The Fat Vagina Theory: "Soft Tissue Dystocia"

One common misconception that many birth attendants have about labor in women of size is the "fat vagina" theory.

In this theory, birth attendants believe that women of size have a lot of extra fat tissue internally, crowding the maternal pelvis and birth canal.  Extra fat tissue then supposedly gets in the way and obstructs the passage of the baby through the bony pelvis and/or vagina.

In medical research they call this "soft tissue dystocia," but sometimes doctors will patronizingly explain it to patients as the "fat vagina" theory.

[Note: "Fat Vagina" is an actual term used by an OB to tell a friend of mine from ICAN why she had a cesarean ─ she was told her morbid obesity made her vagina "too fat" to let the baby out during labor ─ and she's not the only one who has been told that.  So although my sarcasm meter is certainly on when I use it, that's really a term that has been used with some women of size.]

Alas, it's not just doctors who buy into the "fat vagina" theory.  One TV birth show featuring a midwifery practice even promoted this same theory, encouraging a fat woman who wanted a Vaginal Birth After Cesarean (VBAC) at their birthing center to lose weight during pregnancy in order to "decrease the fat deposits in the vagina" and "make it possible" for the woman to have a VBAC. (As if no fat woman has ever had a VBAC without losing weight!  Sorry, they have. And so have I.)

Or there's the story one of my blog's readers emailed to me:

[The midwife] said I will have a large fat buildup around my birth canal and uterus so baby will get stuck in a natural birth. 

Alas, this "fat vagina" belief is a theory near and dear to many birth attendants' hearts.  They have been taught in medical or midwifery school that "soft tissue dystocia" is the cause of many cesareans in fat women, and they believe it with all of their hearts.  It's very difficult to get them to question its existence. 

But what does the evidence say? 

The Research, Such As It Is

One of the most frustrating aspects of the whole "fat vagina" theory is how little actual evidence there is on it, yet it is taught as if it is clearly established science. 

One study (Crane 1997) describes it as a possible reason for a higher primary cesarean rate in "obese" women:

Perhaps dystocia due to an increased deposition of soft tissues in the maternal pelvis may lead to the observed increase in the cesarean delivery rate.

One review (Vahratian 2005) noted that many studies attribute the increased rate of cesareans in "obese" women to soft tissue dystocia but that few prove it.

Several authors have speculated that this association [between obesity and cesarean rates] may be due to the added soft-tissue deposits in the pelvis of obese women, which coupled with a larger fetus might necessitate more time and stronger contractions to progress through labor.  However, direct evidence of fat deposition in the pelvis is needed to support this assertion.

The study noted that as yet, the authors promoting this theory have failed to provide this evidence.

Most studies discuss increased maternal fat deposits as if it is a proven concept, but no one has actually done much study showing a real difference in maternal fat compartments, let alone proven that it affects labor.

Barau et al., 2006, also favors the concept of soft tissue dystocia but acknowledges that:

There is no direct support of this concept by medical imagery studies.

So why are healthcare providers so devoted to this theory when there is very little actual proof of it? 

Most of the time, practitioners believe in the concept of soft tissue dystocia because they know that visceral fat can build up around internal abdominal organs, so they speculate that a similar thing must be happening in the maternal pelvis.

Furthermore, they often observe more tissue during pap smears in "morbidly obese" women, sometimes needing to use a larger speculum in order to hold open the vaginal vault and do the exam.  Therefore, they jump to the conclusion that there is more fat "crowding" the pelvis of women of size, and that therefore, this must be obstructing the passage of the baby through the mother's pelvis.

But just because a fat woman's vagina tends to have more tissue to hold back with a speculum does not necessarily mean that there is enough fat way up inside the pelvis (pelvic inlet) or at the pelvic outlet to be clinically significant.  

Remember, because the considerable weight of a fat woman's abdomen presses down with the weight of gravity, it may tend to make the vaginal walls more prone to collapse and look overly crowded, but it may not actually be so, or not enough to make much difference. And even if there was somewhat more tissue in a fat woman's vaginal area, remember......fat squishes.  It's unlikely that there would be enough tissue there to actually impede a baby's exit, even in very fat women. 

The one study we do have that actually did medical imagery to discover whether there are extra maternal fat deposits in the pelvis does not support the idea that there is enough extra soft tissue to be relevant to birth.

