Some have argued that obese women are more likely to have stillbirths because their extra fat makes ultrasound scans less accurate, leaving more problems unnoticed. Anne Evans at Oregon Health & Science University in Portland and colleagues suspected the problem might be more fundamental. So they studied what happened to obese Japanese macaques during pregnancy.
They began by feeding 15 female macaques a high-fat diet and nine others a standard "monkey chow" diet. Nine of the macaques on the high-fat diet became obese with insulin resistance, while six remained at a relatively healthy weight.
The team then mated the macaques over several seasons. After 120 days into each pregnancy they measured blood flow into the uterus and placenta – the organ that connects the fetus to the uterine wall and provides it with nutrition.
Blood flow to the uterus was 38 to 56 per cent lower in all the macaques on a high-fat diet compared with the healthy-eating animals. Blood flow to the placenta itself, meanwhile, was 32 per cent lower in obese monkeys than in those on the balanced diet. Samples of placental tissue from obese animals also showed signs of damage.
Ten days later, the monkeys' infants were delivered by caesarean section. Among the obese mothers, seven out of 20 pregnancies were stillborn, compared with only one stillbirth from 26 pregnancies in the healthy diet group, and one in 13 pregnancies from the macaques who maintained a healthy weight, despite the high-fat diet.
According to Evans, poor nutrition can probably damage the placenta, given that high-fat diets, regardless of subsequent obesity, lowered blood flow to the uterus in the monkeys. Subsequently, "maternal obesity and insulin resistance further exacerbates placental dysfunction" resulting in more stillbirths, she says.
David Ellwood, an obstetrician at the Australian National University in Canberra, agrees that "placental dysfunction is the most likely explanation" for the increased risk of stillbirths amongst obese mothers.
He speculates that high-fat diets may increase the fat content of placental tissue, which could block blood flow and so limit nutrition to the fetus.
Journal reference: American Journal of Obstetrics and Gynecology, DOI: 10.1016/j.ajog.2010.10.009
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