ASSESSMENT OF RISK FOR SMALL FOR GESTATIONAL AGE AT MID-GESTATION TO DEFINE SUBSEQUENT CARE.

Abstract

Background: We have previously demonstrated that a competing risks model for prediction of small for gestational age (SGA) neonates, has a superior performance to that of the traditional risk-scoring methods. The Fetal Medicine Foundation (FMF) fetal and neonatal population weight charts are derived from the sonographic estimated fetal weight (EFW), rather than birthweight, because a large proportion of babies born preterm arise from pathological pregnancy. The individualized risk assessment for SGA at mid-gestation could be the basis of an antenatal plan that aims to improve the management of preterm SGA pregnancies with minimum resources.

Objective: To stratify subsequent assessment after 24 weeks' gestation based on the estimated risk for SGA neonates delivering <28, <32 and <36 weeks' gestation by the combination of maternal risk factors with EFW and uterine artery pulsatility index (triple test), assessed at mid-gestation. The rationale of the study is that those at high risk for SGA <28, <32 and <36 weeks would require ultrasound examinations at 26, 30 and 33 weeks' gestation, respectively.

Methods: Our cohort was derived from a prospective, non-intervention study in women with singleton pregnancies attending for a routine ultrasound scan at 19⁺⁰ to 23⁺⁶ weeks' gestation in two UK maternity hospitals. The competing risks model was used to estimate the individual patient-specific risks of delivery with SGA at <36 weeks' gestation from the triple test. Different risk cut-offs were used with the intention of detecting about 80%, 85% and 90% of cases of delivery with SGA at <28, <32 and <36 weeks' gestation. Discrimination measures by means of sensitivity, specificity, positive and negative predictive values were computed for different risk cut-offs. Calibration for risks for delivery with SGA at <36 weeks' gestation was assessed by plotting the observed incidence of SGA against the predicted incidence.

Results: The study population of 134,443 singleton pregnancies contained 16,813 (12.51%) pregnancies that subsequently delivered SGA neonates <10^th percentile, as defined by the FMF chart, including 196 (0.15%), 566 (0.42%), and 1787 (1.33%) that delivered at <28, <32 and <36 weeks, respectively. If the objective of screening was to identify about 80% of cases of delivery of SGA neonates <10^th percentile at <28, <32 and <36 weeks' gestation, using the FMF chart to define SGA, the respective screen positive rates would be 9.5%, 19.6% and 29.6%. If the objective of screening was to identify about 80% of cases of delivery of SGA neonates <3^rd percentile at <28, <32 and <36 weeks' gestation, using the FMF chart to define SGA, the respective screen positive rates would be 6.5%, 13.0% and 21.6%. The calibration plots demonstrated good agreement between the predicted risk and the observed incidence of SGA.

Conclusions: Assessment of risk for birth of SGA neonates at mid-gestation is useful to identify the subgroups that require monitoring at 26, 30 and 33 weeks' gestation, in addition to a routine scan at 36 weeks. The FMF competing risk model for SGA can be customised to the desired detection rate and to the clinical resources availability.