Fetal Medicine Foundation charts for fetal growth in twins

Predicting Adverse Perinatal Outcomes in Dichorionic Twin Pregnancies: A Multicentre Cohort Study

OBJECTIVE

Dichorionic twin pregnancies are associated with increased risks of stillbirth or medically indicated early preterm birth (ePTB) to avoid stillbirths. This study evaluated the predictive value of fetal estimated weight (EFW) and Doppler indices before adverse perinatal outcomes.

DESIGN

Retrospective multicentre cohort study.

SETTING

Three tertiary centres in the UK, Italy and Belgium.

POPULATION

The study included 1294 dichorionic twin pregnancies managed between 2013 and 2023.

METHODS

Univariable and multivariable analyses assessed the association and the predictive accuracy between EFW and Doppler indices taken within 2 weeks of birth or adverse perinatal outcomes.

MAIN OUTCOME MEASURES

Stillbirths (of one or both twins) or medically indicated ePTB before 34 weeks' gestation for fetal indications.

RESULTS

The study identified 58 pregnancies (4.5%) complicated by adverse perinatal outcomes. There were significant differences (all p < 0.001) between twins with adverse perinatal outcomes and liveborn twins for small for gestational age foetuses (89.5% vs. 59.3%), EFW discordance (31.8% vs. 8.4%), umbilical artery (UA) pulsatility index (PI) discordance (39.7% vs. 12.6%) and middle cerebral artery PI discordance (27.6% vs. 13.3%). These associations remained significant after adjusting for maternal characteristics and gestational age. The best predictive model included EFW discordance and UA PI discordance, with an area under the curve of 0.90.

CONCLUSIONS

The integration of intertwin EFW and UA PI discordance can effectively predict stillbirths or the need for medically indicated ePTB. After external validation in larger populations, this model could provide effective risk stratification of dichorionic pregnancies to enable targeted interventions to improve clinical outcomes.

Fetal biometry reference ranges derived from prospective twin population and evaluation of adverse perinatal outcome

OBJECTIVES

Ultrasound-derived estimates of fetal size play an integral role in the prenatal management of twin pregnancy. These biometric measurements are conventionally plotted against singleton standards. We sought to establish fetal growth references for abdominal circumference, head circumference, biparietal diameter, femur diaphysis length and estimated fetal weight (EFW) in twin pregnancy. We also aimed to determine whether the performance of a twin fetal growth reference was superior to a singleton reference in the prediction of adverse perinatal outcome in twin pregnancies.

METHODS

This was a retrospective analysis of data collected prospectively in the Evaluation of Sonographic Predictors of Restricted growth in Twins (ESPRiT) study, which was conducted at eight academic perinatal centers in Ireland, all with tertiary neonatal intensive care facilities. Only diamniotic twin pregnancies with two live fetuses were eligible for inclusion. Exclusion criteria were monoamnionicity, congenital abnormality, twin-to-twin transfusion syndrome or previable fetal demise (< 24 weeks' gestation). Using serial ultrasound observations, we applied fractional polynomial multilevel models to derive an equation for fetal centile determination. We compared these centiles with published singleton and twin fetal references, with particular focus on the Fetal Medicine Foundation (FMF) references. Using the last ultrasound examinations before delivery, we determined associations between biometric measures and a composite measure of adverse perinatal outcome (intraventricular hemorrhage, periventricular leukomalacia, hypoxic ischemic encephalopathy, necrotizing enterocolitis, bronchopulmonary dysplasia, sepsis or perinatal death), neonatal intensive care unit admission, preterm delivery (< 34 weeks) and birth-weight discordance ≥ 25%, based on the varied prevalence of these outcomes. We compared our results with the singleton and twin FMF reference ranges and the twin reference of the Southwest Thames Obstetric Research Collaborative (STORK) study.

RESULTS

Among the 948 twin pairs that met the inclusion criteria, 776 (81.9%) dichorionic and 172 (18.1%) monochorionic twin pairs completed the prospective 2-weekly ultrasound surveillance program. Fetal biometric measurements were obtained in 15 274 ultrasound assessments (12 279 in dichorionic and 2995 in monochorionic twin pairs) from serial ultrasound assessments. The median number of ultrasound assessments per pregnancy was 8 (interquartile range, 7-9). Growth trajectories in this cohort were consistent with the FMF and STORK published twin cohorts and notably less consistent with the FMF singleton standard. Compared with the FMF singleton standards, the 50th centiles for twins were greater early in pregnancy and lower later in pregnancy for all biometric measures, in both dichorionic and monochorionic twin pregnancies. This crossover in growth occurred at approximately 28 weeks' gestation for dichorionic twins and earlier for monochorionic twins. The 50th centiles for EFW were comparable to the FMF twin standards for both monochorionic and dichorionic twins, but with lower 10th centiles for dichorionic twins in the third trimester. The current (ESPRiT) twin reference ranges, the STORK twin reference ranges and the FMF twin reference ranges showed larger and statistically significant (P < 0.01) odds ratios for multiple biometric measures and multiple adverse perinatal outcomes, for both monochorionic and dichorionic twins, not observed with the FMF singleton reference standard.

