For most women, gestational diabetes doesn't cause noticeable signs or symptoms.

Researchers don't know why some women develop gestational diabetes. To understand how gestational diabetes occurs, it can help to understand how pregnancy affects your body's glucose processing.

Your body digests the food you eat to produce sugar (glucose) that enters your bloodstream. In response, your pancreas — a large gland behind your stomach — produces insulin. Insulin is a hormone that helps glucose move from your bloodstream into your body's cells, where it's used as energy.

During pregnancy, the placenta, which connects your baby to your blood supply, produces high levels of various other hormones. Almost all of them impair the action of insulin in your cells, raising your blood sugar. Modest elevation of blood sugar after meals is normal during pregnancy.

As your baby grows, the placenta produces more and more insulin-counteracting hormones. In gestational diabetes, the placental hormones provoke a rise in blood sugar to a level that can affect the growth and welfare of your baby. Gestational diabetes usually develops during the last half of pregnancy — sometimes as early as the 20th week, but generally not until later.

Any woman can develop gestational diabetes, but some women are at greater risk. Risk factors for gestational diabetes include:

• Age greater than 25. Women older than age 25 are more likely to develop gestational diabetes.
• Family or personal health history. Your risk of developing gestational diabetes increases if you have prediabetes — slightly elevated blood sugar that may be a precursor to type 2 diabetes — or if a close family member, such as a parent or sibling, has type 2 diabetes. You're also more likely to develop gestational diabetes if you had it during a previous pregnancy, if you delivered a baby who weighed more than 9 pounds (4.1 kilograms) or if you had an unexplained stillbirth.
• Excess weight. You're more likely to develop gestational diabetes if you're significantly overweight with a body mass index (BMI) of 30 or higher.
• Nonwhite race. For reasons that aren't clear, women who are black, Hispanic, American Indian or Asian are at higher risk to develop gestational diabetes.

Most women who have gestational diabetes deliver healthy babies. However, gestational diabetes that's not carefully managed can lead to uncontrolled blood sugar levels and cause problems for you and your baby, including an increased likelihood of needing a C-section to deliver.

Complications that may affect your baby

If you have gestational diabetes, your baby may be at increased risk of:

• Excessive birth weight. Extra glucose in your bloodstream crosses the placenta, which triggers your baby's pancreas to make extra insulin. This can cause your baby to grow too large (macrosomia). Very large babies — those that weigh 9 pounds or more — are more likely to become wedged in the birth canal, sustain birth injuries or require a C-section birth.
• Early (preterm) birth and respiratory distress syndrome. A mother's high blood sugar may increase her risk of early labor and delivering her baby before the baby's due date. Or her doctor may recommend early delivery because the baby is large.

Babies born early may experience respiratory distress syndrome — a condition that makes breathing difficult. Babies with this syndrome may need help breathing until their lungs mature and become stronger. Babies of mothers with gestational diabetes may experience respiratory distress syndrome even if they're not born early.

• Low blood sugar (hypoglycemia). Sometimes babies of mothers with gestational diabetes develop low blood sugar (hypoglycemia) shortly after birth because their own insulin production is high. Severe episodes of hypoglycemia may provoke seizures in the baby. Prompt feedings and sometimes an intravenous glucose solution can return the baby's blood sugar level to normal.
• Type 2 diabetes later in life. Babies of mothers who have gestational diabetes have a higher risk of developing obesity and type 2 diabetes later in life.

Untreated gestational diabetes can result in a baby's death either before or shortly after birth.

Complications that may affect you

Gestational diabetes may also increase the mother's risk of:

• High blood pressure and preeclampsia. Gestational diabetes raises your risk of high blood pressure, as well as preeclampsia — a serious complication of pregnancy that causes high blood pressure and other symptoms that can threaten the lives of both mother and baby.
• Future diabetes. If you have gestational diabetes, you're more likely to get it again during a future pregnancy. You're also more likely to develop type 2 diabetes as you get older. However, making healthy lifestyle choices such as eating healthy foods and exercising can help reduce the risk of future type 2 diabetes.

Of those women with a history of gestational diabetes who reach their ideal body weight after delivery, fewer than 1 in 4 eventually develop type 2 diabetes.