Higher BMI Linked to Pregnancy Risks in Women With Type 2 Diabetes, Study Finds

Higher pre-pregnancy body mass index (BMI) may be linked to increased pregnancy risks among women with pre-existing Type 2 Diabetes (T2DM), according to an Australian study. Researchers found that each 1 kg/m² increase in BMI was associated with an 8.2% increase in the odds of pre-eclampsia among women with T2DM. Higher BMI was also associated with preterm delivery in this group.

The study examined 264 singleton pregnancies affected by pre-existing diabetes, including both type 1 diabetes (T1DM) and type 2 diabetes. The findings suggest that BMI may influence pregnancy outcomes differently depending on the type of diabetes involved, highlighting the importance of individualised care before and during pregnancy.

What Did the Study Examine?

Researchers analysed prospectively collected pregnancy data retrospectively from a quaternary hospital in metropolitan Sydney, Australia. The pregnancies were managed between 2010 and 2019.

The study included 264 singleton pregnancies involving women with pre-existing diabetes. Of these, 124 pregnancies were affected by type 1 diabetes and 140 by type 2 diabetes.

The researchers separately examined the T1DM and T2DM groups to understand whether increasing BMI was associated with different maternal and pregnancy outcomes.

Overweight and obesity were substantially more common among women with T2DM. Around 90% of women in the T2DM group had overweight or obesity, compared with 46% among women with T1DM.

Higher BMI and Pre-Eclampsia Risk in Type 2 Diabetes

One of the key findings was the association between BMI and pre-eclampsia among women with type 2 diabetes.

The researchers found that every 1 kg/m² increase in BMI was associated with an 8.2% higher odds of pre-eclampsia in the T2DM group.

Pre-eclampsia is a pregnancy complication characterised by high blood pressure and other signs of organ involvement, usually developing after 20 weeks of pregnancy. It can become serious for both the mother and baby if not recognised and managed appropriately.

Women with pre-existing diabetes already require close monitoring during pregnancy because diabetes can increase the likelihood of several pregnancy complications. The study suggests that higher BMI may add another layer of risk among women with T2DM.

Preterm Delivery Also Linked to Increasing BMI in T2DM

The study also found an association between increasing BMI and preterm delivery among women with type 2 diabetes.

Preterm birth refers to birth occurring before 37 completed weeks of pregnancy. Babies born prematurely can face an increased risk of complications, particularly when birth occurs significantly earlier than expected.

Importantly, an association observed in an observational study does not necessarily prove that increasing BMI directly causes preterm delivery. Pregnancy outcomes can be influenced by several factors, including maternal health, diabetes control, blood pressure, medications, socioeconomic circumstances and pregnancy complications.

What Happened in Type 1 Diabetes?

The relationship between BMI and pregnancy outcomes was different among women with type 1 diabetes.

In the T1DM group, overweight and obesity were associated with poorer attainment of first-trimester HbA1c targets. HbA1c provides an estimate of average blood glucose levels over the preceding weeks to months and is an important measure of diabetes management.

Maintaining appropriate glucose levels before conception and throughout pregnancy is an important part of reducing diabetes-related pregnancy complications. The finding suggests that weight status may be relevant to early glycaemic management in women with T1DM.

Higher BMI was also associated with a greater likelihood of caesarean delivery among women with type 1 diabetes.

Why Does Pre-Pregnancy BMI Matter?

BMI is commonly used as a screening measure to classify weight status according to height and weight. While it does not directly measure body fat or fully capture an individual’s metabolic health, it is frequently used in population-level research.

Pre-pregnancy BMI can be particularly relevant in women with diabetes because pregnancy already creates major metabolic changes. Insulin requirements, blood glucose management, blood pressure and nutritional needs can all change as pregnancy progresses.

When diabetes and higher BMI occur together, clinicians may need to consider multiple risk factors rather than focusing on glucose levels alone.

Pregnancy With Pre-Existing Diabetes Requires Close Monitoring

Both type 1 and type 2 diabetes can increase the risk of pregnancy complications. These may include problems related to blood glucose control, high blood pressure, pre-eclampsia, delivery and newborn health.

The Australian findings are therefore relevant to the broader question of how pregnancy care can be personalised according to a woman’s metabolic profile.

For women planning pregnancy while living with diabetes, preconception care can provide an opportunity to review glucose management, medications, blood pressure, nutrition and other health factors before conception.

