How Your Pregnancy Weight Directly Affects Your Baby's Size

Research is getting closer to understanding what causes the pregnant person's weight to impact how big baby is at birth—and what that means for your child's future health.

You've probably already been advised by your doctor to maintain a healthy weight while trying to conceive and during pregnancy. But although doctors know that a pregnant person's weight is linked to baby's, the reasons behind the association haven't been fully understood.

One study in the medical journal JAMA, though, sheds some light on the factors that connect the two—and found clear evidence that the pregnant person's weight, blood sugar levels, and blood pressure directly affect their baby's weight. Learn more about how your pregnancy weight directly impacts the size of your baby.

Why the Pregnant Person's Weight Matters

Scientists looked at data from more than 30,000 pregnant people and their babies in 18 studies.

"Many studies have shown that mothers who are [of a higher weight] at the start of their pregnancy have babies who are more likely to be heavier—but is it not clear whether the mother being [heavier] causes her baby to be bigger at birth," study authors Rachel Freathy, Ph.D., of the University of Exeter Medical School, and Debbie Lawlor of the University of Bristol, both in the U.K., said.

"If mothers' [excess weight] does cause their babies to be heavier at birth, why this happens is not clear." By analyzing the data, they found that being overweight or obese in pregnancy does cause babies to be born larger, and so does having higher blood sugar levels, which can lead to gestational diabetes.

But, the actual levels of fat in the blood (lipids), which lead to high cholesterol, do not seem to have an effect on baby's weight. High blood pressure actually causes infants to be smaller, likely because decreased blood flow allows fewer nutrients to get to baby.

What makes this study unique was that it used genetics instead of just relying on observations of the women's weight.

"In our study, we analyzed a set of gene variants known to be associated with [body mass index] and we assigned a 'genetic score' to each woman, based on how many BMI-raising variants she carried," Freathy and Lawlor say. "BMI can be influenced by many things besides genetics, for example diet and exercise levels. However, the genetic scores cannot be changed by lifestyle factors.

So when we saw an association between the mother's genetic score for BMI and birth weight, we could be confident that the effect on birth weight was due to her BMI, and not due to lifestyle or other factors. We did the same with different genetic scores for other factors such as glucose and lipid levels." By examining these genetic associations, the researchers found stronger evidence for a cause and effect on baby's size.

Why Your Weight Impacts Your Baby's

Once they proved the cause and effect, the researchers looked at the question of why. "Women who weigh more tend to have higher levels of glucose [sugar] in their blood, and this extra sugar is one of the key factors linking a heavier mother to a heavier baby," Freathy and Lawlor say. The sugar causes the baby to produce more insulin, which acts a growth hormone.

But, the authors say, this is only part of the story. "Other mechanisms are likely to be at play, and at present we do not know what these are," they say.

After testing for levels of certain fats [triglycerides] and high cholesterol, they found no evidence that they influenced the growth of the baby. "There are many other factors related to being overweight or obese, for example the levels of fatty acids in the blood, not available to test in our study, which could be important for growth of the baby," Freathy and Lawlor say. "These will need to be tested in future studies."

In the case of higher blood pressure, its effects seemed to counteract the effects of the women's BMI. "A key finding from our study was that even though we know women who are heavier tend to have higher blood pressure, the blood pressure itself causes babies to be born smaller, which contrasts with higher BMI or glucose causing larger babies," Freathy and Lawlor say. "This shows there are complicated factors at play, and demonstrates that monitoring blood pressure in pregnancy is just as important as maintaining a healthy weight or glucose levels."

How to keep baby's size on track

With this clearer proof for how mom's weight, blood sugar and blood pressure affect baby's birth weight, the researchers hope doctors will better be able to manage these factors in their patients. Higher and lower birth weights have risks for newborns, such as birth injury and blood sugar problems for big babies, and breathing and developmental issues for small babies.

Also, "being born very small or very large is associated with risk of diseases such as type 2 diabetes in later life, but the causes underlying these associations are poorly understood," Freathy and Lawlor say. "It may be that the environment in the womb is responsible for these increased risks, and this is something we would like to test in future studies."

If you're overweight or obese and are trying to conceive or are already pregnant, talk to your doctor about how best to manage your weight in order to keep your baby's size on target. Pregnancy is not a time for dieting, but your doctor can give you strategies for healthy eating and moderate exercise that can help ensure a healthy birth weight for your baby. In addition, the authors say to make sure to have routine prenatal checkups, which include checking blood pressure and glucose.

"A healthy birth weight reflects how well the baby has grown and developed in the womb," Freathy and Lawlor say. "It is important for babies' health and well being in the first year of their life," and beyond.

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