But how would obesity impede labor and result in more cesareans, you ask?
The usual reasons given (based on assumptions or poor research) were soft tissue dystocia (i.e., the fat vagina theory), or inefficient uterine contractility due to high leptin or cholesterol levels (seriously, that's a current theory still floating around, despite evidence that contradicts it).
Yet no one was asking whether the way labor was managed in "obese" women contributed to this high cesarean rate.
Now, for the first time, FINALLY someone is starting to ask these questions! A Canadian study out earlier this year examined labor management of obese women compared with other women.
And guess what?! As I've been saying for years, they found that the labors of women of size are indeed managed differently, with more interventions and a much lower threshold for surgery.
Furthermore, when the study controlled for the use of interventions, the relationship between obesity and cesareans was "markedly attenuated."
Details from the Study
There are a couple of interesting items in the study worth a closer look.
Induction Rates
First, induction rates went up strongly as BMI increased. Here's a summary of induction rates by BMI category (delivery BMI):
- "Normal" BMI (20-24.9) 23.7% induced
- "Overweight" BMI (25-29.9) 29.3% induced
- "Obese" BMI (30-39.9) 37.2% induced
- "Morbidly Obese" BMI (40+) 50.0% induced
A great deal of research has shown that induction of labor is linked to higher cesarean rates. This is particularly true for first-time mothers or women who have never had a vaginal birth before, or whose cervix was not ripe before the induction.
So why don't any researchers (including this one) connect the dots between such an extremely high induction rate in women of size and a resulting high cesarean rate?
The authors don't really comment on the induction rates or question them at all; most research never does. Most authors assume that all these inductions are truly indicated, especially in women of size. But frankly, they need to question such a high rate of induction more closely.
How many of these inductions were for real medical indications, and how many were for dubious indications like suspected macrosomia or provider fear?
We know from research that inducing early for a suspected big baby does not improve outcomes, and actually strongly increases the cesarean rate in many studies. Yet it is common practice still among clinicians to induce labor early if a big baby is suspected, especially in women of size.
So when you see the 50% induction rate in "morbidly obese" women, how many were for "soft" indications like suspected macrosomia? And what was the cesarean rate among those induced for "soft" indications? I would love for researchers to look more carefully at induction indications and how that influences cesarean rates in women of size.
We know from another recently published study that high induction rates definitely do have a strong influence on cesarean rates in obese women. According to the authors of that study:
We conclude that morbid obesity is associated with a significantly higher risk of pre-existing medical conditions, developing antenatal complications, induction of labour, caesarean section and greater birth weight. However, there was no significant difference in caesarean section rates when adjusted for induction of labour.More research is needed to further clarify the impact of high induction rates on cesarean rates in obese women, and researchers need to finally start questioning the validity of many of these inductions.
Cervical Status Upon Admission
Another interesting finding that deserves further investigation is a major difference in cervical status upon admission and what might be influencing this.
Women whose cervix is more dilated upon admission tend to have shorter labors and a lower cesarean rate; those whose cervix is less dilated at admission usually have longer labors and more cesareans because their bodies aren't ready to labor yet.
In this study, 37.9% of women of average BMI had minimal cervical dilation (2 cm or less) upon admission to the hospital. In comparison, 55.7% of "morbidly obese" women had minimal cervical dilation upon admission.
Yes, this is surely partly due to a higher rate of inductions and therefore less spontaneous labors in the high-BMI group, but it also suggests that perhaps this group is far less ready for labor when being induced. Again, many authors have noted this and have blamed it on "inefficient uterine contractility" or hormonal deficits, but what if there are other factors they are not considering?
Studies have shown that high-BMI women have longer menstrual cycles and longer gestations; perhaps what is happening is that their due dates are not being sufficiently adjusted for their longer cycle length and as a result, their bodies are less ready for labor when the doctors think they "should" be going into labor. And, as a result, they have more inductions, less cervical ripeness when induced, and more cesareans when the induction doesn't work.
More Interventions
The study found that there was more use of oxytocin augmentation and epidurals as BMI increased. The pit augmentation increase may reflect the lower level of cervical ripeness before labor, but it may also reflect the common perception among some clinicians that obese women won't labor sufficiently on their own. So they automatically just start pitocin augmentation, without ever looking at whether it's needed or not.
The increased epidural rate may simply reflect the higher rate of inductions and pitocin augmentations; it's hard to go through such induced and augmented labors with little mobility and not need some pain relief. However, it may also reflect the common practice of strongly encouraging early epidural placement in obese women to avoid a difficult placement later if a cesarean is needed.
