Maternal Weight, Blood Glucose and Blood Pressure Affect Baby’s Birth Weight Interview with:
Debbie Lawlor PhD
School of Social and Community Medicine
University of Bristol, Oakfield House, Oakfield Grove
Medical Research Council Integrative Epidemiology Unit
University of Bristol, UK and
Rachel Freathy PhD,
University of Exeter, Institute of Biomedical and Clinical Science,
Royal Devon and Exeter Hospital,  Exeter  UK What is the background for this study? What are the main findings?

Response: A healthy birth weight is important for babies’ health and wellbeing in the first year of their life. It reflects how well the baby has grown and developed in the womb. The experience of fetuses in the womb and how well they grow and develop might also determine their future health, even into adulthood. Both being too light or too heavy at birth is not good for the baby. Lots of studies have shown that mothers who are fatter at the start of their pregnancy have babies who are more likely to be heavier. But is it not clear whether the mother being fatter causes their baby to be bigger at birth. If mothers’ fatness does cause their baby to be heavier at birth, why this happens is not clear.

We used genes to find out whether being fatter in pregnancy causes babies to be born heavier. We also tested whether risk factors in the mother that are affected by her fatness, such as her blood pressure, and the level of glucose (sugar) and lipids (fats) in her blood stream affect how heavy her baby is.

Our results showed that being fatter during pregnancy did cause a mothers’ baby to be born heavier. We also showed that having higher blood levels of glucose in pregnancy also caused a mothers’ baby to be heavier. But we did not find any effect of mothers’ blood levels of lipids in pregnancy on their baby’s weight. Whilst mothers who are heavier in pregnancy will tend to have higher blood pressure in pregnancy we found that higher blood pressure caused the women’s babies to be lighter. What should clinicians and patients take away from your report?

Response: We want babies to be a healthy weight, not too heavy and not too light. As populations have been getting fatter over the last 10-20 years, pregnant women have also got fatter and this has been associated with more babies being born unhealthily heavy. Our study shows this is cause and effect and that if mothers were lighter on average their babies would possibly be lighter.

But our study also showed that things are a little bit more complicated because the heavier mothers tend to have higher blood levels of glucose which causes babies to be heavier, but they also tend to have higher levels of blood pressure which cause their babies to be lighter. So it might be just as important to monitor mother’s blood glucose and blood pressure during pregnancy to make sure they are both at levels that support healthy growth of their baby. What recommendations do you have for future research as a result of this study?

Response: In this study we have used a method called Mendelian randomization which uses genes to tell us about the true causal effects of modifiable (non-genetic) risk factors, like fatness, glucose and blood pressure.

As well as being interested in whether mother’s fatness and other risk factors in pregnancy are associated with birth size we are interested in whether there is a lasting effect across their child’s life. i.e. we are interested to know if the children of mothers who are fatter and those with higher glucose in pregnancy are also more likely to be fatter and have problems with diabetes and heart disease throughout their lives. If mothers’ pregnancy risk factors affect their children throughout their lives it becomes even more important to ensure women are very healthy in their pregnancies. If being fatter in pregnancy meant that a mothers’ children were also fatter for all of their lives then their daughters would start their pregnancies fatter and the problem could cycle through generations and make it extremely hard to stop the obesity epidemic. But we do not know if this happens or not.

In the future we would like to use our genetic method to test whether there is this long-term effect – including seeing if it continues through to grand-children. But that will require even bigger numbers of participants than we have used in this study. Is there anything else you would like to add?

Response: Just to say that this study would not have been possible without the collaboration of a large number of scientists from different countries and with the support of a lot of research funders and we thank all of them. Most importantly we thank all of the participants. This research was done in a number of what we call ‘birth or pregnancy cohorts’ which are studies where we recruit pregnant women and then follow them, their children and sometimes other family members for as long as we can, collecting lots of information and clinic tests on them. These cohorts make a big difference to health research and we are very thankful for everyone who gives up their time to participate in these studies. Thank you for your contribution to the community.


Tyrrell J, Richmond RC, Palmer TM, et al. Genetic Evidence for Causal Relationships Between Maternal Obesity-Related Traits and Birth Weight. JAMA. 2016;315(11):1129-1140. doi:10.1001/jama.2016.1975.

Note: Content is Not intended as medical advice. Please consult your health care provider regarding your specific medical condition and questions.

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Dr. Debbie Lawlor and Dr. Debbie Lawlor (2016). Maternal Weight, Blood Glucose and Blood Pressure Affect Baby’s Birth Weight

Last Updated on March 18, 2016 by Marie Benz MD FAAD