Pregnancy is a time of many anatomical and physiological changes in the body. A woman’s biology, together with environmental and genetic factors, predisposes her to pelvic floor weakening. This is characterized by loss of tone in the muscles, ligaments and fascia of the pelvic floor. Eventually pelvic floor weakening may lead to urinary incontinence.
Increasing and prolonged pressure on the pelvic floor by the fetus during pregnancy can contribute to pelvic floor weakening. There is also an anatomical effect of the pregnant uterus on the bladder.
Pelvic floor weakening through the trimesters
In the first trimester, the increased weight of the uterus presses directly on the bladder and can compress it, predisposing to urinary leakage.
In the second trimester, the uterus enlarges and rises up and out of the pelvis to become an abdominal organ. The pressure on the bladder lessens.
In the third trimester, after part of the fetus descends into the pelvis, there is again direct pressure on the bladder. At this stage, the pressure on the bladder and the reduced space for the bladder to expand as it fills with urine can cause a tendency to leak urine.
Other causes of pelvic floor weakening
Pelvic floor weakening is likely when the mother gains a lot of weight during pregnancy, or if a multiple birth is expected. A multiple birth pregnancy introduces extraordinary physical demands on the mother's body. These women have a remarkably high prevalence of pelvic floor weakening.
Pregnancy contributes to pelvic floor weakening more than childbirth
Pregnancy itself, more than childbirth, appears to contribute to altered long term pelvic floor function in later life. A possible cause is the influence of the hormones of pregnancy. The hormone relaxin, with levels highest in the second trimester, may have the most significance. This hormone has been associated with relaxation and structural effects on the connective tissue of the pelvic floor during the third trimester and childbirth. These effects increase pelvic floor weakening to help ease the birthing process.
Obstetric factors can cause pelvic floor weakening
The evidence also shows that some obstetric factors may cause partial damage to the pelvic floor nerves in some women. Effects on a particular nerve—the pudendal nerve—can cause pelvic floor weakening and relaxation of the external urethral sphincter, resulting in urinary incontinence. Strengthening the pelvic floor muscles can play a fundamental role in maintaining continence and integrity of the pelvic floor.
For many women, pelvic floor weakening will be alleviated after delivery. In others, factors may make incontinence more likely during the immediate postpartum period or later in life. Such factors include stretching of tissues beyond physiological limits during delivery, consecutive pregnancies, short pregnancy intervals, hereditary factors, and ageing.