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Pregnancy & Prenatal Testing

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Second Trimester: Blood Glucose

During pregnancy, up to 3% of women develop gestational diabetes (increased glucose levels). Although it can occur at any time, most cases will develop during the later part of the pregnancy. If increased blood sugar levels in the pregnant woman are uncontrolled, they can cause the fetus to increase in size and weight. They can also cause the baby to be born with very low glucose levels and to have breathing difficulties.

Most women are checked for gestational diabetes between 24 and 28 weeks of pregnancy. Sometimes a test for diabetes is done earlier in the pregnancy if a woman is suspected of having pre-existing diabetes or is considered to be at higher than average risk.

Risk factors for diabetes include:

  • Older age
  • Ethnicity with higher rate of diabetes such as Hispanic, Native American, South or East Asian, African American, or Pacific Islands descent
  • Obesity
  • Family history of diabetes
  • Personal history of gestational diabetes with a previous pregnancy


The U.S. Preventive Services Task Force, the American Academy of Family Physicians, the American College of Obstetricians and Gynecologists (ACOG), and the American Diabetes Association (ADA) recommend that all pregnant women be screened for gestational diabetes after 24 weeks of pregnancy.

ACOG recommends screening using patient history, risk factors, or laboratory testing.

In the U.S., laboratory testing may involve either a one-step or two-step approach:

  • One-step 2-hour oral glucose tolerance test (OGTT): After a fasting glucose level is measured, a woman is given a 75-gram dose of glucose to drink and her glucose levels are measured at 1 hour and 2 hours after the dose. Only one of the values needs to be above a cutoff value for diagnosis.
  • Two-step
    • Perform a glucose challenge test as a screen: a woman is given a 50-gram glucose dose to drink and her blood glucose level is measured after 1 hour.
    • If the challenge test is abnormal, perform a 3-hour oral glucose tolerance test. After a woman's fasting glucose level is measured, she is given a 100-gram glucose dose and her glucose is measured at timed intervals. If at least two of the glucose levels at fasting, 1 hour, 2 hour, or 3 hour are above a certain level, then a diagnosis of gestational diabetes is made.

ACOG currently recommends the two-step approach while the ADA says either the one-step or two-step approach is appropriate.

For women with gestational diabetes, diet control and/or insulin injections throughout the rest of the pregnancy may be required to bring glucose levels down to normal levels. In most cases, gestational diabetes will go away after delivery, but women who have gestational diabetes will be at an increased risk of having it again with subsequent pregnancies and of developing diabetes in the future. Some organizations, including ACOG and the ADA, recommend that women diagnosed with gestational diabetes be screened 6-12 weeks after delivery for diabetes that persists.

National Diabetes Information Clearinghouse: What I Need to Know about Gestational Diabetes
American College of Obstetricians and Gynecologists: Gestational Diabetes  
Centers for Disease Control and Prevention: Type 1 or Type 2 Diabetes and Pregnancy