Gestational diabetes develops when your body can’t make enough insulin to overcome the natural insulin resistance that builds during pregnancy. About 8.3% of pregnancies in the United States are affected, up from 6% just five years earlier. Every pregnant woman becomes somewhat insulin resistant as pregnancy progresses, but gestational diabetes happens when the pancreas can’t keep up with the rising demand.
Why Pregnancy Creates Insulin Resistance
Your placenta is essentially a hormone factory. As it grows, it releases increasing amounts of hormones that interfere with how your cells respond to insulin. The key players include human placental lactogen, human placental growth hormone, progesterone, estrogen, and cortisol. These hormones serve important purposes: they help direct more glucose to your developing baby. But they also make your own cells less responsive to insulin, meaning your pancreas has to work harder to keep blood sugar in a normal range.
Progesterone, for example, suppresses one of the molecular pathways your cells use to pull glucose out of the bloodstream. Estrogen at high concentrations diminishes insulin sensitivity directly. Human placental growth hormone can trigger the same blood-sugar-raising effects as regular growth hormone, impairing your cells’ ability to absorb glucose and prompting your liver to release more of it. Cortisol, which rises steadily throughout pregnancy, is the only one of these hormones that researchers have found to correlate directly with the degree of insulin sensitivity loss.
This resistance ramps up as the placenta grows. By the third trimester, insulin sensitivity can drop to roughly 50% of its normal level. That’s why gestational diabetes is typically screened for between weeks 24 and 28, when the hormonal load is high enough to reveal whether your pancreas can compensate.
When the Pancreas Can’t Keep Up
Here’s the critical distinction: every pregnant woman develops some degree of insulin resistance, but only about 1 in 10 develops gestational diabetes. The difference comes down to what happens in the insulin-producing beta cells of the pancreas. In a healthy pregnancy, these cells multiply and ramp up insulin output to match the increased resistance. In women who develop gestational diabetes, the beta cells fail to expand or secrete enough insulin to compensate.
This failure, called beta-cell decompensation, can happen in several ways. Some women’s beta cells simply don’t replicate fast enough. Others produce beta cells that don’t sense glucose properly or that die off at higher-than-normal rates. In some cases, toxic byproducts enter the beta cells and damage their energy-producing machinery, reducing their ability to release insulin when blood sugar rises. The end result is the same: not enough insulin to handle the glucose load, and blood sugar climbs.
Genetics Play a Significant Role
Your genes have a meaningful influence on whether your beta cells can rise to the challenge. A systematic review of genetic studies identified several gene variants consistently linked to higher gestational diabetes risk. The strongest association was with variants in a gene called TCF7L2, which is involved in insulin secretion. Carrying certain variants of this gene raises gestational diabetes risk by about 44 to 46%. Variants in the glucokinase gene (GCK), which helps beta cells sense glucose levels, increase risk by about 29%.
Other implicated genes affect insulin signaling, the internal clock of beta cells, and the receptors that tell beta cells to multiply during pregnancy. For instance, mutations in the prolactin receptor gene, which normally signals beta cells to expand during pregnancy, are associated with higher gestational diabetes risk. Women whose cells carry these mutations may simply lack the biological signal that says “make more insulin.”
This genetic component explains why family history of diabetes (either gestational or type 2) is one of the strongest predictors of developing gestational diabetes yourself.
Inflammation and Body Fat
Excess body fat, particularly before and early in pregnancy, adds a layer of insulin resistance on top of what pregnancy itself creates. Fat tissue isn’t just a passive energy store. It houses immune cells called macrophages that release inflammatory molecules, including TNF-alpha and interleukin-6. These molecules interfere with insulin signaling at the cellular level, degrading key proteins your cells need to respond to insulin.
TNF-alpha in particular shows a strong correlation with insulin resistance. Women with gestational diabetes also show increased expression of these inflammatory molecules in the placenta itself, creating a double source of insulin-blocking signals. Higher levels of free fatty acids in women with obesity early in pregnancy further inhibit glucose uptake and stimulate the liver to pump out more glucose, compounding the problem.
Risk Factors That Raise Your Odds
Several well-established factors increase your likelihood of developing gestational diabetes:
- BMI above 25 (or above 23 for Asian Americans) before or early in pregnancy
- Family history of type 2 diabetes or a previous pregnancy with gestational diabetes
- Ethnicity: African American, Latino, Native American, Asian American, and Pacific Islander women face higher rates
- PCOS: polycystic ovary syndrome involves insulin resistance even outside of pregnancy, and the added hormonal burden of pregnancy can push blood sugar past the threshold
- Having had multiple pregnancies (higher parity is independently associated with risk)
Research shows the strongest associations cluster around family history, previous gestational diabetes, Southeast Asian ethnicity, number of prior pregnancies, and high BMI. Women with a BMI of 30 or higher and at least one additional risk factor are typically screened for type 2 diabetes at their very first prenatal visit, rather than waiting until the standard 24-to-28-week window.
The Gut Microbiome Connection
An emerging piece of the puzzle involves the trillions of bacteria living in your gut. A meta-analysis of 23 studies found that women with gestational diabetes consistently showed lower diversity in their gut bacteria compared to pregnant women without the condition. The ratio of two major bacterial groups, Bacteroidetes and Firmicutes, appears to shift in ways associated with abnormal glucose metabolism.
In one striking experiment, researchers transplanted stool samples from women with gestational diabetes into germ-free mice. The recipient mice developed the same impaired glucose tolerance as the human donors, while mice that received samples from women without gestational diabetes did not. This suggests the bacterial imbalance isn’t just a side effect of high blood sugar but may actively contribute to it. Lower levels of beneficial Bifidobacteria and reduced production of short-chain fatty acids (compounds gut bacteria make that help regulate metabolism) appear to be part of this process.
How These Causes Work Together
Gestational diabetes rarely has a single cause. It’s typically a collision of factors: the universal insulin resistance of pregnancy, layered with genetic susceptibility that limits beta-cell expansion, amplified by inflammation from excess body fat, and possibly shaped by gut bacteria. A woman with strong genetic risk might develop gestational diabetes at a normal weight, while a woman with no family history might develop it only if she enters pregnancy with significant obesity. The common thread is always the same: insulin demand outpaces insulin supply, and blood sugar rises as a result.