Aim. To compare the impact of the time and method of diagnosis on gestational diabetes mellitus (GDM) in women who gave birth at the Hospital of the Lithuanian University of Health Sciences (LUHS) Kauno klinikos. Methods. A retrospective study was performed using data from the Department of Obstetrics and Gynecology of the LUHS Birth Registry to analyze the data of women who gave birth and had GDM in 2020–2021. The subjects were divided based on the type of diagnosis: GDM was diagnosed either at the first antenatal visit when fasting plasma glycemia (FPG) was ≥5.1 mmol/L (early diagnosis group) or after OGTT at 24 + 0 − 28 + 6 weeks of gestation when at least one pathological glycemic index was observed: fasting glycemia 5.1–6.9 mmol/L or 1-h glycemia ≥10.0 mmol/L or 2 h glycemia 8.5–11.0 mmol/L (late diagnosis group). The results were processed using IBM SPSS. Results. The early diagnosis group had 1254 (65.7%) women, the late diagnosis group had 654 (34.3%). More primigravida women were in the late diagnosis group (p = 0.017) while more multigravida were in the early diagnosis group (p = 0.033). The early diagnosis group had more obese women (p = 0.001), including those with a BMI > 40 (p = 0.001). In the early diagnosis group, GDM was more frequently diagnosed in women who gained <11 kg (p = 0.005), while in the late diagnosis group—>16 kg (p = 0.001). FPG was higher in the early diagnosis group (p = 0.001). Glycemia was more commonly corrected with lifestyle changes in the late diagnosis group (p = 0.001), and with additional insulin therapy in the early diagnosis group (p = 0.001). Polyhydramnios and preeclampsia were more common in the late diagnosis group (p = 0.027 and p = 0.009). There were more large-for-gestational-age neonates in the late diagnosis group (p = 0.005). Macrosomia was more common in the late diagnosis group (p = 0.008). Conclusions. GDM is more commonly diagnosed with OGTT in primigravida women. Higher pregestational weight and BMI has an impact on the early diagnosis of GDM and need for insulin therapy with lifestyle changes. Late diagnosis of GDM is connected with obstetric complications.