OBJECTIVE: To evaluate the effects of ionizing radiation exposure during the first trimester of pregnancy in usual clinical situations. STUDY DESIGN: We conducted a prospective observational cohort study using data collected between 1987 and 2014. This database was authorized by the French "Commission Nationale de l'Informatique et des Libertés". The exposed group consisted of 319 pregnant women exposed to sub diaphragmatic ionizing radiations for diagnostic purposes, during the first trimester of pregnancy, and the control group consisted of 319 pregnant women without any exposure or exposed to non-teratogenic agents. Data on maternal history and radiations exposure were collected on first contact, and pregnancy outcomes were documented at follow-up. An univariate analysis was performed to compare both groups for the main outcomes. RESULTS: Exposure to sub diaphragmatic ionizing radiation for diagnosis purpose (median fetal dose of 3.1 mGy [0.2-130.0]) during the first trimester of pregnancy was not significantly associated with an increased risk of malformations (1.5% vs 1.8%, p = 1.00), miscarriage (7.8% vs 7.2%, p = 0.88), in utero fetal death (0.3% vs 0%, p = 1.00) or fetal growth restriction (5.4% vs 3.5%, p = 0.62). CONCLUSION: Pregnant women exposed to irradiant diagnostic procedures do not present a higher risk of malformations, miscarriage, in utero fetal death or fetal growth restriction and should be reassured, even if the examination focused on the pelvis.