Untreated gestational diabetes exposes both mother and baby to serious, sometimes life-threatening complications. High blood sugar during pregnancy forces the baby’s body to overproduce insulin, which drives excessive growth, disrupts organ development, and creates dangerous conditions during and after delivery. The risks escalate the longer blood sugar stays uncontrolled, particularly in the third trimester when fetal growth accelerates.
How Uncontrolled Blood Sugar Affects the Baby in the Womb
When a mother’s blood sugar stays consistently high, glucose crosses the placenta freely. The baby’s pancreas responds by producing extra insulin to process all that sugar. Insulin is also a growth hormone, so the baby starts gaining weight at an abnormal rate, a condition called macrosomia. Babies are generally considered macrosomic at a birth weight above 8 pounds 13 ounces, though some providers use a threshold of 9 pounds 14 ounces. Untreated gestational diabetes carries a macrosomia risk as high as 20%.
The excess growth isn’t proportional. Babies of mothers with uncontrolled diabetes tend to develop disproportionately large shoulders and upper bodies. This specific pattern of growth is what makes delivery so much more dangerous, because even if the head delivers normally, the shoulders can become trapped behind the mother’s pelvic bone.
Beyond size, the baby’s lungs are directly affected. High blood sugar and high insulin levels in the womb interfere with the production of surfactant, the substance that coats the inside of the lungs and allows them to inflate properly after birth. Research shows that babies born to diabetic mothers have a 5.6 times greater risk of developing respiratory distress syndrome compared to babies from non-diabetic pregnancies. The combination of excess glucose and insulin reduces the production of key proteins the lungs need to function, essentially delaying lung maturation even when the baby is otherwise full-term.
Dangerous Complications During Delivery
Shoulder dystocia, where the baby’s shoulders get stuck during vaginal delivery, occurs in 1 to 2% of all deliveries. That rate climbs significantly with a macrosomic baby. When shoulder dystocia happens, the delivery team has seconds to minutes to free the baby using emergency maneuvers. The stakes are high: the baby can sustain nerve damage to the arm and shoulder (brachial plexus injury), broken collarbones, or broken upper arm bones. In rare cases, oxygen deprivation during a prolonged delivery can cause brain damage or death. Permanent nerve injury occurs in roughly 1 in 10,000 deliveries.
Mothers face their own physical risks during a complicated delivery. Shoulder dystocia and large babies increase the likelihood of severe perineal tearing, rectal injury, postpartum hemorrhage, and nerve damage to the mother’s pelvic floor. These injuries can require surgical repair and extend recovery by weeks or months. An emergency cesarean section becomes more likely when labor stalls or complications arise, which carries its own set of surgical risks.
What Happens to the Baby After Birth
One of the most immediate dangers after delivery is a sudden drop in the baby’s blood sugar. For months, the baby’s pancreas has been churning out extra insulin to match the mother’s high glucose supply. The moment the umbilical cord is cut, that glucose supply disappears, but the baby’s insulin production doesn’t shut off immediately. The result is a rapid crash in blood sugar called neonatal hypoglycemia, which can cause seizures, breathing problems, and in severe cases, brain injury if not caught and treated quickly.
Breathing difficulties are another common concern. Because surfactant production may be delayed, even full-term babies born to mothers with uncontrolled gestational diabetes can struggle to breathe on their own. Some need supplemental oxygen or time in a neonatal intensive care unit. This is one of the reasons these babies are monitored closely in the hours after birth, even when the delivery itself goes smoothly.
Stillbirth Risk in Later Pregnancy
The most devastating possible outcome of untreated gestational diabetes is stillbirth. Data from a large retrospective study using U.S. birth and death records found that the risk of stillbirth was higher at every week of term pregnancy in women with diabetes compared to a low-risk population. The gap widened substantially at 40 and 41 weeks, which is why providers managing diabetic pregnancies often recommend delivery before the due date rather than waiting for labor to start naturally.
The exact mechanism isn’t fully understood, but chronically high blood sugar is thought to damage the placenta over time, reducing its ability to deliver oxygen and nutrients to the baby. By the final weeks of pregnancy, a compromised placenta may simply fail.
Long-Term Consequences for the Mother
Gestational diabetes doesn’t end at delivery, at least not in terms of what it signals about a mother’s metabolic health. A large Finnish registry study tracking nearly 100,000 women with prior gestational diabetes found that 4.1% developed type 2 diabetes within an average follow-up period of about 7 years. That’s roughly ten times the rate seen in women who had normal blood sugar during pregnancy, where only 0.4% developed type 2 diabetes over a similar timeframe.
Women who had gestational diabetes that was poorly controlled or went untreated face the higher end of that risk spectrum. The same insulin resistance that caused blood sugar problems during pregnancy often persists or worsens after delivery, especially without lifestyle changes. Maintaining a healthy weight, staying physically active, and getting screened for diabetes regularly after a gestational diabetes pregnancy are the most effective ways to delay or prevent the progression to type 2.
How Gestational Diabetes Gets Caught
Standard screening happens between 24 and 28 weeks of pregnancy, typically through a glucose challenge test where you drink a sugary solution and have your blood drawn an hour later. A result of 140 or higher triggers a longer, fasting version of the test to confirm the diagnosis. Women with risk factors like obesity, a family history of diabetes, or a previous pregnancy with gestational diabetes may be tested as early as their first prenatal visit.
This timing matters because gestational diabetes usually develops in the second half of pregnancy as placental hormones increasingly block insulin’s effectiveness. Women who skip or delay prenatal care are the most likely to have gestational diabetes go undiagnosed and untreated, which is part of why early and consistent prenatal visits are so important. The condition itself rarely causes noticeable symptoms. Most women feel completely normal, which means the only reliable way to catch it is through routine screening.
What Treatment Actually Looks Like
The good news is that the complications described above are largely preventable. Most women with gestational diabetes manage their blood sugar successfully through dietary changes and regular physical activity. The typical approach involves monitoring blood sugar several times a day, eating smaller and more frequent meals, choosing foods that don’t spike blood sugar rapidly, and walking or doing other moderate exercise most days. For women whose blood sugar doesn’t respond to these changes, insulin or oral medication brings it under control.
Treatment dramatically reduces the risk of macrosomia, shoulder dystocia, neonatal hypoglycemia, and the other complications that make untreated gestational diabetes so dangerous. The difference between a managed and an unmanaged case is substantial. That’s why the screening test at 24 to 28 weeks exists: catching it early enough to act is the whole strategy.