Briffa, C; Stirrup, O; Huddy, C; Richards, J; Shetty, S; Reed, K; Khalil, A; (2021) Twin chorionicity-specific population birth-weight charts adjusted for estimated fetal weight. Ultrasound in Obstetrics and Gynecology , 58 (3) pp. 439-449. 10.1002/uog.23606.

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Abstract

OBJECTIVE: To construct chorionicity-specific birthweight reference charts for dichorionic diamniotic (DCDA) and monchorionic diamniotic (MCDA) twin pregnancies incorporating estimated fetal weight (EFW) data in order to adjust for the relationship between suboptimal growth and premature delivery. An additional aim of this study was to determine if the inclusion of complicated twin pregnancies impacts the reference charts produced. METHODS: The Inclusion criteria were twin pregnancy of known chorionicity, known pregnancy outcome, last ultrasound scan within 14 days of birth which took place between 25 and 38 weeks (Analysis A). An analysis was also conducted excluding any pregnancies with complications recorded (Analysis B). The analysis makes use of previously published twin EFW reference ranges. A joint statistical model for EFW and observed birthweight for each pregnancy was created in order to estimate population birthweight reference ranges corresponding to the distribution expected were all pregnancies to deliver at any given gestation. It was not assumed that the median EFW was equal for any given gestation. The models were fitted using a Bayesian approach. RESULTS: We retrieved data on 1664 twin pregnancies, of which 707 DCDA and 241 MCDA pregnancies met the inclusion criteria. The estimate population median birthweight was similar to median EFW around 27 weeks but fell below the EFW values with increasing gestation to 156g lower in both DCDA and MCDA pregnancies at 35 weeks; this finding was confirmed by direct comparison of last EFW and birthweight values in each pregnancy. When the analysis was repeated after excluding the complicated twin pregnancies, there was very little difference between the results obtained when comparing the median EFW across gestations, in Analysis A and those in Analysis B. The largest absolute difference in DCDA twins being a decreased median birthweight of 9g in the Analysis A cohort at 31, 32- and 33-weeks, when compared to Analysis B. The largest absolute difference in MCDA was greater showing an increased median birthweight of 25.3g in the Analysis B cohort at 25 weeks, when compared to Analysis A. CONCLUSION: We established population reference chorionicity-specific birthweight charts for all twin pregnancies, corresponding to the range expected were all pregnancies to deliver at any given gestational age. In this population the median birthweight for twins is consistently lower than singletons and there is a variation in the median birthweight at different gestational ages for chorionicity. This article is protected by copyright. All rights reserved.

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