Wischnik (1992) did a study on the "fatty pelvis" to see whether fat compartments within the pelvis actually resulted in "functional reduction of the birth channel diameters."  They found slightly more fat compartments, but did not find it to be clinically relevant.  They concluded:

The common assumption can no longer be maintained, that adiposity necessarily causes soft tissue dystokia  [sic] due to larger fat compartments within the small pelvis.

So from a strictly evidence-based point of view, there is no substantiated medical evidence of enough increased deposits to be medically significant. 

All we have is the observation that there is slightly more tissue pushing in and thus more need for a stronger speculum during the pap smears of women of size. 

I think most medical providers then take a leap of logic and conclude that if fat women's babies don't come out as easily as skinny women's babies, it must be due to fat blocking the way....and the need for a bigger speculum during a GYN exam in some women of size just seems to confirm that bias. 

But I think they need to look more closely at this assumption.

Alternative Explanations

Unfortunately, despite a glaring lack of evidence, soft tissue dystocia is a concept that continues to be taught and widely believed. 

Barau 2006 argued that although there is no hard proof of soft tissue dystocia, it must exist because there is an increase in cesareans due to prolonged labor in obese women compared to average-sized women with similarly-sized babies. 

However, I will point out again that a higher rate of malpositioned babies among women of size could also explain their longer labors and increased rate of cesareans due to labor dystocia.

Babies who face their mothers' tummies (occiput posterior) don't fit as easily through the pelvis, present with a larger head diameter, and often experience long, slow, hard labors.  Research clearly shows that posterior babies have longer and slower labors and have a much higher cesarean rate than anterior babies. 

Although the subject needs further study, some research has found a higher rate of malpositioned babies in "obese" women, and very old obstetric research also often notes in passing a higher rate of occiput posterior and other malpositions in women of size as well.

And anecdotally, women of size often do seem to have more malpositions, especially occiput posterior, many of which result in cesareans.

Many of these fat women have been told they had cesareans because of their "fat vagina" or "too small" pelvis.  Yet if you read their stories carefully, they had all the signs and symptoms of a malpositioned baby instead. 

Furthermore, many women who had cesareans for "fat vaginas" went on to have subsequent vaginal births with babies that were even larger than their cesarean babies....without losing weight first. This would be impossible if the problem really were a "fat vagina." 

The key was having a well-positioned baby, not losing weight or reducing maternal pelvic fat deposits. 

Another problem with the concept of the "fat vagina" is the inconsistency with which this diagnosis is applied. If a ~275 lb. woman (like my friend from ICAN) is told that her vagina is "too fat" and prevented her baby from getting out, then it follows that a 350 lb. woman should not be able to birth a similarly-sized baby vaginally. Yet we know that women of that size can and do have vaginal births. I have birth stories on my website of vaginal births to women at 300, 350, and 400 lbs.  Yes, women of this size have a high cesarean rate because they are almost never given a real chance to actually have a vaginal birth.....but it can happen when they are given a realistic chance.

If fat vaginas truly prevented vaginal birth as much as some providers think they do, there would be NO vaginal birthers above a certain size. Sure, vaginal birth is always a combo of factors, including pelvic size/shape, the baby's size and position, (passenger, powers, position, etc.), so some variability in who gives birth vaginally is logical, but if soft tissue dystocia were a really significant factor, really supersized women would never birth vaginally.  Yet many can and do, if they have a well-positioned baby and are given a realistic chance.

In my opinion, the real issue behind a higher rate of "dystocia" cesareans in women of size is probably fetal position, not fat vaginas. 

However, because doctors are trained to blame obesity as the go-to diagnosis when they don't have another explanation, soft tissue dystocia gets blamed for "blocking" the baby's way out. 

But that's just lazy thinking, not actual proof of soft tissue dystocia.

What About Shoulder Dystocia?

Another fear that many doctors and midwives have is that extra soft tissue in a fat woman's vagina might cause such a tight fit for the baby that the baby's shoulders will get stuck ("shoulder dystocia"), which can be a true obstetric emergency.

This fear seems borne out by studies which have shown higher rates of shoulder dystocia in "obese" women. Yet many of these studies did not control for other factors, like macrosomia, diabetes, forceps/vacuum extractor, or induction, all of which increase the risk for shoulder dystocia, and most of which occur at higher rates in women of size. 

