Serial changes in the 50-g oral glucose test in pregnancy: implications for screening.

To quantitate the effect of advancing pregnancy on the screening test for gestational diabetes, 550 patients were given a 50-g oral glucose screening test at 20, 28, and 34 weeks' gestation. A significant increase in the plasma glucose value was found, with a mean increase of 1.1 +/- 1.9 mg/dL per week from 20-34 weeks' gestation (r = 0.39, P less than .01). Only 34 patients (6.2%) had a positive screening test at 20 weeks' gestation, and it appears that routine screening in a low-risk population at 20 weeks is not warranted. Sixty patients (10.9%) with a negative screening test at 20 weeks had a positive test at 28 weeks. Forty-four patients (8.0%) with a negative test at 28 weeks had a subsequent positive test at 34 weeks. The overall incidence of gestational diabetes in this population was 4.9%. Currently, screening is recommended at 24-28 weeks for all pregnancies that have not been previously identified as having glucose intolerance. Eleven percent of the gestational diabetics in our population would have been undetected had we screened only at 24-28 weeks. This quantitative information may be helpful in deciding whether rescreening later in pregnancy is indicated.