Gestational diabetes typically develops after 20 weeks of pregnancy, with most cases appearing between 24 and 28 weeks. However, some women show signs of abnormal blood sugar much earlier, sometimes in the first trimester, which often signals undiagnosed type 2 diabetes that existed before pregnancy or a more severe form of glucose intolerance that carries higher risks.
Why Most Cases Start After 20 Weeks
Gestational diabetes is driven by hormones produced by the placenta. As the placenta grows, it releases increasing amounts of estrogen, cortisol, and a hormone called human placental lactogen. These hormones partially block insulin’s ability to move sugar out of your blood and into your cells. This blocking effect, sometimes called contra-insulin resistance, typically kicks in around 20 to 24 weeks of pregnancy.
If your body can compensate by producing extra insulin, your blood sugar stays normal. If it can’t keep up with the rising demand, glucose builds up in your bloodstream and gestational diabetes develops. This is why standard screening happens between 24 and 28 weeks: that’s the window when placental hormones are ramping up enough to reveal the problem. Both the American College of Obstetricians and Gynecologists and the U.S. Preventive Services Task Force recommend this timing for all pregnant women without a prior diabetes diagnosis.
When Abnormal Blood Sugar Shows Up Earlier
Some women test positive for high blood sugar well before 24 weeks. When glucose levels are elevated in the first trimester, it usually means one of two things: either the woman had type 2 diabetes before becoming pregnant and didn’t know it, or she has an early and more aggressive form of gestational diabetes tied to significant insulin resistance that was already present before pregnancy.
Distinguishing between these two scenarios matters clinically but can be tricky. A first-trimester A1c (a blood test reflecting average blood sugar over the past two to three months) of 6.5% or higher is generally used to diagnose pre-existing diabetes. An A1c between 5.7% and 6.4% falls into a gray zone. Research has found that an early A1c at or above 5.7% is associated with substantially higher odds of needing insulin during pregnancy, a seven-fold increase in the risk of having a large baby, and a greater chance of shoulder complications during delivery.
Who Gets Screened Before 24 Weeks
Not every pregnant woman is tested in early pregnancy. First-visit screening is recommended for women who are overweight or obese (a BMI above 25, or above 23 for Asian Americans) and have at least one additional risk factor. Those risk factors include:
- Family history: a parent or sibling with diabetes
- Previous gestational diabetes in an earlier pregnancy
- Prior large baby: a previous birth weight of 9 pounds or more
- Race or ethnicity: African American, Latino, Native American, Asian American, or Pacific Islander background
- Polycystic ovary syndrome (PCOS)
- Physical inactivity
- High blood pressure or use of blood pressure medication
- Prediabetes: a previous A1c of 5.7% or higher, or abnormal fasting glucose
- BMI above 40 before pregnancy
If you have one or more of these factors, your provider will likely check your blood sugar at your first prenatal visit using the same diagnostic criteria applied to non-pregnant adults. If results come back normal at that early visit, you’ll still be screened again at 24 to 28 weeks, because placental hormone changes later in pregnancy can trigger gestational diabetes even when early tests were fine.
Early-Onset Cases Carry Greater Risks
A large secondary analysis published in Diabetes Care compared outcomes in women diagnosed before 24 weeks (early-onset) versus those diagnosed at 24 to 28 weeks (late-onset). Even when both groups received treatment starting at 24 to 28 weeks, women with early-onset gestational diabetes had a 59% higher rate of a composite of adverse outcomes, including higher birth weight percentiles, more preterm births, and more newborn jaundice. The late-onset group, by contrast, did not show a statistically significant increase in these complications compared to women without gestational diabetes.
Women with early-onset disease also had a greater need for insulin or other medication to control blood sugar, and their BMI tended to be higher than women diagnosed later. The researchers concluded that the weeks of uncontrolled high blood sugar between early onset and the start of treatment at 24 to 28 weeks likely contributed to the worse outcomes, suggesting that earlier treatment could make a real difference.
Why You Probably Won’t Notice Symptoms
Gestational diabetes rarely announces itself with obvious signs, regardless of when it starts. Most women feel perfectly normal. When symptoms do appear, they overlap heavily with ordinary pregnancy experiences: fatigue, frequent urination, increased thirst, and nausea. These are easy to dismiss as standard first- or second-trimester discomforts, which is exactly why screening tests matter more than symptom monitoring for catching this condition.
If you have risk factors for early-onset gestational diabetes, the most useful thing you can do is make sure your provider knows your full medical and family history at that first prenatal appointment. Early testing won’t prevent gestational diabetes from developing, but catching high blood sugar sooner gives you more time to manage it and reduces the chance of complications for both you and your baby.