Small Fetal Abdominal Circumference Associated with Delivery of Small-for-Gestational Age Neonates
Small Fetal Abdominal Circumference Associated with Delivery of Small-for-Gestational Age Neonates
Abstract & Commentary
By John C. Hobbins, MD, Professor and Chief of Obstetrics, University of Colorado Health Sciences Center, Denver, is Associate Editor for OB/GYN Clinical Alert.
Dr. Hobbins reports no financial relationship to this field of study.
Synopsis: Low blood sugars and small abdominal circumference are often associated with small-for-date babies.
Source: Beinstock JL, et al. Small fetal abdominal circumference in the second trimester and subsequent low maternal plasma glucose after a glucose challenge test is associated with the delivery of a small for gestational age neonate. Ultrasound Obstet Gynecol. 2008;31:517-519.
Often there are articles in the ultrasound literature that have diagnostic implications extending far past the ultrasound findings that were the centerpieces of these papers. One such article recently appeared in the Ultrasound in Obstetrics and Gynecology Journal. The authors conducted a simple retrospective study to correlate two variables—the fetal abdominal circumference (AC) and a one-hour glucose screen—with the incidence of small-for-gestational age (SGA) neonates.1
Data were abstracted from the records of 576 consecutive pregnancies, which included the results from one-hour glucose challenge tests, ACs obtained during routine ultrasound examinations between 18 and 22 weeks, and birth weights. The study was so simple one wonders why this had not been done before, especially since the information obtained from the study was well worth the rather uncomplicated effort in extracting the data.
One-hour 50 g screens were done on every patient between 24 and 30 weeks of gestation and the ACs were obtained as part of a standard second trimester ultrasound evaluation between 18 and 22 weeks. SGA was defined as an infant weight of <10th percentile and a low one-hour glucose value was considered to be below 100 mg %.
Fifty out of 576 patients delivered SGA infants (8.7%). Those with low glucose values had an incidence of an SGA of 13%, vs those with normal glucose values, 6.25% of whom delivered SGA infants (p = 0.005). Those patients with fetal ACs below the 10th percentile had an incidence of SGA of 17%, vs 8% in those with adequate sized ACs (p = 0.018). In those with small fetal ACs and low glucoses (19 patients), 32% delivered SGA infants, vs 0% in those with small ACs and glucose levels above 100 mg% (p= 0.018). If the fetus had a small AC, 100% of mothers eventually delivering infants with weights below the 10th percentile had low glucoses. Also, 100% of those patients with normal glucoses and normal fetal ACs delivered neonates who were of appropriate size for gestation.
Commentary
Tons of studies have addressed the relationship of elevated blood glucose levels and fetal macrosomia, but very few studies have dealt with the relationship between low blood sugar and fetal size. Similarly, many studies have correlated fetal AC with macrosomia and/ or diabetes, but less attention has been directed to AC measurements in the second trimester as predictors of later SGA, despite its obvious potential.
In later pregnancies, many of our colleagues outside the U.S. have used the AC alone as an indicator of SGA, rather than using estimated fetal weight, the formula that takes into account the size of the fetal head and femurs, in addition to the AC. The size of the head is related to genetic tendencies, as is the size of the fetal femur, and, in most cases, neither has much to do with mild to moderate fetal deprivation. The head is rarely affected in a major way in the common varieties of SGA, and the fetal femur, although falling off the growth curve early in pregnancy in severe fetal deprivation, generally is affected little in the more common type of placentally mediated SGA. On the other hand, the AC incorporates the size of the fetal liver and the amount of subcutaneous fat deposition, both of which are diminished in growth restriction. Interestingly, on the opposite end of the scale, the AC is the biometric parameter affected most by maternal glucose intolerance and plain old macrosomia.
So, when reviewing the results of a second trimester ultrasound it is important to look at the AC percentile, and not just the estimated fetal weight. If it is small, then particular attention should be paid to the results of the one-hour glucose screen to follow, since the likelihood of SGA is very high if the results of both are in the low range. This, in turn, should trigger serial fetal growth assessments to identify the fetus with true growth restriction. The importance of identifying SGA fetuses in utero has been highlighted by Lindqvist in 2005, who demonstrated that neonatal morbidity and mortality rates were fourfold higher in SGA fetuses who were not diagnosed in utero, compared with those who were (with ultrasound).2
References
- Beinstock JL, et al. Small fetal abdominal circumference in the second trimester and subsequent low maternal plasma glucose after a glucose challenge test is associated with the delivery of a small for gestational age neonate. Ultrasound Obstet Gynecol. 2008;31:517-519.
- Lindqvist PJ, Molin J. Does antinatal identification of small for gestational age fetuses significantly improve their outcome? Ultrasound Obstet Gynecol. 2005; 25:258-264.
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