What Pregnancy Data Reveals About Blood Sugar Control and Birth Weight in Type 1 Diabetes

Pregnancy with Type 1 diabetes requires careful management, but even women with excellent care and modern tools—insulin pumps and continuous glucose monitors—still face higher risks of complications than women without diabetes. One serious complication is babies being significantly larger than expected at birth, a condition called large-for-gestational-age (LGA) or macrosomia. Understanding which blood sugar patterns most strongly predict this outcome could help guide pregnancy care.

Researchers in Europe followed 55 pregnant women with Type 1 diabetes from 2018 to 2023, tracking their glucose control throughout pregnancy using both traditional HbA1c measurements and newer time-in-range metrics from their glucose monitors.

The median HbA1c at the start of pregnancy was 6.1%—a solid number by most standards. Women maintained similar control throughout the three trimesters, averaging around 5.9–6.0%. However, only about one-third of women hit the recommended HbA1c targets consistently.

When researchers looked at time in range (the percentage of the day glucose stayed in the recommended zone), the picture was less rosy. Across all three trimesters, time in range was at or below 70%—not quite meeting the 70% target many diabetes specialists recommend.

Despite this level of control, the outcomes were striking: nearly half of all babies (48%) were born larger than expected for their age, and one-third were macrosomic (very large). These rates are higher than in the general population.

The study identified a critical threshold: a mean blood glucose level of 124 mg/dL or higher in the third trimester was the single strongest predictor of large-for-gestational-age babies. Women whose average glucose was above this level had nearly 12 times the risk of delivering an LGA infant compared to those with lower averages.

This finding suggests that what happens in the final months of pregnancy—not just overall HbA1c—may be especially important in determining infant size. It also hints that time-in-range tracking might capture something important that HbA1c alone does not: the day-to-day pattern of glucose levels.

HbA1c is a three-month average and doesn't show fluctuations. Time in range, available through continuous glucose monitors, shows what percentage of the day glucose stays in a target zone. The authors conclude that time in range appeared to be a better predictor of pregnancy complications, including LGA, than HbA1c alone.

The good news: women who achieved the recommended time-in-range target had babies with lower birth weights on average. The catch: they had a higher rate of jaundice (yellowing of newborn skin), suggesting the relationship between glucose control and infant outcomes is complex.

This research underscores an important truth: even with modern insulin pumps, continuous glucose monitors, and specialist care, women with Type 1 diabetes do face poorer obstetric outcomes than those without diabetes. Good control—by traditional measures—does not eliminate the risk of large babies.

The study suggests that paying close attention to time-in-range goals and, importantly, to mean glucose levels in the third trimester may help reduce complications. If you are planning pregnancy or are currently pregnant with Type 1 diabetes, these findings reinforce the value of frequent glucose monitoring and close collaboration with your diabetes and obstetric care team. Every pregnancy is different, and personalized management—not one-size-fits-all targets—remains the gold standard.

Source: Diagnostics (Basel, Switzerland). Evidence type: PubMed indexed literature. Type1Cure is an information and intelligence hub, not a medical advice service. This article summarizes published research and does not provide diagnosis, treatment, or personal medical guidance. Always talk to your own care team before changing anything about your Type 1 diabetes management.