Assessment of risk for small for gestational age at midgestation to define subsequent care.

Background: Previous studies have demonstrated that a competing risks model for the prediction of small-for-gestational-age neonates has a superior performance than traditional risk scoring methods. The Fetal Medicine Foundation fetal and neonatal population weight charts are derived from sonographic estimated fetal weight rather than birthweight because a large proportion of neonates born before term result from pathologic pregnancy. The individualized risk assessment for small for gestational age at midgestation could be the basis of an antenatal plan that aims to improve the management of preterm pregnancies with small for gestational age with minimum resources.

Objective: This study aimed to stratify subsequent assessments after 24 weeks of gestation based on the estimated risk of delivery of small-for-gestational-age neonates at <28, <32, and <36 weeks of gestation using the combination of maternal risk factors, with estimated fetal weight and uterine artery pulsatility index (triple test) assessed at midgestation. The rationale of the study is that pregnancies at high risk of small for gestational age at <28, <32 and <36 weeks of gestation would require ultrasound examinations at 26, 30, and 33 weeks of gestation, respectively.

Study design: The study cohort was derived from a prospective, nonintervention study in women with singleton pregnancies attending for a routine ultrasound scan between 19 0/7 and 23 6/7 weeks of gestation in 2 United Kingdom maternity hospitals. The competing risks model was used to estimate the individual patient-specific risks of delivery of a small-for-gestational-age neonate at <36 weeks of gestation from the triple test. Different risk cutoffs were used with the intention of detecting 80%, 85%, and 90% of cases of delivery with small for gestational age at <28, <32, and <36 weeks of gestation. Discrimination measures using sensitivities, specificities, and positive and negative predictive values were computed for different risk cutoffs. The calibration of risks of delivery of small for gestational age at <36 weeks of gestation was assessed by plotting the observed incidence of small for gestational age against the predicted incidence.

Results: The study population of 134,443 singleton pregnancies contained 16,813 pregnant women (12.51%) who subsequently delivered small-for-gestational-age neonates with birthweights of <10th percentile, as defined by the Fetal Medicine Foundation chart, including 196 (0.15%), 566 (0.42%), and 1787 (1.33%) pregnant women who delivered at <28, <32, and <36 weeks of gestation, respectively. Using the Fetal Medicine Foundation chart to define small for gestational age, if the objective of screening was to identify approximately 80% of cases of delivery of small-for-gestational-age neonates with birthweights of <10th percentile at <28, <32, and <36 weeks of gestation, the respective screen-positive rates would be 9.5%, 19.6%, and 29.6%, respectively. Using the Fetal Medicine Foundation chart to define small for gestational age, if the objective of screening was to identify approximately 80% of cases of delivery of small-for-gestational-age neonates with birthweights of <3rd percentile at <28, <32, and <36 weeks of gestation, the respective screen-positive rates would be 6.5%, 13.0%, and 21.6%, respectively. The calibration plots demonstrated good agreement between the predicted risk and the observed incidence of small for gestational age.

Conclusion: In addition to a routine scan at 36 weeks of gestation, assessment of the risk of birth of small-for-gestational-age neonates at midgestation is useful to identify the subgroups that require monitoring at 26, 30, and 33 weeks of gestation. The Fetal Medicine Foundation competing risks model for small for gestational age can be customized to the desired detection rate and availability of clinical resources.