The topic of pelvic floor damage in childbirth is attracting more and more attention from non-medical people. This is not surprising- after all, a large porportion of women who have given birth naturally are affected, and so are their partners. Until recently we thought that ‘pelvic floor trauma’ meant perineal and vaginal tears, and damage to the anal sphincter, the muscle that surrounds the end of the back passage. In developing countries especially this includes fistulae, that is, abnormal connections between vagina and bladder (vesicovaginal fistula) or back passage (rectovaginal fistula), but these are uncommon in developed countries with good medical care.
The left hand image is the patient’s right side, the top of the image is the pubic bone, and the bottom of the image is close to the end of the spine and the back passage. Vagina, urethra and back passage fill the space inside the V and have been digitally ‘removed’. On playing the video you can see that the V of the muscle gets wider higher up, that is, further up the vagina. The area that is most likely to suffer in childbirth is the thickest, lowermost part, because it has to stretch most, and fastest (during crowning of the head). Normally, skeletal muscle (the kind we usually mean when we talk of muscle) can't stretch to more than twice its length without tearing. The puborectalis muscle often has to stretch by much more. The miracle is that in many women it doesn’t tear and even returns to normal after letting the baby through. Nobody understands how this is possible, but we assume it has something to do with hormonal effects of pregnancy.
However, it happens often enough that the muscle does tear, something that was first shown in 1943, only to be forgotten for 60 years. That sounds incredible, but it's true. We can sometimes even see the damage in labour ward, in women who have vaginal tears. It’s just that nobody ever looked for damage to the underlying muscle, and nobody seems to ever have tried to fix it, until 2006 when this was first tried and documented, at Hawkesbury Hospital in Western Sydney. Commonly, the puborectalis muscle comes off the bone of the pelvic sidewall during crowning of the baby’s head. This happens towards the very end, when the baby’s head is just visible in the vagina. Women who need a Caesarean Section in labour, regardless of the timing, never have this kind of pelvic floor damage- unless somebody has tried a Forceps before the Caesarean, which happens very occasionally.
Over the last ten years we have learned how to detect tears of the muscle, not just at birth, but also much later, with magnetic resonance imaging and ultrasound, and even by just using our fingers. A 4D pelvic floor ultrasound is the best method for seeing pelvic floor structures. It shows the entire pelvic floor in real-time, and we can see what happens when someone pushes or bears down, stretching the muscle (see Video 2).
In some women, even some who have never given birth the normal way, this muscle stretches a lot (see Figure 1) on pushing or just coughing. This is probably good for having babies, but it means a higher risk of prolapse of the womb, bladder and back passage. These conditions mean that someone feels a lump in the vagina that may even stick out, and there can also be bladder symptoms or problems with the back passage.
Whether someone suffers damage to this muscle in labour or not seems to depend on many things. The older a mum is at the time of the first vaginal birth, the higher the risk of major damage (see Figure 2). That’s very important. People should know about this when they think about having kids. The longer one waits with having children, the lower the likelihood of falling pregnant without help, and the higher the risk of needing a Caesarean or Vacuum or Forceps. And even if a woman manages to avoid a Caesarean, the risk of pelvic floor muscle trauma rises by about 10% with every year of delay in having your first child.
Incidentally, it’s the first baby that matters the most. Whether someone has one child born the normal way or three doesn’t seem to make much of a difference to the pelvic floor. A second baby does not seem to cause any additional damage- unless the first was born by Caesarean, and the second the normal way (vaginal birth after Caesarean, VBAC).
The more difficult a vaginal delivery, the higher the risk of pelvic floor injury, and other factors such as a long labour, OP presentation (when the baby faces upwards rather than downwards) and a big baby probably increase the risk as well. The main problem however is Forceps. There are studies from all over the world showing that it triples the risk of pelvic floor muscle tears, and rotational Forceps is particularly bad. That's important because the Forceps rate in the UK has doubled over the last ten years, and it's now going up in Australia as well- see Forceps link. An Epidural seems to protect the pelvic floor to some degree.
Figure 3-5 show what this damage looks like on magnetic resonance and ultrasound imaging three months after a normal delivery. So far, nobody has been able to repair this kind of damage successfully at birth, although we've tried. Immediately after childbirth the muscle is over-stretched and thin, and stitches seem to simply come loose soon after the repair. There may be other ways of fixing the damage, or just of compensating for it, but such research is still in the initial stages.
Some childbirth-related damage, such as small perineal tears, are often not repaired because we think they’ll heal well by themselves. So, do those pelvic floor muscle tears heal? We have now followed up hundreds of women after their first birth, and there seems to be very little change over time. Once the muscle is pulled off the bone it shrinks and pulls back towards the back passage (it ‘retracts’), and there is no way it can move back to where it came from. In some women the tears are not complete, and scar tissue can bridge a partial tear, but once the muscle is completely off the bone, the defect probably won’t heal. In some women this happens on both sides, making matters worse (see Video 3).
Do defects of the pelvic floor muscle (‘avulsions’) matter? After all, Obstetricians and Gynaecologists seem to have completely missed them until now! In fact, there are many thousands of women in the community who have suffered this kind of trauma in childbirth, without noticing any problems. Partly this may be because such problems sometimes take a long time to develop, but in others those parts of the pelvic floor muscle that are higher up (mainly the iliococcygeus muscle) can compensate and take most of the load of the pelvic organs.
What we can say right now is that pelvic floor muscle trauma (‘avulsion’)
-weakens the muscle by about 1/3 on average
-makes the muscle more stretchy by about 50%
-enlarges the opening of the pelvic floor (the ‘hiatus’) by about 1/4
-more than doubles the risk of bladder prolapse
-triples the risk of prolapse of the uterus (the womb).
-triples the risk of a prolapse returning after pelvic floor surgery
Video 1: This is what the pelvic floor muscle looks like normally, seen from below.
Video 2: Pelvic floor ultrasound during bearing down. This is a thick, healthy muscle that doesn’t stretch much.
Figure 1: This is how much the elasticity of the pelvic floor muscle can vary from one person to the next.
Figure 2: The relationship between age of the mother at her first delivery and pelvic floor muscle trauma (from: Dietz and Simpson, ANZJOG 2007; 491-495).
Figure 3: Right- sided tear of the puborectalis muscle (indicated by star) on MR (left) and 3D ultrasound (right), seen 3 months after a normal delivery.
Figure 4: Left- sided tear of the puborectalis muscle on tomographic 3D ultrasound. The defect is about 1.5 cm wide.
Figure 5: Bilateral tear of the puborectalis muscle (indicated by stars) on MR (left) and 3D ultrasound (right), seen 6 months after a Vacuum delivery.
Video 3: Severe over-stretching of the pelvic floor muscle in a patient with bilateral pelvic floor muscle tears. The effect is most visible on the bottom right image.