Ultrasound is the screening modality of choice for the fetal imaging. However, there are circumstances in which an alternative imaging technique is needed for additional information regarding fetal anatomy and pathology as well as different maternal conditions.

Magnetic resonance imaging (MRI) is being increasingly used as correlative imaging modality in pregnancy because it uses no ionizing radiation, provides excellent soft-tissue contrast, and has multiple planes for reconstruction and large field of view, allowing better depiction of anatomy in fetuses with large or complex anomalies.

In this review, we attempted to identify strengths and weaknesses of each modality both from the literature and our own working experience, and to propose to some practical recommendations on when to use which imaging modality.

Both ultrasonography and MRI are operator-dependant and neither technique obviates the need for thorough knowledge of normal and abnormal anatomy. In early pregnancy, and where repeated assessment is needed, ultrasound has the obvious advantage. In circumstances where ultrasound examination is difficult, as in the obese patient or severe oligohydramnion, better images might be obtained by MRI examination. MRI might also identify early fetal ischemic lesions after an insult, such as maternal trauma or death of a monochorionic co-twin. From the published literature, it would appear that MRI may provide additional diagnostic information to that given by ultrasound in 25 to 55% of cases, which in turn may have influence on parental counseling and/or management of affected pregnancies. Individual circumstances and expertise influence the accuracy of both modalities. Ultrasound and MRI should be performed to the highest possible standard, and the final diagnosis should be made in a multidisciplinary setting.