Positive predictive value of suspected growth aberration in twin gestations

OBJECTIVE: Our purpose was to determine the positive predictive value of ul trasonographic surveillance for growth abnormalities in twin gestations as a function of gestational age. STUDY DESIGN: Women with twin gestations and delivery between January 1992 and March 1998 who had a 20- to 24-week sonogram with normal fetal anatomic findings and who had at least 1 sonogram showing abnormal growth were iden tified. Abnormal growth on ultrasonography was defined as an estimated feta l weight <10th percentile, abdominal circumference <5th percentile, or twin discordance (>20% difference in twin weights as a function of the heavier twin). Birth weights were then assessed for evidence of twin discordance or growth restriction. RESULTS: The positive predictive value for the occurrence of a growth abnor mality at birth, after an abnormal growth finding on ultrasonography at any time during gestation, was 47.7%. The positive predictive value was greate st (85%) when suspected growth restriction was first documented at 20 to 24 weeks of gestation and decreased with increasing gestational age. Even tho ugh sonograms were obtained at a mean interval of 4.4 +/- 2.0 weeks, those gestations with normal growth at 20 to 24 weeks had an elapsed time of 10.3 +/- 3.9 weeks until a growth abnormality was subsequently detected. CONCLUSION: In twin gestations the positive predictive value of a sonogram for a growth abnormality at birth is significantly decreased after normal f indings on a 20- to 24-week sonogram. This finding suggests that a routine 2- to 4-week interval between sonograms for all twin gestations may be unwa rranted.