The hemodynamics of late-onset intrauterine growth restriction by MRI.

Background: Late-onset intrauterine growth restriction (IUGR) results from a failure of the placenta to supply adequate nutrients and oxygen to the rapidly growing late-gestation fetus. Limitations in current monitoring methods present the need for additional techniques for more accurate diagnosis of IUGR in utero. New magnetic resonance imaging (MRI) technology now provides a noninvasive technique for fetal hemodynamic assessment, which could provide additional information over conventional Doppler methods.

Objective: The objective of the study was to use new MRI techniques to measure hemodynamic parameters and brain growth in late-onset IUGR fetuses.

Study Design: This was a prospective observational case control study to compare the flow and T2 of blood in the major fetal vessels and brain imaging findings using MRI. Indexed fetal oxygen delivery and consumption were calculated. Middle cerebral artery and umbilical artery pulsatility indexes and cerebroplacental ratio were acquired using ultrasound. A score of ≥ 2 of the 4 following parameters defined IUGR: (1) birthweight the third centile or less or 20% or greater drop in the centile in estimated fetal weight; (2) lowest cerebroplacental ratio after 30 weeks less than the fifth centile; (3) ponderal index < 2.2; and (4) placental histology meets predefined criteria for placental underperfusion. Measurements were compared between the 2 groups (Student t test) and correlations between parameters were analyzed (Pearson's correlation). MRI measurements were compared with Doppler parameters for identifying IUGR defined by postnatal criteria (birthweight, placental histology, ponderal index) using receiver-operating characteristic curves.

Results: We studied 14 IUGR and 26 non-IUGR fetuses at 35 weeks' gestation. IUGR fetuses had lower umbilical vein (P = .004) and pulmonary blood flow (P = .01) and higher superior vena caval flow (P < .0001) by MRI. IUGR fetuses had asymmetric growth but smaller brains than normal fetuses (P < .0001). Newborns with IUGR also had smaller brains with otherwise essentially normal findings on MRI. Vessel T2s, oxygen delivery, oxygen consumption, middle cerebral artery pulsatility index, and cerebroplacental ratio were all significantly lower in IUGR fetuses, whereas there was no significant difference in umbilical artery pulsatility index. IUGR score correlated positively with superior vena caval flow and inversely with oxygen delivery, oxygen consumption, umbilical vein T2, and cerebroplacental ratio. Receiver-operating characteristic curves revealed equivalent performance of MRI and Doppler techniques in identifying IUGR that was defined based on postnatal parameters with superior vena caval flow area under the curve of 0.94 (95% confidence interval, 0.87-1.00) vs a cerebroplacental ratio area under the curve of 0.80 (95% confidence interval, 0.64-0.97).

Conclusion: MRI revealed the expected circulatory redistribution in response to hypoxia in IUGR fetuses. The reduced oxygen delivery in IUGR fetuses indicated impaired placental oxygen transport, whereas reduced oxygen consumption presumably reflected metabolic adaptation to diminished substrate delivery, resulting in slower fetal growth. Despite brain sparing, placental insufficiency limits fetal brain growth. Superior vena caval flow and umbilical vein T2 by MRI may be useful new markers of late-onset IUGR.