The earlier tests, Nahum said, allow women at highest risk for gestational diabetes to be identified and treated earlier, possibly heading off complications in both mother and baby. He is lead author of a study on the findings that appears in the August 2002 issue of the Journal of Reproductive Medicine.
Obstetricians have usually screened women in their third trimester because most doctors believe that screening later in pregnancy is more accurate. However, he said, "Screening at 16 weeks is a better predictor of gestational diabetes. It's more sensitive than screening later, and allows us to focus earlier on women who are at greatest risk. It's also a more practical screening technique because blood samples drawn during early pregnancy for other tests can also be used for this purpose."
Nahum and colleagues from the University of Hawaii School of Medicine and California State University studied 255 pregnant women who visited a private obstetrics practice in Honolulu.
All of the women were given a glucose screening test between 14 and 18 weeks of pregnancy. At initial screening, 14 of the 255 women had blood plasma glucose levels higher than 135 milligrams of glucose per deciliter of blood (mg/dL)and were sent for additional testing to confirm a gestational diabetes diagnosis.
The remaining 241 women were screened again at between 24 and 32 weeks. At either point, if women tested higher than 135 mg/dL, they were sent for additional formal glucose tolerance testing.
"Women who had a 16-week glucose screening test result of less than 110 mg/dL had a 99.4 percent chance of not developing gestational diabetes," said Nahum. "None of the women who had a result of less than 104 mg/dL developed gestational diabetes. This gives us a cutoff point. Women who test in this range at 16 weeks are almost guaranteed not to develop gestational diabetes."
Nahum said that for women who fall in the intermediate range (111 mg/dL to 134 mg/dL) at 16 weeks, a repeat glucose screening test should be given at 28 weeks to assess whether the woman should be sent for more in-depth testing.
While it is a natural assumption to say that earlier screenings should lead to better outcomes for mother and baby, Nahum said more studies are needed to confirm that.
"We don't know how earlier diagnosis of gestational diabetes will impact maternal-fetal outcomes because we didn't examine that in this study, but that is certainly something to focus on in future research," he said.
Gestational diabetes occurs in 2 percent to 7 percent of all pregnancies in the United States. It develops when a pregnant woman is not able to produce enough insulin to keep her blood sugar, also known as glucose, within a normal range that is safe for her and her developing baby.
Complications from gestational diabetes include above-average weight gain for the baby that can result in injury to the nerves in the baby's neck from a difficult delivery, an increased risk of newborn fractures, a higher likelihood of fetal distress and possible hypoglycemia, jaundice or breathing problems in the baby.
Changes in diet and exercise are usually enough to help keep blood sugar levels within the normal range, but in some cases insulin injections are needed. Most women who develop diabetes during pregnancy go back to having normal glucose tolerance after the birth of the baby; however, these women may be at greater risk for adult onset diabetes mellitus later in life.
Co-authors include Stephen B. Wilson, M.D., University of Hawaii School of Medicine; and Harold Stanislaw, Ph.D., California State University.