That's the conclusion of researchers from the Kaiser Foundation Hospital in California, where studies found the test has such a high rate of false positive results that it does little in the way of predicting who is really at risk.
"Traditionally, the prenatal exam includes a fasting plasma glucose screening, which measures the level of sugar in the blood after a period of fasting," explains Dr. Michael Silverstein, an assistant clinical professor of obstetrics and gynecology at New York University Medical Center.
What this new study shows, Silverstein adds, is that the screening may serve little purpose -- other than uncovering a hidden insulin problem that may have existed before the pregnancy.
"According to the research, the screening does not appear to be that accurate in predicting gestational diabetes," Silverstein says.
Much like regular diabetes, gestational diabetes impairs the ability of the hormone insulin to properly move sugar from the blood to the tissues and organs where it is needed to produce energy.
In the beginning of a pregnancy, a woman's natural insulin production overpowers the placental hormones, so sugar levels don't soar. However, experts say that as a baby grows, so much of the insulin-destroying placental hormones is made that they eventually overpower the mother's insulin production.
When this occurs, gestational diabetes develops -- usually somewhere around the 24th week of pregnancy. According to the American Diabetes Foundation, it's a problem that affects 200,000 women a year.
Although a fasting plasma glucose screening administered during the first prenatal visit was always thought to help identify those women at risk for this problem, the new study shows testing this early may be a waste of time.
In the new report, study author Dr. David Sacks says the high false positive rate "makes it an inefficient screening test."
The study, published in the June issue of Obstetrics and Gynecology, involved 4,507 women, all of whom were tested for diabetes using the fasting plasma glucose screening early in their pregnancy. Those whose tests revealed sugar levels greater than 126 milligrams/deciliter of blood were re-tested. If sugar levels remained high, they were automatically referred for high-risk pregnancy diabetes care.
In the final analysis, 302 women were diagnosed with gestational diabetes. Of those, 12 were in the first testing group, where blood sugar levels initially measured 126 or higher. An additional 34 women were identified before their 24th week of pregnancy after a fasting glucose test revealed levels between 100 and 126.
However, the study also found a high false positive rate of some 57 percent among the women who tested early on. This, say researchers, reduced the "sensitivity" of the test to just 80 percent, a relatively low number in terms of accuracy.
The final conclusion: Fasting plasma glucose screening early on in pregnancy is not sensitive enough to identify those at risk for this problem.
"I believe that this is an important finding and one that we need to consider very carefully when deciding the right time to screen a pregnant woman for gestational diabetes," Silverstein says. He also believes that, unless risk factors are present, screening early in the third trimester would be the best time to reflect the most accurate diagnosis.
Risk factors for gestational diabetes include obesity, family history of diabetes, previous birth of a large baby or a stillbirth, or previous birth of a child with birth defects.
Normally, gestational diabetes is controlled via diet and exercise, and sugar levels usually return to normal shortly after delivery.
However, even when controlled, babies born to mothers with gestational diabetes can be larger than normal -- a condition known as "macrosomia," often making a Caesarean delivery necessary. In addition, these babies may have an increased risk of low blood sugar following birth, as well as an increased risk of jaundice and an increased risk of respiratory distress syndrome, a disorder that can make it hard for them to breathe. Later in life, they may also have a higher risk of diabetes and obesity.
To learn more about blood sugar testing, visit Your Family Doctor. To find out more about gestational diabetes, check out The National Institute of Child Health and Human Development.