An observational study comparing 2-hour 75-g oral glucose tolerance with fasting plasma glucose in pregnant women: both poorly predictive of birth weight.

BACKGROUND The definition and treatment of glucose intolerance during pregnancy are matters of intense controversy. Our goal was to examine the value of the 75-g oral glucose tolerance test (OGTT) in terms of its ability to predict birth weight percentile in a group of women with singleton pregnancies who received minimal treatment for their glucose intolerance. METHODS We reviewed the results of OGTTs performed between 24 and 28 weeks' gestation in a group of 300 consecutive high-risk women (mean age 29.5 years [95% confidence interval, CI, 28.9-30.1]; parity 1.5 [95% CI 1.4-1.7]) whose plasma glucose level 1 hour after a randomly administered 50-g glucose load was 8.0 mmol/L or above. These data were compared with results for a randomly selected control group of 300 women whose plasma glucose level 1 hour after a 50-g glucose load was less than 8.0 mmol/L (mean age 28.0 years [95% CI 27.4-28.6]; parity 1.5 [95% CI 1.3-1.6]). RESULTS For 76 (25.3%) of the 300 high-risk women, the plasma glucose level 2 hours after a 75-g glucose load (confirmatory OGTT) was 7.8 mmol/L or more, but only 6 of these were treated with insulin, which emphasizes the low level of intervention in this group. Thirty (10.0%) of the neonates in this group were large for gestational age (LGA; adjusted weight at or above the 90th percentile). This proportion did not significantly differ from the proportion for the control group (25 or 8.3%). After exclusion of the 6 insulin-treated women, simple correlations between birth weight percentile and fasting or 2-hour plasma glucose levels were very weak (r = 0.23 and 0.16 respectively; p < 0.01). The correlation between birth weight percentile and fasting or 2-hour plasma glucose persisted in a multiple regression analysis that included the following maternal variables: age, prepregnancy weight, weight gain during pregnancy, parity and smoking. In the multivariate models, the standardized coefficients for fasting and 2-hour plasma glucose levels were low (r = 0.19 [p < 0.001] and r = 0.13 [p = 0.02] respectively). These multivariate models could not explain more than 22% of the total variability in birth weight percentile. INTERPRETATION In this population of pregnant, untreated diabetic women, plasma glucose levels (either fasting or after various glucose loads) were independently but poorly correlated with birth weight; no more than 3% to 5% of birth weight variability could be explained by changes in glucose tolerance. Fasting plasma glucose was consistently but marginally better than the plasma glucose level 2 hours after 75-g glucose load for predicting LGA neonates. We conclude that neonatal macrosomia is influenced by variables that are largely independent of plasma glucose concentrations.