Increased perinatal morbidity-mortality are associated with gestational diabetes mellitus (GDM). We studied 69 non-diabetic pregnancies (age 30 +/- 5 years) by repeating oral glucose tolerance test (OGTT, 100 g; area under glycemic, AUGC as g min/dl, and insulinemic, AUIC as mU min/ml, curves were calculated) and HbA1c measurement at 14, 24 and 33 weeks. In the 3rd trimester, 7 women had abnormal OGTT, but none of the 12 mothers of large babies (> 3.9 kg) had GDM. Among 15 pregnants with basal body mass index (BMI) > 25 kg/m2, 2 developed GDM, 5 had babies > 3.9 kg, 8 had normal birthweight babies. Those pregnants who showed after-load hyperglycemia despite normal insulin secretory response (insulin resistance) developed GDM, but delivered normal birthweight babies. Large neonates were delivered from women with the greatest both gestational weight gain and insulin sensitivity, but normal glucose tolerance. The heaviest pregnants with normal both glucose tolerance and insulin sensitivity had normal weight gain and normal birthweight infants. Neonatal body weight was correlated with maternal gestational weight gain, placental weight, 3rd trimester AUIC/AUGC ratio and 1st-2nd trimester HbA1c.