FIGURE 44-6 Correlation of sonographic fetal weight estimation using abdominal circumference (A C) and actual birthweight. (Data from pregnancies managed at Parkland Hospital.) that second-trimester sonography is superior to first-trimester scans for predicting SGA neonates. At Parkland Hospital, we provide midpregnancy sonographic screening examination of all pregnancies. Additional sonographic evaluations of fetal growth are performed as clinically indicated.
With sonography, the most common method for identiying poor fetal growth is estimation of weight using multiple fetal biometrical measurements. Combining head, abdomen, and femur dimensions provides optimum accuracy, whereas little incremental improvement is gained by adding other biometrical measurements (Platz, 2008). Of the dimensions, femur length measurement is technically the easiest and the most reproducible. Biparietal diameter and head circumference measurements are dependent on the plane of section and may also be afected by deformative pressures on the skull. Last, abdominal circumference measurements are more variable. However, these are most frequently abnormal with fetal-growth restriction because soft tissue predominates in this dimension (Fig. 44-6). Shown in Figure 44-7 is an example of a severely growth-restricted newborn.
Some studies have reported a significant predictive value for small abdominal circumference with respect to lagging fetal growth. One study screened nearly 4000 pregnancies using either clinically indicated or universal sonography in the third trimester (Sovio, 2015) . Universal sonography raised the rate of detection of SGA from 20 percent to 57 percent. Importantly, however, the neonatal morbidity rate was increased only if the abdominal circumference growth velocity was in the lowest decile.
Sonographic estimates of fetal weight and actual weight may be discordant by 20 percent or more, leading to both false-positive and false-negative indings. Dashe and associates (2000) studied 8400 live births at Parkland Hospital in which fetal sonographic evaluation had been performed within 4 weeks of delivery. They reported that 30 percent of growth-restricted fetuses were not detected. In a study of 2586 women with low-risk pregnancies randomly assigned to sonography at 32 or 36 weeks' gestation, sensitivity to identiy grow restriction was improved at the later gestational age (Roma, 2015). Still, nearly 40 percent of cases of growth restriction deined as birthweight <3rd percentile were missed. A Cochrane database analysis of 13 trials with 34,980 women concluded that routine late pregnancy ultrasound for a low-risk or an unselected population is not associated with maternal or fetal benefit (Bricker, 2015).
FIGURE 44-7 A 36-week newborn with severe fetal-growth restriction. (Used with permission from Dr. Roxane Holt.)
An association between pathological fetal-growth restriction and oligohydramnios has long been recognized. Petrozella and associates (201r1) reported that decreased amnionic fluid volume between 24 and 34 weeks' gestation was signiicantly associated with malformations. In the absence of malformations, a birthweight < 3rd percentile was seen in 37 percent of pregnancies with oligohydramnios, in 21 percent with borderline amnionic fluid volume, but in only 4 percent with normal volumes. Also, from a recent metaanalysis of 15 studies involving more than 35,000 pregnancies, high-risk pregnancies with oligohydramnios were more likely to be complicated by low birthweight compared with low-risk pregnancies with oligohydramnios (Rabie, 2017). Hypoxia and diminished renal blood low are proposed explanations for oligohydramnios.