Gestational diabetes develops when your body can’t produce enough insulin to overcome the natural insulin resistance that builds during pregnancy. Every pregnant person experiences some degree of this resistance, but roughly 10% of pregnancies tip into gestational diabetes because of a combination of hormonal shifts, genetics, body composition, and lifestyle factors. Understanding what drives this process can help you assess your own risk and take steps to lower it.
Why Pregnancy Creates Insulin Resistance
As pregnancy progresses, the placenta releases a surge of hormones, including estrogen, progesterone, cortisol, and placental lactogen. These hormones serve an important purpose: they slightly elevate blood sugar so that glucose crosses the placenta more easily and fuels the growing baby. But the trade-off is that your cells become less responsive to insulin. Your body also ramps up its own glucose production and starts breaking down fat stores, pushing blood sugar and fatty acid levels higher still.
In most pregnancies, the pancreas compensates by producing significantly more insulin. The insulin-producing cells actually multiply in response to placental lactogen, keeping blood sugar in a healthy range despite the increased resistance. Gestational diabetes develops when those cells can’t keep up. They fail to scale their insulin output to match the demand, and blood sugar stays elevated. The core question of “what determines gestational diabetes” comes down to what makes some pancreases unable to compensate while others manage just fine.
Body Weight Is the Strongest Modifiable Factor
Pre-pregnancy weight has a larger influence on gestational diabetes risk than any other factor you can change. In a large study comparing BMI categories to maternal age, obesity before conception (BMI of 30 or higher) nearly tripled the odds of developing gestational diabetes. Even being overweight (BMI 25 to 29.9) raised risk more than being 35 or older, which has long been considered a primary risk factor.
The numbers are striking when you look at actual prevalence. Among women younger than 35, gestational diabetes rates climbed from 16.4% in those with a normal pre-pregnancy weight to 23% in those who were overweight and 38.5% in those with obesity. For women 35 and older, the pattern was steeper: 20.4% at normal weight, 37.2% when overweight, and 51.4% with obesity. In other words, more than half of older women with obesity developed the condition.
Early pregnancy weight gain matters too. For normal-weight women, every standard deviation of first-trimester weight gain above what was predicted for their body raised the odds of gestational diabetes by 23%. A similar pattern held for women with moderate obesity. Gaining weight faster than expected in early pregnancy appears to compound the insulin resistance that’s already building.
Genetics and Family History
Your genetic background plays a significant role in how well your pancreas adapts to pregnancy’s demands. Researchers have identified over 100 genes linked to type 2 diabetes susceptibility, and many of the same genetic pathways influence gestational diabetes risk. The clearest signal comes from family history: if a first-degree relative (parent or sibling) has type 2 diabetes, gestational diabetes prevalence jumps to about 26.6%, compared to 15.9% in women with no family history. Having affected relatives on both sides of the family is even more powerful, with prevalence reaching 33.3% and nearly five times the odds compared to women without any diabetic relatives.
The interaction between parents matters in an interesting way. Having a mother with type 2 diabetes was associated with worse metabolic profiles during pregnancy, but the risk of gestational diabetes itself only rose dramatically when both parents were affected. This suggests the genetic contribution is complex, likely involving dozens of genes that each nudge the pancreas toward underperformance under stress.
Ethnicity and Geographic Background
Gestational diabetes rates vary considerably across ethnic groups, even after accounting for weight and age. Asian Americans have the highest overall prevalence of any racial or ethnic group in the United States, at 12.3%. Within Asian populations, the rates differ too: Asian Indians have the highest at roughly 129 per 1,000 live births, while Japanese (7.3%), Korean (9.1%), and Chinese (10.9%) populations have lower rates.
An unusual finding involves immigration patterns. Foreign-born women who have lived in the U.S. for more than 10 years have higher gestational diabetes rates (as high as 15.7% in some studies) than both recent immigrants and U.S.-born women. This suggests that adopting a Western diet and lifestyle over time layers additional metabolic risk on top of whatever genetic predisposition a person already carries.
PCOS and Pre-existing Metabolic Conditions
Polycystic ovary syndrome roughly doubles the odds of developing gestational diabetes. About 40% of pregnancies in women with PCOS result in the condition. The connection isn’t surprising: PCOS is closely tied to insulin resistance even outside of pregnancy. The hormonal shifts of pregnancy amplify a metabolic vulnerability that already exists.
Not all forms of PCOS carry equal risk. Women with the “classic” type, marked by higher levels of male hormones, face substantially greater odds (up to 2.7 times higher) than women with forms of PCOS that don’t involve hormonal imbalance. This distinction is increasingly recognized as important for counseling and early screening.
Diet Before and During Pregnancy
What you eat before you conceive appears to shape gestational diabetes risk independently of body weight. Dietary patterns rich in vegetables, fruits, whole grains, legumes, nuts, and fish are consistently linked to lower risk. Diets heavy in red and processed meat, sugar-sweetened beverages, and trans fats push risk in the other direction. The evidence is strongest for pre-pregnancy eating patterns, suggesting that metabolic health heading into conception sets the stage for how your body handles the insulin demands ahead.
Age and Prior Gestational Diabetes
Being 35 or older at the time of pregnancy is an established risk factor, raising the odds of gestational diabetes by about 45% compared to younger women. While this effect is real, it’s consistently smaller than the effect of pre-pregnancy weight. Age likely contributes because insulin-producing cells gradually lose some of their ability to ramp up output, and background insulin resistance tends to increase as you get older.
If you’ve had gestational diabetes in a previous pregnancy, the recurrence rate is high. A large meta-analysis found that about 51% of women who had gestational diabetes in one pregnancy developed it again in a subsequent one, with individual studies reporting rates ranging from 31% to 74% depending on the population and diagnostic criteria used. A prior history is one of the strongest single predictors and is the reason most guidelines recommend early glucose screening in subsequent pregnancies rather than waiting until the standard 24 to 28 week window.
How These Factors Work Together
No single factor “causes” gestational diabetes in isolation. The condition emerges from a collision between the insulin resistance every pregnancy creates and your body’s capacity to compensate for it. That capacity is shaped by genetics you inherited, weight you carry going into pregnancy, metabolic conditions like PCOS, your age, and the dietary and activity patterns that influence your baseline metabolic health. Women with several overlapping risk factors face the steepest odds, while those with just one or two may never develop the condition at all. The interplay between genes and environment explains why gestational diabetes strikes some women who appear low-risk and spares others who seem to have every reason to develop it.