Objective
The gestation-adjusted projection method (GAP method) uses third trimester ultrasound fetal weight to predict birthweight. Our study sought to assess if the accuracy of the GAP method in morbidly obese women depended on (1) ultrasound timing or (2) extreme elevations in maternal body mass index (BMI).
Study Design
We conducted a diagnostic accuracy study from 2007 to 2012 of all singleton pregnancies with BMI >40 kg/m 2 at the time of delivery that had fetal growth assessment between 30+0 and 35+0 weeks (EARLY) and greater than 35+0 weeks (LATE). Next, a ‘reference’ BMI group (30-35) was compared with subcategories of women with BMI ≥40. Paired t tests, χ 2 tests and analysis of variance were used to determine significance.
Results
A total of 235 pregnancies for the first objective and 430 for the second objective were included. The mean absolute percent error was comparable between the 2 periods (EARLY 7.9 ± 6.7%, LATE 7.4 ± 5.6%; P = .33) and across BMI categories (30-35: 7.6 ± 6.3%, 40-50: 7.2 ± 5.7%, >50: 7.8 ± 6.0%; P = .79). The sensitivity and specificity of the GAP method to predict macrosomia during the EARLY time period was 46% (95% confidence interval 28–66) and 97% (94–99) and in the 40-50 BMI subcategory was 72% (47–90) and 96% (90–99), respectively.
Conclusion
Prediction of birthweight using the GAP method in obese women does not appear to be influenced by timing of ultrasound or extreme BMI. In a population where clinical estimated fetal weight is difficult, the GAP method may aid in delivery planning.
Obstetricians often use sonographic and clinical estimates of fetal weights to determine mode and timing of delivery. Ultrasound technology to determine fetal weight during the antenatal period has been in use since the early 1980s. Early studies using ultrasound to predict birthweight typically performed the assessment immediately before delivery.
For many high-risk pregnancies, antenatal surveillance includes regular assessment of fetal growth by ultrasound in the third trimester. Pressman et al found that ultrasound performed between 34 + 0 and 36 + 6 weeks’ gestational age provided a more accurate assessment of fetal weight than one performed at 37 weeks and beyond. Using the gestation-adjusted projection method (GAP method), one can use an ultrasound performed in this near term period to estimate the expected birthweight at term through extrapolation.
Ultrasound in women with a normal body mass index (BMI) can approximate birthweight within 7 to 13%, whereas accurate estimation of fetal weight by ultrasound among morbidly obese women is unclear. Obesity is known to interfere with optimal visualization of anatomic structures. Although previous studies have shown no difference in ultrasound (performed within 5 days of delivery) to predict birthweight based on BMI, these studies classified ‘obesity’ as a BMI of >29 kg/m 2 . Another study, which used the GAP method and ultrasound performed between 34 + 0 and 36 + 6 weeks, illustrated similar findings but only included 16 women with a BMI >50 kg/m 2 . Among twin gestations, a decrease in accuracy of birthweight prediction by ultrasound was found with increasing BMI. The rapid increase in morbid obesity (BMI >40 kg/m 2 ) in the United States, including among reproductive-aged women, means that this technology and extrapolation from its estimates will likely be used more frequently, as clinical estimation of fetal weight in these women is not practical.
We conducted this study to determine whether the ultrasound time period (early vs late third trimester) affected accuracy of the GAP method to predict birthweight in morbidly obese women and second, to determine whether extreme elevations in maternal BMI impacted the accuracy of birthweight prediction.
Materials and Methods
We conducted a diagnostic accuracy study of women who had obstetric ultrasound at Duke University Maternal Fetal Medicine between January 2007 and December 2012. The Duke University Medical Center institutional review board approved this study. The first objective was to determine whether birthweight estimates using the GAP method among women with a BMI of 40 kg/m 2 or greater were more accurate if the ultrasound was performed during the ‘EARLY’ or ‘LATE’ third trimester. The ‘EARLY’ third trimester was defined as 30 + 0 to 35 + 0 weeks, although the ‘LATE’ third trimester was defined as after 35 + 0 weeks. Patients met inclusion criteria if they had at least 1 ultrasound in both the ‘EARLY’ and ‘LATE’ third trimester where biometry was performed, the infant was liveborn, and data on outcome of the delivery was available. If a patient had more than 1 ultrasound performed in either of the specified time periods, the ultrasound performed later in gestation was used. Exclusion criteria were multiple gestations and a BMI <40 kg/m 2 for the first objective. The second study objective was to determine whether birthweight estimates using the GAP method varied by BMI subcategories. Inclusion criteria was similar to the first objective with the exception that the ultrasound performed was between 34 + 0 to 36 + 6 weeks’ gestation. Each patient was categorized into 1 of the following BMI groups; 30-35 kg/m 2 , 40-50 kg/m 2 , and >50 kg/m 2 . Patients with multiple gestations and a BMI less than 30 kg/m 2 and between 35-40 kg/m 2 were excluded (because of the numbers of patients who met criteria in the BMI 30-35 kg/m 2 category). All ultrasound procedures were performed at Duke University Maternal Fetal Medicine.
Ultrasound biometry was obtained by 1 of 9 experienced ultrasonographers using standard techniques on GE Voluson E8 machines (General Electric, Waukesha, WI) and fetal weight was calculated using Hadlock’s formula (head circumference, abdominal circumference, and femur length). Data were collected from review of the medical record and included: maternal age, parity, race/ethnicity, gestational age at each sonogram, fetal biometry and estimated fetal weights, gestational age at delivery, birthweight, presence of fetal anomalies, suspicion of growth restriction or macrosomia, maternal diabetes or hypertension, and the presence of oligohydramnios (amniotic fluid index <5.0 cm). Suspicion for fetal growth restriction (FGR) was based on the ultrasound closest to delivery within the time period specified and defined as an estimated fetal weight (EFW) of less than 10th percentile and suspicion for macrosomia was defined as an EFW of greater than 90th percentile.
The GAP method uses EFW by ultrasound to predict birthweight for a specified estimated gestational age of delivery.
( Estimated Fetal Weight Median Fetal Weight at Gestational age of US ) = ( Birthweight Median Fetal Weight at Gestational Age of Delivery )
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