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The link between pregnancy weight gain and big babies

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A close friend told me recently that at one of her early pregnancy check-ups, her obstetrician told her she shouldn’t gain more than 20 pounds. My friend is a thin, incredibly active and fit soon-to-be mother of three, so I was surprised to hear such strict advice.

But her doctor is not alone in scrutinizing pregnancy pounds. Over 60 percent of women in the U.S. gain more weight than recommended for their body mass index, according to the Institute of Medicine. This trend, alongside growing evidence that a mom’s weight affects her baby in the womb, has brought the issue front and center for a lot of physicians.

A study published this year in The Lancet – a collaboration between Columbia University and Children’s Hospital Boston – was the latest to show that mom’s weight gain can affect her baby’s health. The researchers tracked more than 500,000 women and over a million babies, finding a significant correlation between mom and baby’s weight.

Women who gained upwards of 53 pounds were more than twice as likely to have a heavy baby (at least 8.8 pounds) as women who gained between 18 and 22. Women who gained between 44 and 49 pounds were almost twice as likely to have a heavy baby.

Since parents pass on weight tendencies through their DNA, the researchers had to sift apart the effects of genes from the effects of the moms’ weight gain. So they compared birth weights of multiple kids with the same mom, and the relationship between pregnancy weight and birth weight still held up. If a mom gained more with one baby than the other, that baby was more likely to be heavy.

Birth weight and BMI (body mass index) later in life are related, so scientists and public health researchers think pre-programming of a baby’s weight before he’s born could be contributing to the nation’s obesity epidemic. It’s a cycle, they say – as the population gets heavier, more babies are coming out of the gate destined to be bigger and at higher risk for heart disease and diabetes.

Needless to say, moms don’t like being scolded for the size of their bellies. It’s a double whammy – weight is already a sensitive issue for many women, and the overwhelming amount of advice and warnings we get from the womb police while we’re pregnant is just piled on. It’s a vulnerable time, so adding a tsk-tsk to your regular weigh-in feels horrible.

We can’t ignore the science, but maybe we need to put things in perspective. Yes, there is strong evidence that pregnancy weight has an impact on babies’ health (the study populations are large, and new data keeps re-affirming the connection). Scientists think that a mom’s fat stores send chemical signals that cross the placenta and become part of the uterine environment, affecting the way the baby’s own fat cells and metabolism develop and possibly altering the brain in small ways that translate into weight tendencies.

But the real concern is on the extreme ends of the spectrum. For example, in the early part of the 20th century, when weight recommendations were closer to 15 pounds, the risk was underweight babies and, accordingly, in the 1970s doctors started encouraging moms to pack it on a little more. Now doctors are most concerned with the opposite – overweight moms who tip the scales too much in the other direction.

In a way, it’s not such a controversial idea after all: The extremes aren’t good for us. Or our babies.

In fact, researchers say it may be a woman’s pre-pregnancy weight that matters as much or more than what she gains with her baby, which is also part of the problem with telling moms to tighten up their belts. You can’t just set ideal weight standards and tell pregnant women to figure it out – moms need support for staying healthy in pregnancy. Sure, there is an element of indulging for some women who finally feel they can cut loose when they get pregnant. But we can’t deny that the same barriers to health for the general population – like a food system set up around processed foods, not whole ones – is also to blame.

But here’s something that might help take the spotlight off mom for a minute: Last month, a Nature paper suggested that dad’s diet affects his child’s health, too. Through chemical tweaks that take place in the sperm, fathers were found to pass on unhealthy dietary effects to their babies.

Researchers at the University of New South Wales in Sydney, Australia fed one group of mice an unhealthy diet and the others a well-balanced one. As expected, the ones they beefed up got fat and showed signs of type 2 diabetes. When the groups went on to have babies, though, the dads who had been overfed were more likely than the healthy ones to have babies with symptoms of type 2 diabetes.

It’s a provocative idea – that in addition to transferring genes that code for weight tendencies, dad could pass on the effects of his lifestyle, healthy or unhealthy. The researchers’ guess that ‘epigenetic’ factors are at play – these are not changes to a dad’s DNA code, but changes to the signals that turn genes on and off.

In the end, the question of weight is complex, and studies like these point to differences across the population, not necessarily in your home (which is why “but I gained more weight and my baby is fine” misses the point – that’s not how studies work). Of course we need to approach the topic with sensitivity and empathy for the bind some moms feel. But we can’t just say it’s unfair to focus on pregnancy weight when we’re so diligent about everything else we do for our babies.

The Institute of Medicine’s weight guidelines

BMI pre-pregnancy Recommended pregnancy weight gain
Underweight
<18.5
28 to 40 lbs
Normal
18.5 24.9
25 to 35 lbs
Overweight
25 29
15 to 25 lbs
Obese
>30
11 to 20 lbs

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