Kids whose moms were overweight during pregnancy have increased odds of being overweight themselves – but the connection may be largely genetic, a new study suggests.
Risk factor for complications
The implication, researchers said, is that overweight women are unlikely to influence their kids' future weight by shedding pounds before pregnancy.
But they also stressed that more research is needed to confirm their findings.
And no one is suggesting that a woman's weight before and during pregnancy is unimportant.
There are plenty of reasons to go into pregnancy at the healthiest weight possible, said Rebecca Richmond, the lead researcher on the study.
Read: Pregnant and overweight – the facts
A high body mass index (BMI) raises the risk of pregnancy complications such pre-eclampsia and gestational diabetes, explained Richmond, a senior research associate at the University of Bristol in England.
Plus, she said, those extra pounds boost the odds of having an abnormally large newborn – another risk factor for complications.
The new study, published online in PLOS Medicine, tried to address a question raised by past research: Do pregnancy pounds, in and of themselves, affect a child's weight in the long run?
BMI explained by genes
Pregnancy weight does affect birth size, Richmond said. But, she added, birth weight is not "deterministic", and bigger newborns are not necessarily going to become bigger kids.
Richmond and her colleagues focused on over 6,000 mother-child pairs who were taking part in two long-term health studies. The kids' body mass index (BMI) was recorded throughout childhood and adolescence. BMI is a measure that roughly estimates body fat, using weight and height, and in children, age and sex. In general, the higher someone's BMI, the more body fat they have.
Read: Body Mass Index (BMI)
The researchers found there was a correlation between moms' pre-pregnancy BMI and kids' BMI across the age span.
But it seemed to be mostly explained by genes.
Using blood samples from mothers and their children, the researchers gave each pair a "genetic risk score". That was based on 32 gene variants that have been strongly linked to BMI in past studies.
In the end, Richmond's team found, the genetic risk score largely accounted for the higher BMI among kids of overweight moms.
However, the findings shouldn't be considered the final word, said Dr Siobhan Dolan, an obstetrician-gynaecologist and medical advisor to the nonprofit March of Dimes.
Risk of chronic conditions
"Disentangling what's genetic and what's environmental is challenging," said Dolan, who wasn't involved in the study.
For example, she said, kids can also "inherit" behaviours such as eating habits from their parents.
Dolan agreed that there are already "compelling reasons" for women to head into pregnancy at the healthiest weight possible. And those reasons go beyond curbing the risk of pregnancy complications.
In the long run, Dolan said, excess weight can raise a woman's risk of chronic conditions such as type 2 diabetes and heart disease.
Read: Prevent type 2 diabetes
Pregnancy may only compound the issue if a woman gains too much weight – those pounds can be tough to shed after giving birth, Dolan noted. That's why, she said, it's important to try to follow guidelines on weight gain during pregnancy.
According to the Institute of Medicine, overweight and obese women should put on fewer pounds during pregnancy, compared with normal-weight and underweight women.
Obese women should gain around 11 to 20 pounds during pregnancy, says the IOM – an expert panel that advises the US federal government. Overweight women can gain a bit more, but no more than 25 pounds.
Another important point, Dolan said, is that genes are not destiny: Even if kids inherit genetic variants that raise the odds of obesity, that can still be countered with a healthy diet and regular exercise, she said.
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