Safety and effectiveness of COVID 19 vaccine among pregnant woman Introduction: Major International Organizations such as the World Health Organization (WHO), Centers for Disease and Prevention (CDC), American College of Obstetrics and Gynecology (ACOG) recommend that use of COVID 19 vaccine for people who are pregnant, breastfeeding and trying to get pregnant. Objective: To determine the safety and effectiveness of COVID 19 vaccine among pregnant woman. Materials and methods: In this prospective study, we collected data from pregnant woman who are receiving their antenatal care at the general hospitals of Ulaanbaatar city and National Center for Maternal and Child Health of Mongolia between July 1, 2021 and August 20, 2021. The survey data were collected four categories of questionnaires. An ultrasound scan performed to screening of fetal anomalies and to assess fetal growth at 12, 20, 28, 36 weeks of pregnancy. Results: Among the total of 420 pregnant woman, 193 (46.0%) have received COVID 19 vaccine and 227 (54.0%) were not. (1:1) The most commonly reported adverse events in pregnant participants were injection site pain (63.2%), myalgia (45.2%), headache (44.9%) and fatigue (40%). Furthermore, complications of pregnancy were 11.5% of vaccinated group and 7.8% of the other group and it has shown that pregnancy complications was not statistically significant in differentiating to the study groups (x^2 = 1.62, p = 0.205) Pregnant woman with COVID 19 infections were 73.6% (n=39) of unvaccinated group and 26.4% (n=14) of vaccinated group. Whether to relate between COVID 19 vaccination and COVID 19 infections were statistically significant in differentiating to the study groups. According to the result, COVID 19 vaccine has an effectiveness of 91% for the pregnant woman if they continue to administer the prevention method of COVID 19 infections (same as before the vaccination: wearing a mask, keep a safe distance). The risk of fetal anomalies at first trimester and 20 weeks gestation were not statistically significant in differentiating to the study groups. These results demonstrate that the COVID 19 vaccine does not affects fetus by now. Conclusions: In this study, COVID 19 vaccine does not increase a risk of pregnancy and medical complications. The prevalence of COVID 19 infection was statistically significant in differentiating to the both vaccinated and unvaccinated groups. It shows that COVID19 vaccines have became a crucial tools for reducing the incidence of COVID 19 infection in pregnancy.