Does childbearing weaken the pelvic floor?

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"Does childbearing weaken the pelvic floor?
Over half of pregnant women report symptoms of urinary incontinence and most studies have found that women who have had children are more likely to suffer from urinary incontinence than women who have never had children (Rortveit, Grodstin). A US study has found that the risk of moderate to severe pelvic floor disorders increases with the number of babies a woman has had. Problems are experienced by 12.8% of women who have never given birth, 18.4% of women who have had one child, 24.6% of women who have had two children, and 32.4% of women who have had three or more children (Nygaard & others).
For many women the symptoms that are experienced in the months after birth do diminish naturally. However, in a large Swedish study, over 1 in 5 women reported symptoms of stress incontinence a year after having a baby (Schytt) and for many they persist for life, worsening with age. A recent Danish study found that urinary incontinence increased significantly during the 12 years following the birth of a woman's first baby to the point where 47% of women in the study group had some level of incontinence (Viktrup 2009).

Some studies suggest that stress incontinence symptoms that appear after the birth, rather than during the pregnancy, are much more likely to persist, with 25% of these women still incontinent one year later.

Can a caesarean birth reduce pelvic floor damage?
Some research does indicate that women who have had caesarean deliveries have a lower rate of incontinence than women who have had vaginal deliveries (Rortveit, Farrell). Several studies have found that a forceps delivery increase a woman's chance of suffering from incontinence after giving birth.

Does pelvic floor exercising after having a baby make a difference?
Definitely, YES! Research shows that a regular program of pelvic floor exercise does make a difference; it reduces the likelihood of ongoing stress incontinence and increases muscle strength, with results of an eight-week program still sustained a year later :

[A program of] pelvic floor muscle strength training programme can add significantly to physical recovery after childbirth. (Morkved & others).

However pelvic floor exercises need to be performed for life to fully protect the pelvic floor. New mothers need to incorporate exercises into their own routines, rather than relying on supervised postnatal exercising which research shows is unlikely to be sustained effectively once the health professional support is no longer available (Agur & others).

Is is possible to reduce the likelihood of damage to the pelvic floor by exercising before or during pregnancy?
A strong pelvic floor is an excellent insurance against stress urinary incontinence at any stage of life.
There is evidence that a program of pelvic floor exercise during the first half of pregnancy can substantially reduce incontinence symptoms in later pregnancy and after birth (Sampselle) for women who are pregnant for the first time. And the strength of the pelvic muscle at 20 weeks of pregnancy is an excellent indicator of whether a woman is likely to suffer from incontinence later.
A pelvic floor exercise program during pregnancy can also have a positive effect on the second stage of labour (Salvesen)."