 X-rays and computerized tomography (CT) are not used, especially in the first trimester, due to the ionizing radiation, which has teratogenic effects on the foetus.No effects of magnetic resonance imaging (MRI) on the foetus have been demonstrated, but this technique is too expensive for routine observation.Instead, obstetric ultrasonography is the imaging method of choice in the first trimester and throughout the pregnancy, because it emits no radiation, is portable, and allows for realtime imaging.