Overdistension and defects of the levator ani muscle observed after vaginal delivery are associated with the pathogenesis of pelvic organ prolapse. Cesarean delivery has been suggested as an alternative, but it does adequately protect against symptomatic pelvic organ prolapse. To explain why delivering by cesarean fails to protect the pelvic floor, it has been suggested that pregnancy itself weakens the support system. Few studies have studied the pelvic floor support system during pregnancy.
The primary aim of this prospective cohort study was to investigate whether pregnancy affects the levator hiatus dimensions and the position and mobility of both the bladder neck and the levator ani muscle in nulliparous pregnant women. A total of 274 nulliparous pregnant women underwent 3-dimensional and 4-dimensional transperineal ultrasonography at rest, during contraction, and during Valsalva maneuver at 21 and 37 weeks of gestation. Render imaging was used to measure the levator hiatus dimensions: the anteroposterior and transverse diameter and levator hiatus area. The bladder neck and levator plate positions were analyzed in the midsagittal plane. Their mobility was measured as displacement of the bladder neck or levator plate from rest to contraction or from rest to Valsalva.
There was a significant increase in all levator hiatus dimension measurements from 21 to 37 weeks of gestation. The greatest change was found for levator hiatus area at rest and during Valsalva maneuver; increases in mean area in each were 17.1% and 21.4%, respectively. A significant increase in bladder neck mobility was found from 21 to 37 weeks of gestation; the most marked change was from rest to contraction (mean, −14 [SD, 0.4] mm).
The increase found during pregnancy in all levator hiatus dimensions and bladder neck mobility suggests that pregnancy itself may be an important risk factor for pelvic organ prolapse in nulliparous pregnant women.