Detection and clinical outcome of small‐for‐gestational‐age fetuses in the third trimester—A comparison between routine ultrasound examination and examination on indication

INTRODUCTION: Fetal growth restriction is a major risk factor for adverse perinatal outcome. As most of the growth‐restricted fetuses are small for gestational age (SGA), an efficient antenatal screening method for SGA fetuses would have a major impact on perinatal health. The aim of this study was to compare the SGA prediction rate achieved with third‐trimester routine ultrasound estimation of fetal weight (EFW) with that obtained using ultrasound examination on indication. The secondary aim was to evaluate the clinical outcome in relation to the SGA screening method. MATERIAL AND METHODS: During 1995–2009, two perinatal centers in southern Sweden offered routine ultrasound examination at 32–34 gestational weeks to 99 265 women with singleton pregnancies. Of these, 59 452 (60%) underwent the ultrasound examination. The other population, comprising 24 868 pregnancies, was cared for in another three centers that used a risk‐based method with ultrasound examinations on indication only. Of them, 5792 (23%) underwent ultrasound examination at 32–36 gestational weeks. The deviation in the EFW from the expected one was expressed as the EFW z‐score, SGA EFW being defined as the EFW z‐score less than −2. SGA prediction ability was assessed by receiver operating characteristic (ROC) curves. Crude and adjusted risk ratios were calculated for selected variables of perinatal outcome when comparing the populations. RESULTS: The SGA prediction ability for routine ultrasound was high, area under the ROC curve was 0.90 (95% CI 0.89–0.91). For an EFW z‐score of −1, the sensitivity was 67.3% and specificity was 90.5% among routinely screened pregnancies; corresponding numbers in the ultrasound on indication population were 34.3% and 96.6%. The screened population had a lower risk of preterm birth, birthweight z‐score less than −3, and Apgar score less than 7 at 5 min with adjusted risk ratios 0.87 (95% CI 0.82–0.92), 0.75 (95% CI 0.61–0.92), and 0.77 (95% CI 0.68–0.87), respectively. No difference in perinatal mortality was detected. There were no differences in perinatal outcome between the two subcohorts of infants born SGA. CONCLUSIONS: Third‐trimester routine ultrasound improves the detection of SGA antenatally compared with ultrasound performed on indication, but no convincing improvement in perinatal outcome was identified.