fetal growth might mandate a decision for delivery. In such cases, the presumption is that extrauterine growth and development in the neonatal nursery would be superior to that achieved in utero. Clinical judgment under such circumstances is of paramount importance in deciding when to effect delivery and when to attempt labor suppression. More detailed considerations for aiding in this decision are beyond the scope of this article.

Option for labor induction in women carrying oversized fetuses

For fetuses delivered before 37 weeks' gestation, fetal macrosomia is a rarity; more than 99% of macrosomic fetuses are the product of term gestations. In general, nearly 95% of fetuses gain 12.7 2.8 g/d from 37-42 weeks' gestation, indicating that an average fetus gains an additional 445 98 grams (1 lb 3 oz) during this period. If a patient is thought to have a term fetus weighing more than 4000 grams and is willing to undergo labor induction, effecting vaginal delivery in these gravidas sooner, rather than awaiting the onset of spontaneous labor and a higher average birth weight at delivery, is often reasonable.

In studies that have attempted to examine this question, labor induction has not been demonstrated conclusively to decrease the fetal and maternal risks of intrapartum complications, and the cesarean delivery rate has been suggested to increase in several studies, whereas it has been purported to be unchanged in others. The difficulty in interpreting these results is that significant differences have been found among the predicted and actual birth weights for patients included for investigation and the power of the studies conducted to date has been insufficient to conclusively demonstrate statistically significant differences in adverse fetal outcomes among different study groups.

As with the case of preterm delivery of underweight fetuses, many considerations, including the size of the maternal pelvis and the weight of previously delivered fetuses, should be taken into consideration. Clinical judgment in these circumstances is of paramount importance in deciding whether or not labor induction is indicated in an attempt to minimize excessive fetal weight at delivery.

Conclusions

Both low birth weight (<2500 g) and high birth weight (>4000 g) are fetal conditions that are associated with increased risks of peripartum morbidity and mortality. Although the absolute risk that fetuses with birth weights of 2000-2500 grams and 4000-4500 grams will have major peripartum complications is not overwhelming, the risk of such complications increases substantially with both decreasing and increasing birth weight relative to these lower and upper limits. Thus, birth weight and gestational age are both important determinants of peripartum outcome. From this standpoint, the optimal range of newborn weight generally is thought to be 3000-4000 grams (6 lb 10 oz to 8 lb 13 oz). As always, the problem is knowing the fetal weight with sufficient accuracy in advance of delivery.

Many factors that impact directly upon birth weight are not modifiable. These include maternal race, height, parity, paternal height, and fetal sex. However, what can be influenced with potentially significant effects upon birth weight are the following:

• Prepregnancy weight

• Pregnancy weight gain

• Glucose control in patients with diabetes or glucose intolerance of pregnancy

• Gestational age at delivery

All of these factors can have significant impacts on fetal weight at delivery. Whereas permitting the delivery of fetuses that weigh 2000-2499 grams typically is not associated with an overwhelming increase in neonatal complications compared with normal-weight neonates, those fetuses weighing less than 2000 grams at birth are at increased risk for perinatal complications in a manner that is commensurate with their weight.

Similarly, whereas allowing a trial of a vaginal delivery for a fetus estimated to weigh 4000-4499 grams may be reasonable in many circumstances, many sources suggest that fetuses with estimated weights of 4500 grams or greater should be delivered by cesarean birth in order to avoid the increased intrapartum risks associated with the vaginal delivery of a macrosomic fetus. This is especially true when gestational diabetes is involved and the fetal conformation may be altered to reflect a larger shoulder girdle or head circumference ratio compared with the offspring of mothers without diabetes.

In the case of macrosomic fetuses, attempts to predict birth weight from fetal measurements obtained via ultrasonography have proven unsuccessful from the standpoint of improving clinical outcomes. Many studies conclude that ultrasonographic fetal biometric assessments are no more predictive of fetal macrosomia than clinical assessments of fetal size by simple external abdominal palpation (see Table 5 and Table 10). Both ultrasonography and manual assessment of fetal size have sensitivities of less than 60% for the prediction of fetal macrosomia, with false-positivity rates greater than 40%. Likewise, for small fetuses less than 1800 grams, ultrasonic fetal weight estimates are often in error by as much as 25%.

By using a birth weight prediction equation that is based on maternal and pregnancy-specific characteristics alone, fetal weight at and near term can be predicted with a high degree of accuracy (7.6%). This approach appears to be at least as reliable for predicting fetal macrosomia in healthy gravidas as both clinical palpation and ultrasonographic fetal biometry, neither of which can be used with any degree of certainty in advance of the date of delivery. Such a quantitative assessment of maternal characteristics serves to objectively quantify the majority of previously recognized clinical variables that have long been employed in subjective clinical assessments and that are thought to be predictive of fetal weight.

By contrast, clinical palpation is a subjective methodology that must be employed at or near the date of delivery, and it is both patient- and clinician-dependent for its success (ie, less accurate for obese than nonobese gravidas, significant for interobserver variation in birth weight predictions even among experienced clinicians).

The disadvantages of ultrasonographic fetal biometry are that the method is both complicated and labor-intensive, potentially being limited by suboptimal visualization of fetal structures. It also requires costly sonographic equipment and specially trained personnel. Although such expensive imaging equipment is widely available in the United States and other industrialized countries, this is generally not the case in developing nations, where medical resources are often scarce.

In the future, combining the different methods of fetal weight prediction to improve their overall accuracy may be possible. Wikstrom et al suggested that by combining the independent information about fetal size obtained from the 3 different approaches (ie, clinical examination, quantitative assessment of maternal characteristics, ultrasonographic fetal biometry), the predictive value of fetal weight estimations can be improved dramatically. In the case of excessive fetal size, combining these methodologies may result in an 80% positive predictive value for the identification of fetal macrosomia, with a sensitivity of 63% and specificity of 95%.

Recently, a quantitative combination of maternal demographic information (of the type incorporated in Equation 1) with the independent information obtained by ultrasonic fetal biometry (AC) has been demonstrated to improve birth weight prediction substantially, with the area under the receiver operating characteristic curve increasing to 0.92. The mean absolute percentage error in birth weight predictions that can be attained using this new combinatorial method is 5.4%.

With the advent of 3-dimensional fetal imaging, optimism that these new technologies can provide even better fetal weight estimations may be justified, but the advantages of estimating fetal weight using these newer techniques have not yet been demonstrated. Using these new approaches, further improvements in the accuracy of fetal weight prediction in the future will permit prospective obstetric intervention to be undertaken more confidently by practicing obstetricians, with the aim of minimizing intrapartum and peripartum risks for both fetuses and mothers.

Caption: Picture 2. Estimation of fetal weight. Curve delineating the trade-off between positive and negative predictive value using Equation 1.

Caption: Picture 3. Estimation of fetal weight. Receiver operating characteristics curve for the prediction of fetal weight of greater than 4000 g using Equation 1. The dashed lines represent the sensitivity and specificity associated with prediction equation cutoff values of 3500 g, 3700 g, and 3920 g, respectively.

Picture Type: Graph

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