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Maternal obesity linked to higher infant mortality

Outcomes better when mums are normal weight before and during pregnancy

Louise Prime

Wednesday, 03 December 2014

Babies are more likely to die in early infancy if their mothers were overweight or obese before and during pregnancy – particularly if they were severely obese – than if they were of normal weight, researchers have reported*. The author of an editorial** linked to the study, which is published today on bmj.com, said midwives and doctors should counsel mothers that outcomes are best when they are of a healthy weight before and during pregnancy.

Previous studies have shown inconsistent links between maternal obesity and infant mortality. So researchers led from the Karolinska University Hospital in Stockholm, Sweden, set out to investigate further. They analysed data from the Swedish Medical Birth Register, on 1,857,822 live single births in Sweden from 1992–2010, to look for associations between infant mortality rate and maternal body mass index before and during pregnancy. Mothers’ BMI in early pregnancy was recorded as underweight (≤18.4), normal weight (18.5-24.9), overweight (25-29.9), obesity grade 1 (30-34.9), obesity grade 2 (35-39.9) or obesity grade 3 (≥40).

During the study period there were 5,428 infant deaths, two-thirds of them during the first 28 days after the baby’s birth; causes of death included congenital anomalies, birth asphyxia, infections and sudden infant death syndrome (SIDS). After the researchers had adjusted for factors including mothers’ age, height, smoking, education and country of birth, as well as year of delivery, they found that among babies born at term, neonatal mortality rates rose with maternal BMI in early pregnancy.

Babies of mothers who in early pregnancy had been obese grade 2 or 3 were more than twice as likely to die as babies of mothers of normal weight (5.8 per 1000 in obesity grade 3, compared with 2.4 per 1000); even being mildly obese was associated with modestly increased risks. Infant mortality was also associated with obesity grade 2-3 in very and moderately preterm births.

Most (81%) of the deaths in term babies were caused by congenital anomalies, birth asphyxia, other neonatal morbidities, SIDS or infections. Compared with babies of normal weight mothers, those born to mothers with obesity grade 2-3 were at increased risk of death from congenital anomalies and SIDS.

The researchers estimated that – if the associations they found were causal – 11% of all infant deaths in their study were attributable to mothers’ overweight and obesity. They argue that there is enough evidence for public health officials now to take action against the obesity epidemic in the interests of improving infant health, until more work, including in other populations, is done to explore the associations.

They conclude: “Maternal overweight and obesity may be an important preventable risk factor for infant mortality in many countries.”

The author of an accompanying editorial also calls for more research into the association between maternal BMI and infant mortality, and into the effectiveness of interventions to reduce risk; though she agrees: “Midwives and doctors who are involved in antenatal care should counsel women about the risks associated with overweight and obesity, and help them set targets for gestational weight gain informed by pre-pregnancy body mass index. Specific guidelines for obese pregnant women should be followed as these pregnancies are higher risk and need monitoring throughout.”

* Stefan Johansson, et al. Maternal overweight and obesity in early pregnancy and risk of infant mortality: a population based cohort study in Sweden. BMJ 2014;349:g6572. doi: http://dx.doi.org/10.1136/bmj.g6572

** Katrine Mari Owe. Weight in early pregnancy and outcomes in early infancy BMJ 2014;349:g6850. doi: http://dx.doi.org/10.1136/bmj.g6850

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