Low-intensity pulsed ultrasound to facilitate uterine involution: prospective multicenter study on clinical efficacy and mechanistic insights.

OBJECTIVE: To assess the clinical efficacy and safety of low-intensity pulsed ultrasound (LIPUS) in facilitating uterine involution after delivery and mitigating postpartum complications, and to explore its molecular mechanisms of action through both in-vitro and in-vivo experimental animal models. METHODS: This multicenter, randomized, double-blind, controlled trial was conducted across three centers in China from January 2023 to December 2024. Participants were randomized to a LIPUS group or a control group. The LIPUS group received treatment at 0.8 MHz with a spatial peak-temporal average intensity of 2.0 W/cm2, focal area of 0.09-0.12 cm2 and pulse duration of 4 ms, for 30 min daily over 10 consecutive days, starting 2 h after delivery. The control group received sham treatment with identical parameters but no acoustic output. The primary outcome, percentage reduction in uterine volume on post-treatment day 10, measured using transabdominal three-dimensional ultrasonography, was compared between the two groups. Secondary outcomes, including the incidence of postpartum complications (secondary postpartum hemorrhage and puerperal infection), duration of lochia and indices of uterine involution (fundal height descent index and uterine dimensions) were also compared. For in-vivo experiments, uterine tissues from postpartum Sprague-Dawley rats that had undergone LIPUS or sham treatment were evaluated using hematoxylin and eosin staining for morphology, and immunohistochemistry and Western blotting to assess distribution of endothelin-1 (ET-1) and oxytocin receptor (OTR) proteins and to quantify their expression. For in-vitro experiments, uterine tissue strips from untreated rats were exposed to LIPUS or sham treatment ex vivo for contractility measurement using a BL-420F biological signal acquisition system. Primary uterine smooth-muscle cells (USMCs) from untreated rats were exposed to LIPUS or sham irradiation in vitro and assessed for intracellular calcium ions (Ca2+) by fluorescence imaging and flow cytometry, and for ET-1 and OTR expression by Western blotting. To assess calcium-channel involvement, nifedipine (an L-type calcium-channel antagonist) was included in the contractility and flow cytometry experiments, establishing four treatment groups: control, LIPUS alone, nifedipine alone and LIPUS + nifedipine. For in-vivo and in-vitro experiments, results were compared between the LIPUS-treated and control groups; for the contractility and flow cytometry assays, comparisons were made among all four groups. RESULTS: In total, 1106 participants were enrolled, with 553 assigned randomly to the control group and 553 to the LIPUS group. After exclusions, 528 women in the LIPUS group and 530 in the control group were included in the final analysis. Compared with controls, the LIPUS-treated participants showed a significantly greater percentage reduction in uterine volume and fundal height descent index, shorter lochia duration, reduced uterine dimensions and a lower incidence of secondary postpartum hemorrhage and puerperal infection (all P < 0.05). In-vitro experiments demonstrated that LIPUS was significantly associated with increased contractility of rat uterine tissue strips and elevated intracellular Ca2+ levels in USMCs (P < 0.001); these effects were attenuated by nifedipine but partially restored by LIPUS. Expression levels of ET-1 and OTR were also significantly higher in LIPUS-treated uterine tissue and USMCs compared with controls (P < 0.05). CONCLUSIONS: LIPUS application is significantly associated with reduced uterine volume and fewer postpartum complications, suggesting its potential to promote uterine involution. This effect may be mediated through enhanced Ca2+ influx and upregulation of ET-1 and OTR in USMCs. These findings support the potential of LIPUS as a non-invasive approach to improve postpartum outcomes. © 2026 International Society of Ultrasound in Obstetrics and Gynecology.