Objective
The objective of the study was to compare prenatal ultrasound parameters for intrauterine growth restriction (IUGR) with newborn percent body fat (%BF).
Study Design
This was a prospective study of 87 pregnancies followed with ultrasound. Subjects were categorized into 3 groups: estimated fetal weight (EFW) less than the 10th percentile, abdominal circumference (AC) less than the fifth percentile, and normal biometry. Neonatal %BF by air displacement plethysmography was compared between each group using multivariable analyses.
Results
The %BF in the EFW less than the 10th percentile group (5.1 ± 2.9%) was significantly lower than either AC less than the fifth percentile (9.5 ± 3.3%) or normal groups (11.6 ± 5.6%). EFW less than the 10th percentile best predicted %BF by regression model. Neonatal morbidity was not significantly higher in the EFW less than the 10th percentile group.
Conclusion
Newborn %BF was significantly lower in infants with EFW less than the 10th percentile compared with AC less than the fifth percentile, an intermediate finding. An AC less than the fifth percentile on ultrasound does not reflect the same severity of IUGR as EFW less than the 10th percentile.
The diagnosis of intrauterine growth restriction (IUGR) by antenatal ultrasonography remains a challenge for obstetrical providers. In clinical practice it is difficult to differentiate between constitutionally small but normally grown and pathologically growth-restricted fetuses using antenatal ultrasonography. Additionally, the consequences of an incorrect determination can be profound. Delaying delivery in a pathologic situation can result in stillbirth or other adverse developmental affects, whereas the unnecessary delivery of a constitutionally small but otherwise normal infant poses fetal hazards associated with iatrogenic preterm delivery.
The American College of Obstetricians and Gynecologists defines IUGR as a fetus whose estimated fetal weight (EFW) is below the 10th percentile for gestational age. Unfortunately, about 70% of the fetuses identified by this criterion will be normal or constitutionally small-for-gestational-age infants that are not at risk for adverse fetal or neonatal outcomes.
Even more problematic is the fact that an EFW above the 10th percentile, based on multiple biometric parameters, does not necessarily confirm that there has been normal fetal growth. Defining IUGR as less than the 10th percentile for gestational age makes reasonable statistical sense, but this has some significant limitations in clinical practice.
As a result, there has been significant debate as to what ultrasound parameters best represent clinically relevant IUGR. Studies have suggested that adverse perinatal outcomes are generally not anticipated unless birthweights are below the fifth or even the third percentiles. Other commonly used definitions include an EFW or abdominal circumference (AC) 2 SDs below the mean or an AC measurement less than the fifth percentile. Because the AC also plays a significant role in the EFW formula, lags in AC and EFW may be early manifestations of brain sparing, in which preferential shunting of blood to the brain leads to slower growth of the AC.
A limitation of these ultrasound-based definitions is that they are all measurements of size rather than mass. It is well established that normal fetal size varies significantly by race/ethnicity, fetal sex, parity, and maternal size as well as other genetic physiologic factors.
This normal variation in size and the relative inability to assess fetal mass explains much of the inaccuracy of prenatal ultrasound in diagnosing pathologic IUGR. An AC less than the fifth percentile, reflecting reduced abdominal fat deposition, suggests that this is the parameter that is most likely to be affected by fetal growth restriction.
Fat content correlates with energy stores, and fat mass and lean body mass are often used in nutritional assessment. Investigations of neonatal body composition have shown that the amount of body fat explains approximately half of the variability in neonatal birthweight. Infants with low percent body fat (%BF) have been shown to be at greater risk for neonatal morbidities associated with IUGR as well as for longer-term adverse outcomes. Prior studies reveal that infants born small because of suboptimal fetal growth are at higher risk for neonatal death and impaired cognitive development in childhood.
The objective of this study was to compare antenatal ultrasound parameters suggestive of IUGR in a cohort of high-risk pregnancies and correlate them with neonatal %BF as a measure of fetal growth restriction.
Materials and Methods
This was a prospective cohort study of 87 pregnancies at risk for IUGR seen at the Medical University of South Carolina between March 2009 and January 2011. Pregnant women were referred after 28 weeks of gestation based on a clinical finding of size less than the dates or a maternal history of a condition known to influence fetal growth such as chronic hypertension, smoking, diabetes, uterine fibroids, autoimmune disease, or renal disease.
Eligibility criteria were a singleton pregnancy without major fetal anomalies or known chromosomal abnormality, gestational age of 28-40 weeks, and reliable gestational dating consisting of a first-trimester crown to rump length or a last menstrual period confirmed (± 10 days) by an ultrasound performed prior to 20 weeks’ gestation. Neonates born at less than 28 weeks of gestation were not included secondary to possible bias because of extreme prematurity and the inability to perform timely testing of the neonatal body composition.
The patients were recruited from a single tertiary care, prenatal diagnosis center. Institutional review board approval was obtained for the study.
Biometry measurements including the biparietal diameter (BPD), head circumference (HC), AC, and femur length (FL) were assessed for each fetus. The EFW was obtained by Hadlock’s formula (log 10 [EFW] = 1.3596 + 0.0064 [HC] + 0.0424 [AC] + 0.174 [FL] + 0.00061 [BPD × AC] – 0.00386 [AC × FL]). The overall EFW percentile was computed according to William’s tables, which are derived from a large population-based study referencing birthweight percentiles by gestational age and sex.
The subjects were assigned to 1 of 3 groups: (1) EFW less than the 10th percentile, (2) EFW greater than the 10th percentile but AC less than the fifth percentile, and (3) EFW greater than the 10th percentile and AC greater than the fifth percentile (normal controls).
Participants in the study were followed up clinically with serial ultrasound assessments of fetal growth for the duration of pregnancy at intervals that varied between 2 and 4 weeks. These ultrasounds were conducted in accordance with the routine management of these high-risk pregnancies. All of the infants in this study analysis had normal umbilical artery Dopplers (S/D ratio) on ultrasound examination based on a referenced gestational age normogram.
The assignment into 1 of the 3 groups was based on the last ultrasound examination prior to delivery, which in no case was more than 3 weeks prior to birth, and the mean time between last ultrasound and delivery was 12 ± 10 days. If abnormal umbilical artery Dopplers developed, this was believed to confirm that a pathologic uteroplacental condition existed and the patient was excluded from analysis.
All deliveries took place at Medical University of South Carolina under the supervision of the Maternal-Fetal Medicine Division. Within 7 days after birth, a neonatal body composition measurement for %BF was performed using air-displacement plethysmography (ADP; PEA POD, Life Measurement, Inc, Concord, CA) ( Figure ). For this assessment, the infant was weighed without clothes and placed in a warm incubator-type chamber. The ADP system assesses a volume measurement to calculate density. Body composition data including weight, body volume, %BF, and lean mass were calculated by the computer system and printed out for each infant.
