Cause
During pregnancy, an organ called the placenta develops in the uterus. The placenta connects the mother and baby and makes sure the baby has enough food and water. It also makes several hormones. Some of these hormones make it hard for insulin to do its job—controlling blood sugar—so the mother's body has to make more insulin to keep sugar levels in a safe range. Gestational diabetes develops when the organ that makes insulin, the pancreas, cannot make enough insulin to keep blood sugar levels within a safe range.

Symptoms
Because gestational diabetes does not cause symptoms, you need to be tested for the condition. This is usually done between the 24th and 28th weeks of pregnancy. You may be surprised if your test shows a high blood sugar. It is important for you to be tested for gestational diabetes, because high blood sugar can cause problems for both you and your baby.
Sometimes, a pregnant woman has been living with diabetes without knowing it. If you have symptoms from diabetes, they may include:
Increased thirst.
Increased urination.
Increased hunger.
Blurred vision.
Pregnancy causes most women to urinate more often and to feel more hungry, so having these symptoms does not always mean that a woman has diabetes. Talk with your doctor if you have these symptoms, so that you can be tested for diabetes.

What Happens
Most women find out they have gestational diabetes after being tested between the 24th and 28th weeks of their pregnancy. After you know you have gestational diabetes, you will need to make certain changes in the way you eat and how often you exercise to help keep your blood sugar level within a safe range. As you get farther along in your pregnancy, your body will continue to make more and more hormones. This can make it harder and harder to control your blood sugar. If it is not possible to control your blood sugar with food and exercise, you may also need to give yourself shots of insulin.
Just because you have diabetes does not mean that your baby will have diabetes. Most women with gestational diabetes give birth to healthy babies. If you are able to keep your blood sugar level within a safe range, your chances of having problems during pregnancy or birth are the same as if you didn't have gestational diabetes.
In rare cases, a mother or her baby has problems because of high blood sugar. These problems include:
High blood pressure in the mother caused by preeclampsia.
A baby that grows too large. If an unborn baby receives too much sugar, the sugar can turn into fat, causing the baby to grow larger than normal. A large baby can be injured during vaginal birth and may need to be delivered surgically (C-section).
After the baby is born, extra insulin may cause the baby's blood sugar level to drop below the safe range. If the baby's blood sugar level drops too low, he or she may need to be given extra sugar. Babies can also develop other treatable problems after birth, including low blood calcium levels, high bilirubin levels, and too many red blood cells.
Most of the time, gestational diabetes goes away after a baby is born. But if you have had gestational diabetes, you have a greater chance of having it in a future pregnancy and of developing type 2 diabetes. More than half of women who develop gestational diabetes will develop type 2 diabetes later in life.2

What Increases Your Risk
You have an increased chance of developing gestational diabetes if:
You are 25 or older when you become pregnant.
You have had gestational diabetes before.
You have given birth to a baby who weighed more than 9 lb (4 kg).
You weighed more than 9 lb (4 kg) when you were born.
You have a parent or sibling who has type 2 diabetes.
You are not physically active before pregnancy.
You are obese (your body mass index [BMI] is 30 or higher).
You are a member of a racial/ethnic group that has a high risk of developing diabetes, such as Latin Americans, Native Americans, Asian Americans, African Americans, or Pacific Islanders.
You have polycystic ovary syndrome.
You have a dark skin rash on the back or your neck or in folds on your body.
You take corticosteroid medicine.
You have a history of pre-diabetes.
You have a history of problems during pregnancy.