Summary Objectives: Gestational diabetes mellitus (GDM) is defined as any degree of glucose intolerance, first time detected in pregnancy. Early diagnosis of the disease may reduce fetal exposure to maternal hyperglycemia and decrease the risk of LGA. The aim of the study was to examine the influence of time and diagnostic method of GDM on the prevalence of LGA and pregnancy outcome among patients with gestational diabetes. Material and Methods: The study was conducted among 211 women with gestational diabetes mellitus, the patients of 1st Clinic of Obstetrics and Gynecology at the Medical University in Warsaw. We have reviewed the results of fasting plasma glucose, 50-g glucose screening test (GCT) and 2 hour 75-g glucose tolerance test in GDM patients with LGA and eutrophic newborns. The t-student or the Mann-Whitney test was used in order to compare both groups. P<0,05 was deemed statistically significant. Results: LGA was diagnosed in 10,4% of patients. We did not find any significant differences in gestational age when GDM was diagnosed, results of fasting glucose, GCT and OGTT among LGA (M) and control (K) group. However, when we compared the percentage of LGA in groups of women with different time of GDM diagnosis, the highest prevalence was noted in the group of first trimester diagnosis and between 28 and 32 weeks of pregnancy, which we found interesting. We compared the women and the results of the diagnostic tests with the group of standard time of GDM diagnosis (24-28 week of pregnancy) and the only difference was the late diagnosis. If 75-g glucose tolerance test had not included 1-hour after load glucose assignment, GDM would not have been diagnosed at all in 18,2% of female patients with LGA. We have not found any correlations between the results of the diagnostic tests, the time of the diagnosis or the mode of treatment GDM (diet alone or with insulin) and the birth weight. Conclusions: 1. Results of fasting glucose and glucose tolerance tests are not useful in the prediction of LGA in GDM pregnancies. 2. Diagnosis of GDM during the recommended period (between 24 and 28 weeks of pregnancy) may decrease the prevalence of LGA (comparing to later diagnostics). 3. 75-g glucose tolerance test should provide fasting, 1 and 2-hour after load glucose assignment.