Objective: To determine the influence of the interval between fetal measurements on performance of fetal growth velocity for predicting infants with anthropometric features of fetal growth restriction (FGR).
Methods: Two hundred seventy-four low-risk women had serial fetal biometry at scheduled intervals. Growth velocity of the fetal abdominal area for each was calculated with 2-, 4-, and 6-week scan intervals in which the second measurement was the last scan before delivery. Fetal abdominal area velocity over a 4-week interval in the early third trimester also was included. Fetal growth restriction was defined as skinfold thickness under the tenth percentile, ponderal index under the 25th percentile, midarm circumference–to–occipitofrontal circumference ratio of under 21 standard deviation (SD). Test performance was expressed as likelihood ratios with 95% confidence intervals (CI).
Results: Fetal abdominal area velocity calculated over a 4-week interval predicted FGR with a likelihood ratio of 10.4 (95% CI 3.9, 26) for skinfold thickness; 9.5 (95% CI 4.6, 19) for ponderal index; and 4.7 (2.3, 8.4) for midarm circumference–to–occipitofrontal circumference ratio. Intermeasurement intervals of 6 weeks had a likelihood ratio of 8.5 (95% CI 4, 17) for skinfold thickness; 7.5 (95% CI 3.4, 16.1) for ponderal index; and 14 (6.7, 28) for midarm circumference–to–occipitofrontal circumference ratio. The likelihood ratios for the 2-week interval and the early third trimester 4-week interval were all less than 5.
Conclusion: Four- and 6-week measurement intervals were useful for predicting infants with FGR and were superior to a 2-week interval. Fetal growth velocity is influenced by proximity of the last fetal measurement to date of delivery, which adversely affects clinical use of growth velocity for predicting FGR.
Reliable antenatal identification of growth-restricted infants at risk of adverse outcomes might be expected to improve allocation of monitoring resources, with the possibility of improving perinatal outcomes. Growth velocity standards quantify fetal growth from two ultrasound measurements1 provided the interval between measurements and gestational age at second measurement are known. Fetal growth velocity has been described as potentially valuable for antenatal prediction of growth-restricted infants and intrapartum heart rate abnormalities.2,3
The distinction between growth-restricted and constitutionally small infants is well recognized, as is the limited importance of birth weight as an indicator of fetal growth achievement and perinatal outcome.4 Abnormalities of neonatal body constitution appear to be more useful indicators of adverse short- and long-term outcomes than birth weight alone.5–7 Yet, only a few studies have used neonatal anthropometric criteria to diagnose fetal growth restriction (FGR).2,8
Using absence of an increment in fetal abdominal circumference measurement as their definition of FGR, Mongelli et al9 found that shortening of intermeasurement interval resulted in increased false-positive diagnosis rates for FGR. However, that analysis was limited because of the lack of an appropriate standard for the confirmation of FGR. Against that background we decided to investigate the influence of between-measurement interval on diagnostic performance of fetal growth velocity for predicting three different neonatal anthropometric criteria of FGR.