Intrauterine device is a popular, cost-effective method of contraception known worldwide. It is extremely effective, with pregnancy rates comparable to tubal ligation (5-year cumulative rate varying from 0.5 to 1.1). LNG-IUS increases in popularity and may be used as the method of choice for adults because it offers non-contraceptive benefits: slight menstrual bleeding, decreased number of incidents of dysmenorrhea, and reduced pain associated with endometriosis. The levonorgestrel-releasing intrauterine system (LNG-IUS) provides a release rate of 20 μg/24h of levonorgestrel. The hormone released from the intrauterine device causes some systemic changes, however local effects, such as glandular atrophy and stromal decidualization, in addition to foreign body reaction, are dominant. According to the Food and Drug Administration regulations, category X was assigned to LNG. The use of the product is contraindicated in women who are or may become pregnant. We report a case of a 30-year old woman who has already been pregnant before the insertion of a levonorgestrel –releasing intrauterine system (LNG-IUS). The patient was lactating after the previous pregnancy, the first menstruation had not appeared yet. The patient planned to have the IUD inserted but she missed her appointed visits twice. The third visit took place 8 weeks postpartum. During that visit, the doctor asked her about the possibility of being pregnant, but the patient denied. The gynecologist inserted the intrauterine device and performed transvaginal ultrasound examination. The location of LNG-IUS was proper and there was no gestation sac in the uterine cavity. After the next six weeks, transvaginal ultrasonography and manual examination showed an intrauterine pregnancy at 8 weeks of gestation. The intrauterine device was not detectable in the uterine cavity by ultrasound, and the IUD strings were not visualized in the vagina. It was impossible to remove the device without causing miscarriage. Other risks during pregnancy were connected with potential masculinisation of the fetus by levonorgestrel, premature delivery, and preterm rupture of the membranes. The pregnancy progressed normally, and the delivery was uncomplicated. A female infant was born without congenital abnormalities. The pediatrician did not find any indices of infection. The IUD was found in fetal membranes. The postpartum recovery was uneventful. This case report demonstrates that a healthy infant can be delivered at term with an IUS in situ, probably without causing any abnormalities. Due to that fact that adverse effects of fetal exposure to the LNG-IUS have not been yet established, the exclusion of pregnancy before IUD insertion is strictly indicated.