Metformin for gestational diabetes does not increase preterm births, study results suggest 

Pregnant women treated with metformin for gestational diabetes experience no increase in preterm births, study results show.

The study of 535 pregnant women, published in the Journal of the American Medical Association on 3 October 2023, also revealed no differences in adverse neonatal outcomes, including the need for intensive care treatment for newborns, respiratory support, jaundice, congenital anomalies, birth injuries or low sugar levels.

Gestational diabetes, which affects almost 3 million women worldwide each year, is characterised by elevated blood sugar levels during pregnancy, posing increased risk of high blood pressure and pre-eclampsia for mothers, and health risks for their babies. 

In the randomised trial, 268 women received metformin and 267 received placebo.

The study results showed that birth in the two groups of women occurred at the same mean gestational age (39.1 weeks) and there was no evidence of any increase in preterm birth, defined as birth before 37 weeks, among those who received metformin. 

While there was a slight reduction in infant length (0.7cm) in the metformin group, there were no other significant differences in baby measurements. There were slightly more babies who were small at birth but this did not reach statistical significance. 

The researchers also found that women given metformin were 25% less likely to need insulin and, when insulin was necessary, it was started later in the pregnancy.

Lead author Fidelma Dunne, professor of medicine at University of Galway, Ireland, and consultant endocrinologist at Saolta University Health Care Group, described the study as “a significant step forward for women with gestational diabetes”.

A previous trial comparing metformin, which is routinely used in the treatment of type 2 diabetes mellitus, with insulin, published in October 2020, had “found it to be effective, yet concerns remained, especially regarding preterm birth and infant size”, Dunne said.

She added that gestational diabetes has traditionally been managed initially through dietary advice and exercise, with insulin introduced if sugar levels remain suboptimal.

“While effective in reducing poor pregnancy outcomes, insulin use is associated with challenges, including low sugars in both the mother and infant — which may require neonatal intensive care — excess weight gain for mothers, and higher caesarean birth rates,” Dunne said.

Dunne added that metformin had “emerged as an effective alternative for managing gestational diabetes, offering new hope for expectant mothers and healthcare providers worldwide”.

Nuttan Tanna, pharmacist consultant in women’s health and osteoporosis at London North West University Healthcare NHS Trust, said the research would “help us to offer pregnant women with gestational diabetes a choice between two treatment options to ensure a reasonably healthy pregnancy: either insulin or metformin.”

Alanna Linkhorn, midwife for pregnancy charity Tommy’s and senior NHS midwife, said: “It is great to see this research providing evidence for further interventions to support women and birthing people who develop gestational diabetes during pregnancy.”

Results from a study published in The Lancet in July 2015 demonstrated that giving metformin to women with obesity during pregnancy had no significant impact on babies’ birthweight, termination of pregnancy, stillbirth or neonatal death.