Ultrasonography is commonly utilized in obstetric practice to estimate fetal weight at term. The seminal paper by Lubchenco et al introduced into the field of obstetrics what is commonly referred to as the Denver growth curves. These curves established norms for fetal weight and permitted the identification of fetuses with deviation from average growth at a particular gestational age.

The terms large for gestational age (LGA) and small for gestational age (SGA) are common to most obstetricians and refer to these growth aberrancies. With time, though, it eventually became clear that 1 set of growth parameters may not apply to every fetus. For example, male fetuses on average weigh more than female fetuses. Additionally, maternal race and ethnicity may play a role in determining optimal fetal weight, because varying weight distributions are observed across populations.

Although the precise definition of an excessively large fetus may be up for debate, it is clear that there are risks to both mother and fetus associated with excessive fetal weight. Maternal risks include significantly higher rates of cesarean section, operative vaginal delivery, and genital tract lacerations, particularly third- and fourth-degree lacerations. Neonatal morbidities include an increased risk of shoulder dystocia–associated brachial plexus injury, clavicular/humeral fracture, and hypoxic-ischemic encephalopathy. Therefore, the ability to identify the aberrantly large fetus continues to be an area of great interest.

Risk factors for the delivery of an excessively large fetus, such as maternal diabetes and obesity, are well established, but how to prevent the morbidities associated with a large infant is less clear. Most interventions have evaluated the outcome of shoulder dystocia as a surrogate for brachial plexus injury and have focused on the role of cesarean delivery as a means to avoid such birth injuries.

The results, in particular for the nondiabetic pregnancy, have unfortunately been disappointing. One study in particular, found that approximately 2000 cesarean sections would have to be performed to prevent 1 case of permanent brachial plexus injury. This statistic emphasizes that a clear minority of deliveries of large infants will be complicated by shoulder dystocia. Additionally, approximately half of shoulder dystocias occur in infants with birthweights less than 4000 g.

The latter finding, combined with the fact that epidemiologic variables play a role in determining fetal weight, raises several intriguing questions. First, should one arbitrary cutoff be utilized to identify fetuses as large for gestational age? Should the same weight percentile be applied to a female fetus of a short, white woman as to a male fetus of a tall, Hispanic woman? Put another way, is a 3900 g fetus always appropriately grown? Conversely, is a 4100 g fetus always too large for a given pregnancy? Might it be that the former fetus should ideally be 3500 g at term and has exceeded its growth capacity, whereas the latter fetus might optimally be 4200 g at term and has not yet exceeded its expected growth?

An ideal solution is to better identify those at-risk fetuses currently labeled as nonmacrosomic and exclude those infants currently labeled as macrosomic, who are not at risk for obstetric complications. The use of a customized growth curve is one attempt to address this issue. Customized growth curves are being evaluated with increasing frequency. (In the current issue, an intriguing study extends the technique of customized growth curves to identifying excessively large fetuses.)

Larkin et al reported their experience with the use of customized growth standards for evaluating potential LGA fetuses. The authors integrated demographic variables (maternal height, weight, race parity, and fetal sex) into a linear regression model to develop a predictor of ideal birthweight at term. The technique allowed the authors to identify customized cutoffs for LGA and SGA fetuses based on the aforementioned variables. The obstetric outcomes of more than 30,000 pregnancies were then investigated using their predictor. The pregnancies that were considered LGA based on the customized growth curve carried significantly increased risks for shoulder dystocia, third- and fourth-degree lacerations, and cesarean delivery. Their estimator identified a subset of women who gave birth to infants who would not generally be considered LGA by widely used population growth curves and would ordinarily go unrecognized.

The current study merits attention for several reasons. The use of customized growth curves has the potential to identify previously unrecognized at-risk fetuses for complications, such as shoulder dystocia. Additionally, the curve may also reassure practitioners that a 4000 g fetus is not aberrantly large and may prevent an unnecessary cesarean section. With the obesity epidemic only increasing and evidence that the in utero environment can have an impact on the risk for adult-onset disease, it is important to recognize individuals at risk for childhood and adult obesity.

The technique used in this study has the potential to address this public health issue by perhaps targeting individuals at risk, even prior to delivery. As demographics of pregnant women change, along with our enhanced understanding of the determinants of fetal growth, tools for improved fetal development assessment are clearly a welcome sign.