Handheld Ultrasound for Assessing Fetal Size
By John C. Hobbins, MD
Professor, Department of Obstetrics and Gynecology, University of Colorado School of Medicine, Aurora
Dr. Hobbins reports no financial relationships relevant to this field of study.
Synopsis: A recent study has shown abdominal circumference assessments with a portable handheld ultrasound machine to be superior to standard fundal height measurements in the prediction of small for gestational age and large for gestational age fetuses in utero and infants at birth.
Source: Haragan AF, et al. Diagnostic accuracy of fundal height and handheld ultrasound-measured abdominal circumference to screen for fetal growth abnormalities. Am J Obstet Gynecol 2015;212:820-822.
Fundal height measurements for assessment of fetal growth have been the staple of providers seemingly forever, and there probably is not an obstetrical examining room in the country that does not have a paper tape measure within arm’s length of the exam table. Now, there may be another clinical tool housed in the same area: a new portable handheld ultrasound device.
A team from the University of South Carolina pitted the tape measure against this mini-ultrasound instrument on 251 patients who were between 21 and 40 weeks’ gestation. The idea was to see which method was a better predictor of aberrant fetal growth. Each patient studied had a fundal height (FH) measurement by an experienced provider. Any measurement of > 3 cm above gestational age (in cm per week) was considered large for gestational age (LGA). If the measurement was < 3 cm from gestational age, it was labeled as small for gestational age (SGA). In the same patients, a handheld ultrasound device was used by another provider to assess the abdominal circumference (HHAC). LGA was defined as an abdominal circumference > 95th percentile and SGA was < 10th percentile. All patients then had an estimated fetal weight (EFW) later by a registered sonographer using standard equipment and a commonly used formula. At birth, an infant weighing > 90th percentile for gestational age and gender was considered LGA and one weighing < 10th percentile was SGA.
The results showed a high correlation (R = 0.939, P = < 0.001) between the HHAC and the EFW and abdominal circumference by the formal ultrasound examination. Seven fetuses (2.8%) were defined as LGA and seven fetuses (2.8%) were SGA by the formal ultrasound EFW. At the time of birth, 9.56% were LGA and 10.7% were SGA. Sensitivities for in utero SGA (by EFW) were 100% and 42.8% for HHAC and FH, respectively. Specificities were 85.2% vs 92.6%. For in utero (EFW) LGA, sensitivities were 57.1% vs 71.4% for the HHAC and FH. Regarding each technique’s ability to screen for SGA at birth, the sensitivities for HHAC, FH, and EFW were 74%, 37%, and 21.4%, respectively. In the same order, specificities for SGA at birth were 89.7%, 95%, and 99.5%, respectively. For predicting LGA babies at birth, the HHAC had a sensitivity and specificity of 66.7% and 89.9%, compared with FH of 50% and 66.9%, respectively.
In short, the HHAC had the highest sensitivities of all methods when it came to screening for SGA in utero (low EFW) or at birth, but it did so at the highest false-positive rate of 15% and 11%, respectively. The FH picked up only 42.8% of SGAs in utero and 37% at birth. The huge surprise for me was how poorly the ultrasound EFW performed with screening sensitivities at birth for SGA of 21.4% and LGA of 25%.
Fundal height indirectly reflects everything in the uterus, while abdominal circumference goes right to a portion of the fetus that directly relates to how scrawny or corpulent he or she is. In fact, in fetuses that are labeled as SGA by EFW, I am always more interested in the abdominal circumference, which is always small (< the 5th percentile) in true SGA or large (> 95th percentile) in true LGA. The EFW formula can be skewed by a fetus with a head size or femur length that is well outside the mean for gestation — neither reflecting in most cases the nutritional status of the fetus. Also, the biparietal diameter and head circumference, two of the four components of the common formula used to calculate EFW, are often subject to error because of compression due to the fetal position or, late in pregnancy, the head being deep in the pelvis.
This study certainly makes the case for the use of small modern-day devices to screen for fetal size, and at the same time it could check amniotic fluid volume, monitor fetal behavior through biophysical profile, and quickly assess fetal heart rates, etc. This could even become the modern-day obstetrical provider’s stethoscope. (Remember that prop that we now dust off occasionally to hang around our necks for PR pictures?) However, before jumping on this bandwagon, we have to think about one important item: cost.
These gadgets, marketed mostly to emergency department physicians, come in various sizes and some sell for < $5000. Online, I found one in particular that blew me away. The ad featured a small cordless transducer that talks to an iPad. With corporate competition, the cost of these ultrasound mini-machines may even drop down to a point where it would be affordable as a pocket accessory, coordinated to the color of every provider’s scrub suit — the preferred clothing option of todays’ physicians.
For the time being, the paper tape measure still is useful, especially if this is in the hands of an experienced provider who can use this information in conjunction with clinical acumen. More than anything, however, this study shows how useful the abdominal circumference alone is in assessing fetal size.
A recent study has shown abdominal circumference assessments with a portable handheld ultrasound machine to be superior to standard fundal height measurements in the prediction of small for gestational age and large for gestational age fetuses in utero and infants at birth.
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