There's some evidence from earlier research that pelvic muscle weakness could increase the likelihood of having urinary incontinence or other "pelvic floor" dysfunctions, but this study was unable to make that link.

"We don't know the significance of the pelvic muscle weakness," said Dr. Victoria Handa, one of the authors of the study and a professor at Johns Hopkins Medicine in Baltimore.

"What we do know from this paper is that there are some differences in pelvic muscle strength five to 10 years after childbirth by delivery group," Handa told Reuters Health. "But what we don't know is, will those differences in muscle strength translate into a greater chance of problems for women with 'weakened' pelvic muscles?"

Handa said it's clear that childbirth does play a role in the development of pelvic disorders later in life, particularly with regard to incontinence, the inability to hold in urine, and prolapse, an inability to keep organs such as the uterus in place.

One study from Sweden, for instance, found that women who gave birth vaginally were more likely to have incontinence decades later than women who had a C-section (see Reuters Health story of April 12, 2012).

To see whether pelvic muscle weakness could explain the relationship between the type of childbirth a woman had and her later risk for pelvic disorders, Handa's group measured the muscle strength of 666 mothers.

Each of the women had given birth to their first child five to 10 years earlier.

The women were asked to squeeze their pelvic muscles while a probe inserted into the vagina measured the peak pressure of the squeeze and how long they could hold it.

The probe, which contained water, gave readings based on how much liquid was displaced by the compression.

The researchers found that the 362 women who had had a cesarean delivery could, on average, squeeze the probe 39 centimeters, while 255 mothers who had delivered vaginally could squeeze it 29 cm. And 49 women who had forceps involved in a vaginal delivery could squeeze the probe 17 cm.

During a forceps delivery, a clamp is inserted into the birth canal to extract the baby.

"With the use of forceps you're stretching or disrupting muscles potentially at a much greater rate than you would with women who have given birth (without forceps) or by cesarean," which might explain the finding, said Lisa Kane Low, a professor at the University of Michigan School of Nursing, who was not involved in this study.

The study, which is published in the journal Obstetrics & Gynecology, could not prove that the type of childbirth the women had is responsible for the differences in their pelvic muscle strength.

The researchers didn't know the muscle strength of the women before they gave birth, for instance, so it's possible the women who had regular vaginal deliveries and those who needed forceps had weaker muscles to begin with.

Although there were differences in muscle strength between the three groups of mothers, "what we didn't see in this paper is that the difference in strength explains the (increased risk of) pelvic floor disorders" among women who give birth vaginally, Handa added.

Among mothers who delivered their babies vaginally, those with a pelvic disorder did have weaker muscles than did those without any problems, but the same was not always true among mothers who had given birth via cesarean.

That led the researchers to write, "our results raise the question of whether the mechanism for the development of pelvic floor disorders may differ after vaginal compared with cesarean delivery."

The average muscle strength of women in the cesarean group with any type of pelvic disorder was 39 cm, compared to 36 cm among the women without a disorder - such a small difference it could have been due to chance.

In addition, women in the cesarean group who suffered from uterine prolapse actually had greater pelvic muscle strength than the women with good organ support.

Handa acknowledged that she would have expected to see a more clear relationship between muscle weakness and pelvic problems if muscle strength were important in staving off the disorders.

"If we think the pelvic muscles are important, why aren't we seeing more differences between strength and disorders?" Handa said.

"I don't think this paper can allow us to make that conclusion that the pelvic muscle weakness explains the association" between the type of childbirth and later pelvic issues, she said.

Handa added that it would be premature to make any recommendations, based on these findings, about how women should give birth to avoid later problems.

It's still possible that muscle strength plays a role in the development of disorders, she noted, but maybe researchers haven't found the right aspect to measure, or perhaps more time needs to pass before pelvic muscle weakness begins to take its toll.

Her group is continuing a longer-term study to follow mothers as they age to try to answer that question.

SOURCE: bit.ly/QDBGii Obstetrics & Gynecology, online October 5, 2012.

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