In last 20 years the topic of diabetes in pregnancy has become more relevant than it used to be. For a long time, it has been known that pregnant women have a higher chance of giving birth to a macrosomic child and a higher incidence of birth trauma, but the pathophysiological mechanisms were unknown. Research has shown that maternal hyperglycemia leads to hyperglycemia in the fetus which influences a secretion of fetal insulin. The insulin stimulates an excessive fetal growth. The development of hyperglycemia in pregnant women is associated with an insulin resistance which occurs in the second half of pregnancy due to the activity of certain hormones whose concentration continuously grow till the end of a pregnancy. If diabetes occurs under this circumstance it is called a gestational diabetes and it withdraws after a pregnancy. The infants who weight more than 4000 grams are called macrosomic infants. Macrosomic newborns are at a higher risk of suffering birth trauma such as shoulder dystocia as well as a development of hypoglycemia in the first 24 hours which can be very dangerous due to a possibility of brain damage. Numerous complications are associated with diabetes in pregnancy. The most common complications of diabetes in pregnancy are hypertension and preeclampsia, diabetic nephropathy, diabetic retinopathy and an increased tendency of infections, particularly urogenital infections. These complications can cause a huge burden on the health of a pregnant woman and can put the fetus in danger. It is important to establish the diagnosis of diabetes in pregnancy as soon as possible in order to prevent negative outcomes and complications. It is recommended that all pregnant women undergo an oral glucose tolerance test between 24 and 32 weeks of pregnancy. In case of diabetes, pregnant women should be advised to change their diet and the therapy with oral anti-diabetic drugs and/or insulin must be conducted.