Risk Stratification of Foetuses based on Estimated Foetal Weight less than the 40th Percentile and Evaluation of Perinatal Outcomes using Third-trimester Obstetric Ultrasound: A Prospective Cohort Study

Introduction: Perinatal mortality and morbidities constitute a burden on society and healthcare system. One of the major identifiable causes of these conditions in India is Low Birth Weight (LBW) and preterm births. Small for Gestational Age (SGA) infants account for almost 46.9% of LBW infants. They are prone to the morbidities of preterm birth and are the second major contributor to perinatal mortality. Recent studies have reported that higher screening performance for SGA can be achieved through ultrasonographic foetal biometry and Doppler studies during the third trimester. Aim: To stratify foetuses with Estimated Foetal Weight (EFW) below the 40th centile, as determined by obstetric ultrasound, into three categories (low, intermediate, and high-risk), and study the perinatal outcomes in each category. Materials and Methods: The present prospective cohort study was conducted in the Department of Obstetrics and Gynaecology at Vardhaman Mahavir Medical College and Safdarjung Hospital, New Delhi, India from November 2020 to April 2022. A total of 280 antenatal women, at a gestational period between 35 to 36+6 weeks with singleton pregnancies, were studied and based on the foetal biometry and Doppler parameters, women were classified into low (Group A), intermediate (Group B) and high risk (Group C) categories. Risk categorisation was based on EFW and Doppler indices. The low-risk group underwent an ultrasound Doppler scan after four weeks, the intermediate-risk group after two weeks, and the high-risk group was scanned weekly. The high-risk group was delivered at 39 weeks of gestation. Perinatal outcomes, neonatal weights, adverse neonatal outcomes {such as Neonatal Intensive Care Unit (NICU) admissions exceeding 48 hours}, stillbirths, and neonatal deaths were recorded. The Area under Curve (AUC) cut-off for EFW percentiles on ultrasonography was calculated as a predictor for SGA foetuses requiring NICU admissions. Concurrent admissions in the NICU of neonates delivered to unregistered pregnant mothers, who were not screened during the antenatal period, were also recorded. The Chi-square test was used to compare categorical data between groups. The receiver operating characteristic curve was used to determine the cut-off for foetal weight to predict NICU admission. A p-value of <0.05 was considered statistically significant. Results: Among the study population, 231 (82.6%) were between 21-30 years of age, and 250 (88.87%) had a normal Body Mass Index (BMI) with a mean age of 25.78±3.90 years. On risk categorisation, 71 (25.1%), 82 (29%), and 127 (45.9%) women were in the high, intermediate, and low-risk categories, respectively. All the neonates in the high-risk group were classified as SGA according to the INTERGROWTH 21st growth chart. Among the neonates, 18 (6.4%) weighed below 2100 g, 35 (12.4%) weighed between 2101 to 2200 g, 62 (22.3%) weighed between 2201 to 2400 g, and 165 (59%) weighed above 2401 g. NICU admissions occurred in 25 (35.2%), 10 (12%), and 5 (3.8%) neonates in the high, intermediate, and low-risk groups, respectively (p-value < 0.001). At a cut-off of EFW (g) less than or equal to 2122 g (between the 10th and 20th centile) during the first ultrasound, it predicted SGA neonates requiring NICU admissions with a sensitivity of 80% and a specificity of 70%. No stillbirths or neonatal mortality occurred in the study group. Conclusion: A single third-trimester obstetric ultrasound, along with Doppler measurements, should be performed at 35 to 36 weeks as an important adjunct for identifying and stratifying the risk of singleton foetuses. Close monitoring and timely delivery can help reduce adverse perinatal outcomes in SGA foetuses