Does being heavier impact the weight of your newborn? A recent medical journal article says so, finding evidence that a mum’s weight, blood sugar levels and blood pressure directly affect the birth weight of her baby. Scientists looked at data from more than 30,000 women and their babies and found that being overweight or obese in pregnancy causes babies to be born larger. The findings were recently published in JAMA, the Journal of the American Medical Association. It’s the same outcome for babies born to mums with high blood sugar, which can lead to gestational diabetes – yet high blood pressure causes infants to be smaller, because of decreased blood flow delivering fewer nutrients.

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Study authors Rachel Freathy, of the University of Exeter Medical School, and Debbie Lawlor, of the University of Bristol, say previous studies showed that mothers of a higher weight at the start of their pregnancy had babies who were more likely to be heavier, but it was not made clear whether the mother was the cause of her baby being bigger at birth. This unique study used genetics instead of just relying on observations of the women’s weight. “In our study, we analysed a set of gene variants known to be associated with BMI and we assigned a genetic score to each woman, based on how many BMI-raising variants she carried,” the researchers told Fit Pregnancy.


“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. “Women who weigh more tend to have higher levels of glucose in their blood, and this extra sugar is one of the key factors linking a heavier mother to a heavier baby.” However, testing for levels of certain fats and high cholesterol found no evidence that these influenced the growth of the baby. A key finding was that even though women who are heavier tend to have higher blood pressure, the blood pressure causes babies to be born smaller, which contrasts with higher BMI.

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“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,” say Freathy and Lawlor. The researchers hope doctors will be better able to manage these factors in their patients. Higher and lower birth weights carry risks for newborns, such as birth injury and blood sugar problems for big babies and breathing and developmental issues for small babies. Routine prenatal check-ups, eating a healthy diet and doing moderate exercise can ensure a healthy birth weight for your baby. “A healthy birth weight reflects how well the baby has grown and developed in the womb,” Freathy and Lawlor say. Have a read of our recent post that explains what really happens to your body after having a baby and what you can do about it.

By Natalie Esler from via Fit Pregnancy