Objective. To determine the pathophysiological connection between progesterone level, magnesium deficiency and vein varicosity in pregnant women. Material and methods. Progesterone, magnesium and free hydroxiprolin levels in blood serum were measured. Ultrasound investigation of the lower extremities veins was made. Results. The positive correlation between progesterone level and veins elasticity index, the negative correlation between magnesium level in saliva and veins elasticity index and positive correlation between coefficient of magnesium distribution and veins elasticity index was demonstrated in patients with undifferentiated dysplasia of connective tissue at 15—16 weeks of pregnancy. Conclusion. The study showed that repeated pregnancy, advanced age and high degree of  undifferentiated dysplasia of connective tissue are associated with significant increase in progesterone level in blood serum, by increased collagen disintegration and increased concentration of degradation products, by increased elasticity index, magnesium deficiency and risk of varicosity development.