This study determined whether maternal glucose tolerance has a progressive effect on the length of gestation in singleton pregnancies and whether there is an increasing tendency towards spontaneous preterm birth with increasing glucose intolerance. A total of 2,168 consecutive Chinese women with singleton pregnancies who underwent the 75-g oral glucose tolerance test (OGTT) over a 24-month period were categorized by their OGTT 2-hour value (mmol/L) into the following six groups: 5.9 or less, 6.0-6.9, 7.0-7.9, 8.0-8.9, 9.0-10.9, and 11.0 or greater mmol/L. Women with a 2-hour glucose value of 8.0 or more mmol/L were considered to have gestational diabetes mellitus (GDM) and received diet treatment. Women who eventually required insulin were excluded from the final analysis. The mean gestational age, birth weight, incidence of preterm birth, large for gestational age (LGA, birth weight > 90th percentile), and macrosomic (birth weight > or = 4.0 kg) infants were compared among the six groups. The incidence of preterm birth correlated significantly with increasing glucose intolerance. On further analysis, incidence of spontaneous birth before 37 weeks in the lowest to the highest 2-hour value groups was as follows: 5.5%, 2.6%, 3.7%, 4.9%, 8.5%, and 10.3% (P =.015) and that before 32 weeks went from 0.4%, 0.3%, 0.8%, 0.4%, 2.2%, to 3.4% (P =.018), respectively. There was no significant difference in the incidence of LGA or macrosomic infants. Regression analysis confirmed that the OGTT 2-hour glucose value was an independent determinant of gestational length. Gestational glucose intolerance affects gestation length and incidence of preterm birth, which should be considered a confounding factor in the analysis of the neonatal outcome of GDM pregnancies.