It's important to note that other studies have found that obesity is not a risk factor for shoulder dystocia when these other factors are controlled for. 

Furthermore, it is not extra fat crowding the vaginal walls that causes shoulder dystocia. Rather, shoulder dystocia is caused by obstruction by the bony pelvis, not soft tissues. A complex interplay of factors results in shoulder dystocia, including fetal position, maternal position and mobility, pelvic shape, induction, operative delivery, diabetes, and macrosomia. 

But justified or not, fear that "extra" soft tissue will cause shoulder dystocia is a big contributor to the high planned cesarean rate and early induction rate in women of size. 

But What If Soft Tissue Dystocia is Real?

Frankly, soft tissue dystocia as a concept has not been proven at all, but some care providers believe in it with all their heart and refuse to be dissuaded from the possibility, pointing out that it hasn't been disproven conclusively either.  That's a fair point.

To these providers I would argue that IF soft tissue dystocia were indeed real, then it is vital to change how most providers manage the labors of women of size in order to minimize its theoretical impact.

The typical way the labors of many "obese" women is managed often includes inducing early, breaking the bag of waters early, strongly encouraging early placement of an epidural "just in case," strictly limiting mobility, and using semi-sitting or lying back positions for pushing. Yet these measures often limit the amount of pelvic space the baby has to fit through.

If a bunch of extra fat is supposedly crowding things already, these restrictive protocols just make things worse.

IF soft tissue dystocia were real, mobility in labor would be even more important to women of size because it opens up the pelvic dimensions and gives "obese" women the best possible chance to help that baby fit through. Yet "obese" women have the most restrictions on their mobility in labor and are the least encouraged to try alternative positions for birth.

If a provider truly believes that soft tissue dystocia might be real, then the answer is not to pre-emptively induce or section women of size, but to give them every chance at creating more space in the pelvis. 

This means not having her in the usual lying back/semi-sitting position, which puts pressure on the tailbone and presses it into the pelvic cavity, reducing the space available. 

This means not requiring/pressuring her for an early epidural (as many practices encourage with women of size), so that she can move freely during labor and encourage her baby to be in a good position for birth. 

It means not breaking her waters early in labor, so the baby has the watery cushion to help it move into an optimal position. 

It means letting her stay upright as much as possible, so she can use gravity to help bring that baby down and press it through those supposedly fat-crowded walls. 

It means letting her push in whatever position feels comfortable to her, encouraging her to stay mobile, and letting her utilize things that will help her stay mobile more easily (a labor tub, a dangling support or trapeze bar, a squat bar, etc.).

Personally, I don't believe that soft tissue dystocia is clinically relevant, but if you are a provider and you firmly believe in its existence, follow that to its logical conclusion and utilize labor protocols that help that woman maximize her pelvic space, not inhibit it via restrictive protocols. 

Conclusion

Soft tissue dystocia is a belief that is near and dear to many doctors' hearts (and sadly, even to some midwives' hearts). Yet despite no research to support it, it as taught as if it is fact. 

At this point, soft tissue dystocia is NOT fact.  It has not been proven at all, and remains highly speculative as a possible cause of labor issues.

Furthermore, many women of size who have had their cesareans blamed on "fat vaginas" have gone on to have vaginal births in later births----without having lost weight first.  If soft tissue dystocia were really the cause of their cesareans, this would not have been possible. The issue for them was fetal position, not maternal soft tissue. 

The "morbidly obese" friend of mine who was told that her "fat vagina" was the cause of her cesarean?  She has since gone on to have TWO vaginal births.....at the same maternal weight, with similarly-sized babies.  So much for her "fat vagina" preventing the baby from coming out! 

And then there's the commenter on my blog who had a similar experience: 

I was told by my OB, while on the operating table and again at my 6 week check, that 'my vagina was too fat to birth a baby naturally'. Had a VBAC 3 years later though!

[Comment on 10-12-10, on Prenatal Weight Gain: The Importance of Study Design.]

Far too often, "fat vagina" is just a convenient excuse for lazy thinking, a handy scapegoat for the high cesarean rate in women of size, and a convenient excuse for blocking access to VBAC in yet another group.

And unfortunately, it prevents caregivers from examining their own care practices and how iatrogenic influences like weight bias, induction, fear of macrosomia, restrictive protocols, "failure to wait" and fetal position issues are the real factors driving the cesarean rate for women of size.