Factors involved in changes in the levator ani during pregnancy

Levator ani muscle (LAM) dimensions increase during pregnancy to allow the delivery of the fetus. The objective was to investigate which factors are involved in LAM modifications during pregnancy.

A prospective longitudinal observational study was conducted between July 2015 and March 2018. Ninety-nine nulliparous pregnant women were included. Data on the physical examination, 4D transperineal ultrasound and hormonal concentrations (progesterone, oestradiol and relaxin) were collected during the first and third trimesters.

We found higher hiatal dimensions at the beginning of pregnancy than in other studies with nonpregnant women. Increases in the levator ani hiatal (LH) dimensions were observed at contraction (1.01 ±1.96 cm²), rest (0.82 ± 2.51 cm²) and on Valsalva (2.36 ± 3.64 cm²) throughout pregnancy. The distensibility in the third trimester was higher than in the first trimester (5.79 vs 4.24 cm²; p=0); however, the contractility was lower (-3.32 vs -3.5 cm²; p=0.04). Women with lower scores on the Modified Oxford Grading Scale in the third trimester presented with lower contractility in the LAM. A larger LH at the end of pregnancy was associated with age and body mass index. Eleven women developed ballooning during pregnancy; in these women, relaxin was higher in both trimesters than in women without ballooning, but these results were not statistically significant. The linear models to predict third-trimester Valsalva LH, distensibility and contractility were not conclusive and did not show any factors to predict LAM modifications during pregnancy.

Hormones could play a role in modifying the muscle properties of LAM from the beginning of pregnancy, but we did not find an association between LAM measurements and hormone concentration in this study.