Purpose or Case Report: Previous studies show that phase contrast MRI (PC-MRI) can successfully measure fetal blood flow in late gestation singleton fetuses. To date there is a paucity of data quantifying fetal blood flow of twin or early gestational age pregnancies (~19 weeks). Twin-twin transfusion syndrome (TTTS) is a rare complication of monochorionic pregnancies that is believed to result from inequal flow distribution between fetuses. In this novel investigation, we utilized PC-MRI to investigate fetal aortic flow in early gestational age fetuses with TTTS or selective Intrauterine Growth Restriction (sIUGR). We compared flow distribution between the smaller and larger fetus in each twin pair.
Methods & Materials: We retrospectively examined 11 subjects with twin pregnancies (22 fetuses) at early gestational age (> 18 weeks), 8 with TTTS and 3 with sIUGR. Using an established PC-MRI protocol (Ingenia 1.5T) fetal blood flow was quantified in the descending aorta at the diaphragm (DAo) for each fetus. The DAo flow measurement for each fetus (mL/min) was indexed to the fetal weight (mL/min/kg) using the ultrasound estimated weight of each fetus. Comparison between the smaller and larger fetus in each twin pair was performed with Ttest comparison of the indexed DAo flow (p<0.05 significant).
Results: Examined pregnancies had a mean GA of 19.43 weeks. 3 subjects had sIUGR and 8 had TTTS. Of those with TTTS, the majority (n=6) had advanced TTTS defined by Quintero stages 3 or 4. Among those with sIUGR, growth discordance ranged from 4% - 41%. There was no significant difference in the indexed DAo net flow in the TTTS twin pairs (Mean donor vs recipient: 387 mL/min/kg vs 403 mL/min/kg, p: 0.80) nor in the sIUGR twin pairs. Six TTTS patients underwent selective fetoscopic laser photocoagulation. Of these, two experienced donor demise. Those who did not receive intervention returned to their referring institutions and outcomes are unknown.
Conclusions: This study measures fetal blood flow in early gestational age twin pregnancies using PC-MRI, a technique that to date has only been measured in late gestational age singleton pregnancies. Flow measurements obtained in this study provide insight into the flow distribution between twins in complicated twin pregnancies. Although a small sample size, we found that there was proportional DAo indexed flow distribution between twins in the same pregnancy. This suggests that other factors drive fetal size discrepancies in TTTS and sIUGR, not purely inequal flow distribution.