How obesity during pregnancy affects mothers and babies

Our scientists are working on a major trial to deepen understanding of how obesity during pregnancy affects mothers and babies, which we can use to find new and better ways of improving families’ long-term health.
  • Authors list

    Professor Lucilla Poston, Dr Annette Briley, Dr Dharmintra Pasupathy, Dr Nashita Patel, Mr Paul Seed, Claire Singh MSc, the UPBEAT consortium

This project took place at our London centre which operated between 1995 and 2021.

Across the UK, 1 in 5 pregnant women are obese. Being obese (having a BMI over 30) during pregnancy can raise the risks for mothers and children, with lifelong consequences. Our research into obesity aims to make pregnancy safer and lower the risks facing these families.

Miscarriage and stillbirth are more common among women who are obese. When pregnancy does progress, obese mothers-to-be are 3-6 times more likely to have gestational (pregnancy-related) diabetes, and their chances of developing pre-eclampsia are also heightened – both very serious complications which can be life-threatening and raise the risk of premature birth

Obesity during pregnancy increases the likelihood of babies that are too large at birth, which can lead to problems during labour. Babies born to obese mothers are more likely to grow up to be obese, and will be at higher risk of diabetes and heart problems in their later life.

UPBEAT followed thousands of obese mothers all over the UK through pregnancy and beyond to see who experienced problems and test different potential solutions. There have been many strands to the study over many years. For example, our researchers have investigated:

  • Which tests are most accurate in predicting who will develop gestational diabetes, so that professionals can work with expectant mothers to prevent it if possible or treat it if not
  • Levels of stress hormones in obese mums-to-be – not just how and why these differ from healthy pregnancies, but whether they could pass the placenta and pose risks to babies
  • If the glucose (sugar) levels in obese mothers’ blood during pregnancy can be measured and controlled to improve their babies’ chances of being born a healthy weight
  • Whether changing their diet and exercise during pregnancy could help obese mums-to-be reduce their risk of gestational diabetes and improve their babies’ future health
  • Relationships between obese mothers’ weight and their children’s size at birth, and the role gestational diabetes may play in some babies growing more than others
  • Potential links with inflammation and iron deficiency, which other research suggests could explain some of the increased risks associated with obesity during pregnancy
  • When obese mothers are more at risk of postpartum haemorrhage (losing 500ml of blood or more after childbirth) so care staff can deliver their baby in the safest way 
  • How the various diseases related to obesity and pregnancy work at a cellular level, using a national bank of biological samples collected from mothers throughout the study
  • The influence of fathers on the future weight and health of babies born to obese mothers 
  • Why mothers with a higher BMI are more likely to stop breastfeeding sooner, and potential long-term implications for the health of these mothers and their babies
  • Chemical changes in the DNA of babies born to obese mothers, comparing samples from umbilical cord blood at birth to see if obesity can be genetically ‘programmed’ in the womb
  • Lifelong implications for the health of babies with obese mothers, following up with children over the years and tracking vital signs like blood pressure and kidney function

Even though obese mothers are more likely to suffer pregnancy complications, most will be completely fine – but unfortunately it’s often difficult to tell who is really at risk, so have to err on the side of caution, which can involve unnecessary trips to the doctor and stress for all involved. A major focus of UPBEAT has been finding better ways to predict any pregnancy complications, and going forward we will continue testing how to assess risk more accurately, so that mothers all get the right care and health professionals can make the best use of precious resources.

Researchers studied an ethnically and socially diverse group, which meant they could also see if and how these factors affected obese mothers’ health habits. UPBEAT found mums-to-be of different ethnicities tended to follow different eating patterns, and those who were younger or living in poorer areas generally had unhealthier diets. Certain behaviours, such as eating more processed food, made these already high-risk mothers even more likely to develop pregnancy complications.

The results of the trial show that obese pregnant women face challenges in changing their health and habits, but also that they are both willing and able to make these changes. Putting this research into practice, UPBEAT can help professionals to offer personalised care and support to obese mothers throughout pregnancy and beyond.

Tommy's researchers across the UK are investigating the reasons for pregnancy complications and loss. Pregnancy research is under-funded, and we need your support to continue. Sign up here if you're interested in being kept updated about the latest research from Tommy's.