High blood sugar in pregnancy is caused by hormonal changes that a woman's body undergoes while she is pregnant. The placenta, which helps nourish the baby in the mother's uterus, produces hormones that cause the mother's body cells to reject insulin. This is known as insulin resistance, and it increases as the placenta gets bigger throughout the pregnancy. Usually, the mother's pancreas will produce three times the normal amount of insulin to compensate for the high blood sugar in pregnancy. If it is unable to keep up, a condition known as gestational diabetes is diagnosed.

Gestational diabetes is high blood sugar in pregnancy, and it does not mean that a person has diabetes. The hormonal changes from pregnancy itself cause the high blood sugar response, and a mother could have normal blood sugar when not pregnant. Those at highest risk for gestational diabetes include women who are overweight, have a family history of diabetes or are of a certain heritage, such as Hispanic, black, American Indian or Asian. Also at a high risk are women who have sugar in the urine or moderately high blood sugar levels, have given birth to a baby that weighed more than 9 pounds (4.1 kg), have had a stillbirth or have too much amniotic fluid. Some women, however, develop high blood sugar in pregnancy without any known risk factors at all.

The risks of having high blood sugar in pregnancy are numerous. Blood sugar that is out of control early in pregnancy can lead to birth defects of the brain and heart or possible miscarriage. Later high blood sugar can lead to the baby becoming too large. This can lead to complications when delivering, such as shoulder trauma or the need for a Caesarean section instead of vaginal birth. A baby's blood sugar can also drop very low after birth if it is used to a high blood sugar environment while in the mother's womb.

Those at high risk for high blood sugar in pregnancy should be tested as soon as possible for gestational diabetes, but most other women can be screened during the 24th week. The test will involve drinking a sweetened liquid and having the blood sugar checked at regular intervals. If it is found that gestational diabetes is present, a woman will have to monitor her blood sugar four times a day and take insulin to regulate her blood sugar levels. Diet, exercise and following up with a doctor will also be a large part of her safe pregnancy as a gestational diabetic.