Feasibility of MRI assessment of maternal-fetal oxygen transport and consumption relative to maternal position in healthy late gestational pregnancies

Abstract

Late gestational supine positioning reduces maternal cardiac output due to inferior vena caval (IVC) compression, despite increased collateral venous return. However, little is known about the impact of maternal position on oxygen (O₂ ) delivery and consumption of the gravid uterus, fetus, placenta and lower limbs. We studied the effects of maternal positioning on these parameters in 20 healthy pregnant subjects at 36 ± 2 weeks using magnetic resonance imaging (MRI); a follow-up MRI was performed 6-months postpartum (n = 16/20). MRI techniques included phase-contrast and T1/T2 relaxometry for blood flow and oximetry imaging, respectively. O₂ transport was measured in the following vessels (bilateral where appropriate): maternal abdominal descending aorta (DAo_abdo ), IVC, ovarian, paraspinal veins (PSV), uterine artery (UtA) and external iliacs, and umbilical. Maternal cardiac output was measured by summing DAo_thoracic and superior vena cava flows. Supine mothers (n = 6) had lower cardiac output and O₂ delivery in the DAo_abdo , UtA and external iliac arteries, and higher PSV flow than those in either the left (n = 8) or right (n = 6) lateral positions during MRI. However, O₂ consumption in the gravid uterus, fetus, placenta and lower limbs was unaffected by maternal positioning. The ratio of IVC/PSV flow decreased in supine mothers while ovarian venous flow and O₂ saturation were unaltered, suggesting a major route of pelvic venous return unaffected by maternal position. Placental-fetal O₂ transport and consumption were similar between left and right lateral maternal positions. In comparison to non-pregnant findings, DAo_abdo and UtA O₂ delivery and pelvic O₂ consumption increased, while lower-limb consumption remained constant , despite reduced external iliac artery O₂ delivery in late gestation. KEY POINTS: Though sleeping supine during the third trimester is associated with an increased risk of antepartum stillbirth, the underlying biological mechanisms are not fully understood. Maternal cardiac output and uteroplacental flow are reduced in supine mothers due to inferior vena caval compression from the weight of the gravid uterus. This MRI study provides a comprehensive circulatory assessment, demonstrating reduced maternal cardiac output and O₂ delivery (uteroplacental, lower body) in supine compared to lateral positioning; however, O₂ consumption (gravid uterus, fetus, placenta, lower limbs) was preserved. Unlike other mammalian species, the ovarian veins conduct substantial venous return from the human pregnant uterus that is unaffected by maternal positioning. Lumbar paraspinal venous flow increased in supine mothers. These observations may have important considerations during major pelvic surgery in pregnancy (i.e. placenta percreta). Future studies should address the importance of maternal positioning as a potential tool to deliver improved perinatal outcomes in pregnancies with compromised uteroplacental O₂ delivery.

Keywords: MRI oximetry; blood flow; cardiac output; inferior vena cava compression; maternal position; ovarian vein; oxygen consumption; oxygen delivery; paraspinal veins; pelvic blood flow redistribution; uterine artery.