CONCLUSIONS

In this analysis of data from the prospective ESPRiT cohort study, we confirm significant differences between twin fetal growth patterns and singleton standards, consistent with previous studies. Our results also offer some validation of the new FMF reference for EFW in twins. The outcome-based evidence from this study suggests that a twin-specific growth reference should be used in preference to a singleton chart for fetal growth evaluation in twin pregnancy. © 2025 The Author(s). Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology.

Validation of Fetal Medicine Foundation charts for fetal growth in twins: nationwide Danish cohort study

OBJECTIVE

To assess the validity of the Fetal Medicine Foundation (FMF) chorionicity-specific models for fetal growth in twin pregnancy.

METHODS

This was an external validation study of the FMF models using a nationwide Danish cohort of twin pregnancies. The cohort included all dichorionic (DC) and monochorionic diamniotic (MCDA) twin pregnancies with an estimated delivery date between 2008 and 2018, which satisfied the following inclusion criteria: two live fetuses at the first-trimester ultrasound scan (11-14 weeks' gestation); biometric measurements available for the calculation of estimated fetal weight (EFW) using the Hadlock-3 formula; and delivery of two liveborn infants. Validation involved assessing the distributional properties of the models and estimating the mean EFW Z-score deviations. Additionally, the models were applied to pregnancies that delivered preterm and attended non-scheduled visits (complicated pregnancies).

RESULTS

Overall, 8542 DC and 1675 MCDA twin pregnancies met the inclusion criteria. In DC twins, 17 084 fetuses were evaluated at a total of 95 346 ultrasound scans, of which 44.5% were performed at scheduled visits in pregnancies carried to 37 + 0 weeks or later. The median number of growth scans per DC twin fetus from 20 + 0 weeks onwards was four. The model showed good agreement with the validation cohort for scheduled visits in DC twins delivered at 37 + 0 weeks or later (mean ± SD EFW Z-score, -0.14 ± 1.05). In MCDA twins, 3350 fetuses underwent 31 632 eligible ultrasound scans, of which 59.5% were performed at scheduled visits in pregnancies carried to 36 + 0 weeks or later. The median number of growth scans per MCDA twin fetus from 16 + 0 weeks onwards was 10. The model showed favorable agreement with the validation cohort for scheduled visits in MCDA twins delivered at 36 + 0 weeks or later (mean ± SD EFW Z-score, -0.09 ± 1.01). Non-scheduled visits and preterm delivery before 37 + 0 weeks for DC twins and before 36 + 0 weeks for MCDA twins corresponded with smaller weight estimates, which was consistent with the study's definition of complicated pregnancy.

CONCLUSIONS

The FMF models provide a good fit for EFW measurements in our Danish national cohort of uncomplicated twin pregnancies assessed at routine scans. Therefore, the FMF models establish robust criteria for subsequent investigations and potential clinical applications. Future research should focus on exploring the consequences of clinical implementation, particularly regarding the identification of twins that are small-for-gestational age, as they are especially susceptible to adverse perinatal outcome. © 2024 The Author(s). Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology.

Association of fetal growth restriction and stillbirth in twin compared with singleton pregnancies

Linked article: This Correspondence comments on Martínez‐Varea et al. Click here to view the article.

Incidence of neonatal morbidity in small-for-gestational-age twins based on singleton and twin charts

OBJECTIVE

To compare morbidity, as measured by length of stay in the neonatal intensive care unit (NICU), in twin and singleton gestations classified as small-for-gestational age (SGA) according to estimated fetal weight < 10th percentile on twin or singleton growth charts.

METHODS

NICU length of stay was compared in 1150 twins and 29 035 singletons that underwent ultrasound assessment between 35 + 0 and 36 + 6 weeks' gestation. Estimated fetal weight was obtained from measurements of head circumference, abdominal circumference and femur length using the Hadlock formula. Gestational age was derived from the first-trimester crown-rump length measurement, using the larger of the two twins. Singletons and twins were compared in terms of NICU admission rate and length of stay according to classification as SGA by the Fetal Medicine Foundation singleton and twin reference distributions.

RESULTS

The overall proportions of twins and singletons admitted to NICU were similar (7.3% vs 7.4%), but twins tended to have longer lengths of stay in NICU (≥ 7 days: 2.4% vs 0.8%; relative risk (RR), 3.0 (95% CI, 1.6-4.4)). Using the singleton chart, a higher proportion of twins were classified as SGA compared with singletons (37.6% vs 7.0%). However, the proportion of SGA neonates entering NICU was similar (10.2% for twins and 10.1% for singletons) and the proportion of SGA neonates spending ≥ 7 days in NICU was substantially higher for twins compared with singletons (3.7% vs 1.4%; RR, 2.6 (95% CI, 1.4-4.7)).

CONCLUSIONS

When singleton charts are used to define SGA in twins and in singletons, there is a greater degree of growth-related neonatal morbidity amongst SGA twins compared with SGA singletons. Consequently, singleton charts do not inappropriately overdiagnose fetal growth restriction in twins and they should be used for monitoring fetal growth in both twins and singletons. © 2023 International Society of Ultrasound in Obstetrics and Gynecology.