Pre-pregnancy Obesity and Risks

What midwives need to know about pre-pregnancy obesity.We have been discussing pre-pregnancy obesity, and today we turn our attention to the effects of obesity on pregnancy.  Before we begin, I just want to make sure we recognize the difference between pre-pregnancy obesity and excess gestational weight gain. Both bring increased risk to pregnancy, but they are different risks.

Which brings up another point to remember, these are only risks which are measured by the odds of something happening.  A risk is not an absolute, it is only an estimate of the potential for a problem. The odds of an event happening is measured by looking at a large number of people and seeing how many have any specific problem.  But an individual doesn’t get 30% of a problem – they either have the problem or they do not. This is risk.

When I read popular literature about obesity and pregnancy, I read about a very limited set of problems.  These problems begin with the labeling of obesity as a risk category for pregnancy, which allows the mother fewer options and results in unnecessarily high cesarean rate for obese women. My concern with this line of reasoning is not that it is necessarily wrong, but that it is dreadfully incomplete and gives the false impression that obesity adds no REAL risks to the health of mother or baby.

Pregnancy Induced Hypertension and Gestational Diabetes

We know the adipokines (hormones produced by adipose tissue) increase risk for hypertension, and we see this remains true in pregnancy and postpartum.  Women with a BMI >30 have a diminished vascular responsiveness during pregnancy. And Instead of a normal postpartum dip in blood pressure, their blood pressure seems to be reset at a higher level. Estimates of hypertension in pregnancy increase with BMI from less than 1% to about 17% as BMI increases.

In fact, the odds of pregnancy hypertension appears to increase with increasing BMI.  So while overweight women have an odds ratio of 1.99 (about twice as likely to have pregnancy induced hypertension),  women with BMI >40 have an odds ratio of 4.26 or more than 4 times the odds of developing hypertension. In this study on women with BMIs over 40 (33% were having their first baby), they found 1/3 of the women were admitted to the hospital during their antenatal care – 61% of those admissions were for hypertension treatment.

This still means about 80% of women with BMIs >40 will not develop PIH.  But remember, 20% is the same as 1 in 5.  This means that while on average 1 out of 100 normal weight women will develop PIH, 1 in 20 obese women and 1 in 5 women with a BMI >40 will develop PIH.

Just like hypertension, the odds for gestational diabetes increases with increasing BMI.  Women with a BMI over 40 had over 3 times the odds of developing gestational diabetes. This means that while we expect about 1 in 25 normal weight women will develop gestational diabetes, about 1 in 10 obese women will develop gestational diabetes.

And these numbers are probably low estimates.  A different study found the prevalence of gestational diabetes was 19% in women with BMI of 40 or higher.  This means 1 in 5 women with BMI at or above 40 developed gestational diabetes in this study.

We can debate appropriate cut-off levels for hypertension or diabetes.  We can be frustrated that we don’t have better treatment options for women.  But we cannot ignore that the physiological effects of obesity change a woman’s ability to maintain her health in pregnancy.

What about other risks?

Studies repeatedly show risks such as sepsis and thrombo-embolism have a higher odds of occurring for obese women. And this makes sense when we consider the effect of the adipose hormones.  Remember they play a role in immune function and clotting factors – so we could expect if the levels of fat hormones are out of whack we would see more infection and thrombo-embolism problems.

Particularly frightening is the strong associations between obesity and preeclampsia.  The odds ratios are the same as the Pregnancy Induced Hypertension ratios with women in the highest BMI category more than four times as likely to develop preeclampsia.   And while a nullipara had to have BMI>40 to be in the highest risk group, a multip only needed to have a BMI>30.

Remember, some women will only have one problem while others will have multiple.  The best estimates are that 1 out of 3 obese women will not have a normal pregnancy.  That means 2 out of 3 will continue without problems.

And, we know obese women are more likely to give birth via cesarean, with severely obese (BMI>40) with at least 3 times the odds of giving birth via cesarean.  Yes, there is a difference in elective cesarean, but the rates of emergency cesarean (or decision to cesarean made after labor has started) are actually higher than the elective cesarean group.

One of the problems for an obese woman is that it is going to take her longer to labor.  It is not  a difference in uterine contractility. The contractions themselves take longer to open the cervix in obese women. In fact, it even takes longer for an obese woman to even start labor.  So there is one silver lining – obese women actually have a lower rate of spontaneous preterm birth.

I could not find hypotheses about why the contractions of the same strength are not effective on the cervix of an obese woman.  But regardless of the reason, it takes about 1.2 hours more for an obese nullip to reach 10cm. And because of this “slow to start” labor reality, an obese woman is 2.5 times more likely to be induced – but this is also due to the higher rates of induction for preeclampsia.

But the higher rates of cesarean add another concern for obese women.  The odds of epidural failure is higher, but so is the failure rate for tracheal intubation to allow for general anesthetic. Higher rates of wound infection have led to recommendations for higher or longer doses of prophylactic antibiotics and different surgical closure methods.  As we said, an obese woman has much higher odds of a clot. And when she has her next baby, her odds of VBAC failure nearly double.

Risks for the Baby

Obese women are more likely to have infants with congenital deformities like neural tube defects and congenital heart disease. The odds for congenital heart disease increase with increasing BMI, so while there is a 25% increase for women with BMI 30-39, there is a 50% increase for women with BMI >40.

Obese women have at least a 50% increase and possibly over 200% increase in odds for neural tube defects.  In fact, in the UK guidelines now recommend a 5mg daily folic acid supplement for pregnant women with BMI >30.

But dysfunctional placentas are not only affecting development.  They seem to be affecting the ability of the baby to survive.  It turns out, stillbirth is nearly twice as likely in obese compared to non-obese women – In the UK (where better public health records are kept) almost 1/3 of the women who have a stillborn infant or dies in the neonatal period is obese.


The increased risks for both mother and baby with pre-pregnancy obesity are real. But remember, these are risks – not absolutes.  Tomorrow we will talk about what midwives can do to help women reduce the risks.