Objectives: To estimate serum 25-hydroxy vitamin D (25OH-VD) and insulin, and fasting blood glucose (FBG) early in pregnancy, determine the frequency and severity of insulin resistance (IR), gestational diabetes mellitus (GDM) and pre-eclampsia (PE) during pregnancy and the impact of VD supplemental therapy (VD-ST) on these effects. Patients & Medicine: 494 pregnant women fulfilling inclusion criteria were randomly divided into two equal groups: Study group received VD-ST as a daily oral dose of 1000 IU softgels with meal since 6th week till delivery, while Control group did not receive VD-ST. All women gave blood samples for colorometric estimation of FBG and ELISA estimation of serum insulin and 25-OH-VD levels. Evaluated parameters included Body mass index (BMI), VD sufficiency status and Homeostasis model assessment IR (HOMA-IR) score. Oral Glucose Tolerance Test for diagnosis of GDM was performed at the 20th, 28th and 36th week GA and blood pressure was measured regularly at follow-up visits for diagnosis of PE. Results: At time of enrolment, 405 women (81.9%) were overweight-obese, 86 women (17.4%) had IR and only 63 women (12.8%) had sufficient serum 25OH-VD level. At 3rd trimester, 68 women (13.8%) developed GDM, 71 women (14.4%) developed PE and 23 women (4.7%) developed both with significantly lower incidence in women received VD-ST. Frequency of GDM and PE showed positive significant correlation with BMI and HOMA-IR score, while showed negative significant correlation with serum 25OH-VD. ROC curve analysis defined low 25OH-VD level and high HOMA-IR score as significant sensitive predictors for development of both GDM and PE while receiving VD-ST was the significant specific predictor for possibility of amelioration of such event. Kaplan-Meier Regression curve defined a cumulative hazard for developing both GDM and PE of