A large new study provides even more compelling reasons for women planning a pregnancy to make sure they are in the normal weight range.
 
A recently published review of nearly 750,000 women who had babies in Washington State showed that being over or underweight is associated with a small but significant increase in the risk of major health problems affecting the mother, including death.
 
In a retrospective review of all single babies, born in hospital, mothers were matched for a number of other characteristics including age, education level, marital status, ethnicity, smoking during pregnancy and whether they received help to get pregnant. In all cases, having a BMI out of the normal reference range was associated with a worse outcome for these women.
 
Underweight women were 1.2 times more likely to suffer significant complications while overweight women showed a steady weight-related increase in risk up to 1.4 times that of their normal weight counterparts.

While the study was undertaken in the US, Australian figures are likely to be similar and there is no reason to expect the experience in our country to be different.

While this association is important and interesting, the study is not able to say that the increased weight caused the problems, but does suggest a statistically significant association which makes trying to avoid the association worthwhile.
 
What women eat in pregnancy has been known to be important for some time, perhaps it is now important for women to also watch how much they eat.

The study did not make clear when in the pregnancy the BMI was measured and may be a reflection of the health of women when they embark upon a pregnancy, how much weight they put on during the pregnancy or a combination of both.

The absolute risk of major complication or death remains low but this study provides a further incentive for women planning a pregnancy to make sure they are in the normal weight range. Women planning a pregnancy are well advised to discuss issues around weight and diet with their doctor before embarking on a pregnancy.

This study analysed linked population data from all singleton hospital births in Washington State from 2004 – 2013 from hospital reporting systems and the births and death registrations. 743, 630 women were included in the study.

There was a dose response relationship between maternal pre-pregnancy weight and severe maternal morbidity or mortality. These outcomes included life threatening conditions, complications requiring intensive care admission and maternal death.

Women who were underweight, overweight or obese prior to pregnancy had increased risks. Although the risks were significantly increased, the absolute rates of these conditions are uncommon. The absolute increased numbers of severe morbidity or death per 10,000 women were: 28.8 for underweight women, 17.6 for overweight women, and 24.9, 35.8 and 61.1 for women with obesity classes 1, 2 and 3.
 
Large population studies like this are essential to assess the effect of pre-pregnancy BMI on rare but serious outcomes. In Australia 1 in 2 women enter pregnancy above a normal BMI; on a national basis managing weight prior to pregnancy has the potential to save mothers lives.

This very large cohort study of more than 740,000 women giving birth over 10 years is reporting a combined measure of severe health complications for women by pre-pregnancy BMI category. BMI is a ratio of height to weight and used to estimate if a person is underweight, of healthy weight, overweight or obese.

This study identifies a small but significantly increased risk of severe health complications in women considered underweight and women considered overweight or obese compared with women who were of normal weight before pregnancy. The risk increases with increasing BMI category. The severe and life threatening health complications considered included severe blood loss at birth, hysterectomy, amniotic fluid embolism, admission to intensive care and blood clots.

This study also found that the risks of many less severe but more common pregnancy complications increase with maternal pre-pregnancy BMI, including caesarean birth, high blood pressure and gestational diabetes. In this study, women with the highest BMI were 4-5 times more likely to be diagnosed with gestational diabetes and 3-4 times more likely to have pregnancy-induced high blood pressure compared with women with a normal BMI.

Whilst this study highlights the risk of pregnancy complications associated with overweight and obesity, studies are urgently needed to evaluate strategies to reduce these complications including reducing weight and improving health before pregnancy.