OBJECTIVE

To use superb microvascular imaging (SMI) to longitudinally evaluate blood flow in intravillous arterioles (IVA) in pregnancy, and to develop reference ranges for IVA pulsatility index (PI) and peak systolic velocity (PSV) from 12 to 38 weeks' gestation.

METHODS

The data for this study were obtained from the National Institutes of Health-funded Human Placenta Project. Eligible for inclusion were women aged 18-35 years, with a body mass index of < 30 kg/m, without comorbidities, with a singleton pregnancy conceived spontaneously and a gestational age of ≤ 13 + 6 weeks. Exclusion criteria were maternal or neonatal complications, fetal or umbilical cord anomalies, abnormal placental implantation and delivery < 37 weeks. Each participant underwent eight ultrasound examinations during pregnancy. Using SMI technology, the total number of IVA identified was recorded in a sagittal placental section at the level of cord insertion. The PI and PSV were measured in a total of six IVA (two in the central portion of the placenta, two peripherally towards the uterine fundal portion and two peripherally towards the lower uterine segment). Umbilical artery (UA)-PI was also obtained. Multilevel regression models were used to create reference ranges for IVA-PI and IVA-PSV at 12-38 weeks' gestation.

RESULTS

A total of 90 women fulfilled the study criteria. IVA-PI decreased throughout pregnancy from a mean ± SD of 1.04 ± 0.23 in the first trimester (at 12-13 weeks) to 0.75 ± 0.10 in the late third trimester (at 36-37 weeks). Mean IVA-PSV increased from 5.97 ± 1.47 cm/s at 12-13 weeks to 12.65 ± 4.19 cm/s at 28-29 weeks, and then plateaued throughout the third trimester, to 13.09 ± 4.64 cm/s at 36-37 weeks. UA-PI decreased progressively throughout pregnancy, from 1.49 ± 0.22 at 16-17 weeks to 0.89 ± 0.18 at 36-37 weeks. Reference ranges for IVA-PI and IVA-PSV from 12 to 38 weeks' gestation were developed.

CONCLUSION

Placental IVA blood flow can be visualized and quantified from early gestation in normal pregnancies using SMI. © 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.