Despite low estrogen levels, rapid immune and repair responses in lactating animals could help guide new treatments to improve postpartum recovery.

After childbirth, levels of the hormone estrogen plummet and remain low for longer in breastfeeding women. Given estrogen’s role in supporting skeletal muscle repair, researchers at the University of California San Diego expected lactation to delay pelvic floor muscle recovery.

Instead, the team found that lactation accelerated healing in rats with simulated birth injuries by triggering an earlier immune response. This helped injured muscles transition more quickly from inflammation to repair.    

“While we initially expected lactation to hinder recovery because estrogen remains low, we found the opposite,” says Karen L. Christman, professor of bioengineering and co-director of the Sanford Advanced Therapy Center at UC San Diego.

Understanding how pelvic floor muscles heal after childbirth could lead to treatments that improve recovery before permanent damage sets in.

During vaginal delivery, the pelvic floor muscles stretch dramatically, potentially causing injury. This can have lasting effects, including urinary incontinence, pelvic organ prolapse, chronic discomfort, and sexual dysfunction, all of which can significantly reduce quality of life.

Despite the prevalence of postpartum pelvic floor injuries, little is known about how these muscles heal. Most research on muscle regeneration has focused on limb muscles, where estrogen has been shown to regulate inflammation, reduce muscle damage, and support the cells that repair muscle tissue.

But unlike limb muscles, pelvic floor muscles undergo pregnancy-related adaptations that help prepare them for the extreme stretching of childbirth and must then heal in a dramatically altered hormonal environment after birth.

To investigate whether breastfeeding influences pelvic floor muscle recovery, the researchers used a preclinical model. In pregnant rats, they simulated a birth injury designed to mimic the stretching experienced during vaginal delivery. After giving birth, some mothers nursed their pups while others did not.

Within 24 hours, the lactating animals showed a rapid immune response. Their blood contained elevated levels of signaling molecules that recruit immune cells to sites of injury. Those immune cells also appeared earlier in the injured pelvic floor muscles, where they helped initiate the repair process. At the same time, genes associated with inflammation declined sooner, while genes linked to tissue repair became more active. 

“The body responded very quickly to muscle injury,” says study co-lead Marianna Alperin, a urogynecologist and vice chair for research in the Department of Obstetrics, Gynecology, and Reproductive Sciences at UC San Diego.

“Within the first 24 hours, immune cells infiltrated the injured muscle, and we observed a quick increase in differentiated muscle stem cells that are necessary for new muscle formation and faster repair.”

By eight weeks after injury, the lactating rats had developed larger regenerated muscle fibers than the non-lactating animals — a sign of more advanced recovery. 

Inflammation is an essential part of healing, but only if it happens at the right time. Immediately after an injury, immune cells clear away damaged tissue and signal muscle stem cells to begin rebuilding.

“If inflammation continues for too long, it can delay recovery, promote fibrosis, and result in muscle dysfunction,” says Alperin.

“In our study, lactation is associated with an earlier switch to the anti-inflammatory response, meaning the body is resolving inflammation more efficiently and allowing the muscle to get to a reparative phase sooner.”

One of the study’s biggest surprises is that estrogen may not be the dominant factor influencing postpartum muscle recovery after all.

During breastfeeding, estrogen remains low, while other hormones rise or fluctuate. The researchers believe this broader hormonal environment — including prolactin, oxytocin, progesterone and other signaling molecules — may orchestrate the body’s response to injury.

“The postpartum environment is much more hormonally complex than simply low estrogen,” says Christman.

The researchers caution that the study was conducted in rats and examined changes at the tissue level rather than measuring pelvic floor strength or function.

“Evaluation in humans needs to be performed before one can conclude that breastfeeding accelerates muscle recovery in women,” says Christman. “But studies in preclinical models are essential for figuring out what specifically we need to study in humans.” 

Even so, identifying the biological signals that promote healing could eventually lead to therapies that improve pelvic floor recovery, regardless of whether a woman breastfeeds.

The team is now exploring injectable biomaterials that could recreate the pro-regenerative environment seen during lactation and promote faster healing after childbirth.

Reference: Bianca L. Peña et al., Lactation is Associated with Accelerated Postpartum Pelvic Floor Muscle Recovery in a Pregnant Simulated Birth Injury Model. Advanced Science (2026), DOI: 10.1002/advs.202600052

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