Study Findings at a Glance

Does This Mean BMI Alone Determines Pregnancy Outcomes?

No. BMI is only one factor among many that can influence pregnancy outcomes.

The study identifies associations rather than establishing that BMI alone causes pre-eclampsia, preterm birth or caesarean delivery. This distinction is important because women with diabetes can have very different health profiles even when their BMI is similar.

Factors such as age, duration of diabetes, glucose control, blood pressure, kidney health, previous pregnancy history, medications and access to specialised maternity care can all contribute to pregnancy outcomes.

The results should therefore be viewed as evidence supporting closer risk assessment rather than as a prediction of what will happen in an individual pregnancy.

What the Findings Could Mean for Diabetes and Pregnancy Care

The study highlights the importance of looking beyond blood glucose levels when assessing pregnancy risk in women with pre-existing diabetes.

For women with T2DM, higher BMI may identify a group that requires particularly careful surveillance for conditions such as pre-eclampsia and preterm delivery. For women with T1DM, attention to early glycaemic control and delivery planning may be especially important when overweight or obesity is present.

Importantly, pregnancy is not the time for extreme dieting or rapid weight-loss efforts. Any approach to weight management during pregnancy should be discussed with an obstetrician, diabetes specialist or qualified dietitian.

Why Preconception Care Matters

The findings also reinforce the importance of managing diabetes before pregnancy rather than waiting until pregnancy begins.

Preconception consultations can help healthcare professionals review blood glucose control, medications, blood pressure and other factors that may affect pregnancy. Women can also receive personalised advice about nutrition, physical activity and appropriate weight management.

For someone with diabetes who is planning pregnancy, the goal is not simply to achieve a particular number on the weighing scale. A broader approach focuses on metabolic health, safe glucose management and reducing preventable pregnancy complications.

The Bigger Picture: Rising Overweight and Obesity in Diabetes

The high proportion of women with overweight or obesity in the T2DM group reflects a wider challenge in diabetes care. Type 2 diabetes and excess weight frequently occur together, creating a complex metabolic environment that can become even more challenging during pregnancy.

As more women of reproductive age live with type 2 diabetes, understanding how factors such as BMI affect pregnancy outcomes becomes increasingly important.

The study also demonstrates why researchers separated T1DM and T2DM rather than treating all diabetes during pregnancy as one condition. Although both involve problems with blood glucose regulation, their underlying biology, treatment approaches and clinical circumstances can differ considerably.

What Should Women With Diabetes Take From the Study?

The findings should not be interpreted as a reason for fear or blame. Instead, they underline the value of early and individualised medical care.

• Plan ahead: Women with diabetes considering pregnancy can discuss their plans with their healthcare team before conception.
• Monitor glucose: Maintaining appropriate blood glucose levels is an important part of pregnancy care.
• Monitor blood pressure: Regular monitoring can help identify complications such as hypertension and pre-eclampsia.
• Discuss weight management: Weight-related goals should be personalised and medically supervised, particularly before or during pregnancy.
• Attend prenatal appointments: Regular follow-up allows clinicians to monitor both maternal and fetal health.

Study Adds to Evidence on BMI, Diabetes and Pregnancy

The Australian study adds to evidence suggesting that maternal weight status may have different implications depending on the type of diabetes present.

Among women with type 2 diabetes, higher pre-pregnancy BMI was associated with increased odds of pre-eclampsia and greater risk of preterm delivery. Among women with type 1 diabetes, overweight and obesity were associated with poorer first-trimester HbA1c target attainment and higher odds of caesarean delivery.

Because the analysis involved 264 pregnancies from a single Australian hospital and examined associations rather than proving causation, further research involving larger and more diverse populations will be important.

Conclusion

Higher pre-pregnancy BMI may be an important marker of pregnancy risk among women with pre-existing diabetes, particularly those with type 2 diabetes. The study found that each 1 kg/m² increase in BMI was associated with 8.2% higher odds of pre-eclampsia in the T2DM group, while increasing BMI was also linked to preterm delivery.

For women with type 1 diabetes, the findings pointed instead toward poorer early HbA1c target attainment and a greater likelihood of caesarean delivery among those with higher BMI.

The results reinforce a broader message: pregnancy care in women with diabetes needs to consider the whole metabolic picture rather than glucose levels alone. Preconception planning, personalised diabetes management, appropriate weight-related guidance and regular monitoring can help healthcare teams identify and manage risks throughout pregnancy.