Interval between fetal measurements in predicting growth restriction.

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 -1 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.