(And of course, once that epidural is placed, oxytocin augmentation is often needed to compensate for the way that epidurals tend to slow labor. It can be an vicious circle.)
Lower Surgical Threshold
The study also shows that doctors were quicker to terminate labor early and move to a cesarean in "obese" women. In the study, the labors of "morbidly obese" women were terminated about an hour earlier than women of average BMI.
Some of this is understandable; surgery in a very heavy woman is more difficult and takes quite a bit longer than in a woman of average size. Doctors want to avoid an emergency situation where every second counts to save a baby, and especially so in a woman whose extensive adipose layers may require more time to get to the baby in the first place. Therefore, doctors may be more prone to intervene early in women of size, before things get to an emergency situation.
Yet most cesareans are not done under truly emergent conditions, and research shows that many women whose progress is slow are able to give birth vaginally if just given a little more time, and their babies generally do just as well.
So doctors have to walk a fine line between not waiting too late and not intervening too early. From this study, it looks like too many doctors are erring on the side of intervening far too early. And because cesareans are extra risky for women of size, this is a cause for concern ─ and a potentially modifiable variable for reducing the tremendously high cesarean rate in women of size.
Final Thoughts
As I've been saying for years, the high cesarean rate in obese women is not only about obesity itself, but also about the way that obese women are managed during pregnancy.
Sky-high induction rates, increased utilization of interventions during labor, and a very low threshold for surgical intervention all combine to ratchet up the cesarean rate in women of size.
The important thing to note is that these are all potentially modifiable factors for reducing the cesarean rate in this group.
Up till now, the only options most doctors saw for lowering the cesarean rate in women of size involved encouraging weight loss before pregnancy or restricting weight gain during pregnancy. Yet this research suggests that if doctors simply change their management practices and fear levels around women of size, it's likely that the cesarean rate can be lowered in this group without draconian weight restrictions.
In the Canadian study, the authors concluded:
Because of the potential morbidities associated with Caesarean section, we must modify our management approaches to allow equal opportunity for a vaginal birth for all women.Those are strong words for an obstetrical community that's usually pretty mealy-mouthed about these things, and frankly, it's nice to finally hear them from someone other than me. Bravo to these authors for being willing to advocate for vaginal birth for women of size at a time when some doctors are advocating pre-emptive cesareans across the board for this group.
But if doctors really want to get serious about allowing equal opportunity for a vaginal birth for fat women, first and foremost they need to crack down on the insanely high induction rates, as well as re-examining the use of interventions and threshold for surgery in this group.
References
J Obstet Gynaecol Can. 2011 May;33(5):443-8. Higher caesarean section rates in women with higher body mass index: are we managing labour differently? Abenhaim HA, Benjamin A. PMID: 21639963
Background: Higher body mass index has been associated with an increased risk of Caesarean section. The effect of differences in labour management on this association has not yet been evaluated.
Methods: We conducted a cohort study using data from the McGill Obstetrics and Neonatal Database for deliveries taking place during a 10-year period. Women's BMI at delivery was categorized as normal (20 to 24.9), overweight (25 to 29.9), obese (30 to 39.9), or morbidly obese (≥ 40). We evaluated the effect of the management of labour on the need for Caesarean section using unconditional logistic regression models.
Results: Data were available for 11 922 women, of whom 2289 women had normal weight, 5663 were overweight, 3730 were obese, and 240 were morbidly obese. After adjustment for known confounding variables, increased BMI category was associated with an overall increase in the use of oxytocin and in the use of epidural analgesia, and with a decrease in use of forceps and vacuum extraction among second stage deliveries.
Higher BMI was also found to be associated with earlier decisions to perform a Caesarean section in the second stage of labour.
When adjusted for these differences in the management of labour, the increasing rate of Caesarean section observed with increasing BMI category was markedly attenuated (P less than 0.001).
Conclusion: Women with an increased BMI are managed differently in labour than women of normal weight. This difference in management in part explains the increased rate of Caesarean section observed with higher BMI.
Obesity represents a rapidly emerging epidemic amongst pregnant women. Our study looks at the impact of morbid obesity on pregnant singleton nulliparous women in comparison with normal body mass index women. We conclude that morbid obesity is associated with a significantly higher risk of pre-existing medical conditions, developing antenatal complications, induction of labour, caesarean section and greater birth weight. However, there was no significant difference in caesarean section rates when adjusted for induction of labour. We also found no significant difference in length of hospital stay, postnatal complications and neonatal morbidity.
