Isolated fetal umbilical vein varix and the association with intrauterine fetal death and fetal growth restriction: A systematic review, meta-analysis, and nested retrospective cohort study

Objectives: To assess the risk of intrauterine fetal death (IUFD) and fetal growth restriction (FGR) in fetuses with an isolated fetal intra-abdominal umbilical vein varix (i-FIUVV). Methods: A retrospective cohort study combined with a systematic review and meta-analysis of the literature was performed. In the retrospective cohort study, all singleton fetuses with an i-FIUVV in the fetal medicine units of the Amsterdam UMC (between 2007 and 2023) were analyzed. The primary outcome measures were IUFD and FGR. The sample proportions of IUFD and FGR were depicted as risk percentages. The IUFD proportion was compared to the regional reference population and the FGR proportion was compared to the reported proportions in Europe. The secondary outcome measures were gestational age at diagnosis, initial and maximal FIUVV diameter, fetal monitoring in pregnancy, turbulent flow in the varix, thrombus formation in the varix, induction of labor, gestational age at birth, and birthweight centile. The proportion of fetuses with a birthweight below the 10^th centile was compared with that of the regional reference population. The systematic review included all cases from eligible literature published between 2007 and 2023 supplemented by the data of our retrospective cohort study. In the systematic review and meta-analysis, the pooled proportions of IUFD and FGR were assessed in fetuses with i-FIUVV. Results: The retrospective cohort included 43 singletons with an i-FIUVV. The IUFD risk was 0% [Confidence Interval, CI: 0%–8.2%], which did not differ significantly from 0.3% in the reference population, p = 1.0. The risk of FGR was 16.3% [CI: 6.8%–30.7%] in the studied population, which is higher than the reported incidence of FGR in Europe ranging from 5%–10%. The proportion of fetuses with birthweights below the 10^th centile was higher in our cohort compared with the reference population (23.3 vs. 9.9%, p < 0.01). The systematic review included 12 articles, three abstracts, and our current cohort. In total, 513 cases with an i-FIUVV were included. The pooled risk was 0.4% [CI: 0.1%–1.7%] for IUFD and 5.2% [CI: 1.1%–21.3%] for FGR. The mean gestational age at birth did not exceed 39 weeks in neither the cohort (38.7 weeks) nor the pooled literature (37.6 weeks). Conclusion: An i-FIUVV in singletons is not associated with an increased IUFD risk up to 39 weeks of gestation but is possibly associated with FGR. The incidence of FGR in our cohort was higher than in the pooled literature (16.3% vs. 5%) but FGR definitions in the included studies varied. The proportion of birthweights below the 10^th percentile in our cohort was significantly higher than in the reference group. Thus, based on these findings, we suggest conducting sonographic growth assessments while simultaneously assessing the i-FIUVV. No further monitoring and follow-up are indicated up to 39 weeks of gestation. After 39 weeks of gestation, data on fetuses with i-FIUVV and their outcomes are lacking.