MRI was introduced in clinical practice in the early 1980s, and the first attempts to apply it among pregnant women were already reported in 1983 ^[1][2][3]. Commonly described indications for MRI during pregnancy can be classified into three main categories (Figure 1): non-obstetric, obstetric or gynecological, and fetal indications ^[4].

To create images, MRI exploits the interaction between protons of molecules within the body (mainly water) and an external magnetic field. In the absence of a strong magnetic field, protons in the body spin around their axis. The axes of different protons are randomly oriented in space ^[21]. When the body is put inside a high magnetic field (1.5 T or 3T in clinical diagnostics ^[22]), all the protons axes align in the direction of the magnetic field, and a net magnetization, called longitudinal magnetization, is generated ^[23]. During the application of an opportune radiofrequency pulse (RF), the total magnetization is tilted out of alignment, and a new, transverse magnetization component arises. When the RF pulse is removed, both components of the magnetization return, or relax, to their previous states under the influence of the magnetic field. During this relaxation process, the body produces an RF signal, which is detected and processed to produce an image ^[23].

MRI images include areas of diverse shades of gray, which correspond to different body components. Endogenous contrast of MRI is due to chemical, physical, and proton density differences in tissues ^[22]. Because of these differences, tissues have different relaxation times, called longitudinal (T1) and transverse (T2), which are the cause of the different shades of gray in the MRI scan (Figure 2).

To date, there are no data that have definitively demonstrated harmful effects on the fetus caused by MRI exposure of the pregnant woman in any trimester of gestation ^{[3][4][24][25][26]}. However, prospective and longitudinal studies are lacking; therefore, long-term safety has not yet been established ^{[3][4][24][25][26]}. The American College of Radiology (ACR), indeed, recommends pregnancy screening for women before MRI scanning. In the case of pregnancy confirmation, ACR advises the use of MRI only if ^[25]:

• It is not prudent to delay the examination until the completion of pregnancy;
• The information needed cannot be obtained through other non-ionizing techniques;
• The data gained may influence the management of the patient or the fetus.

The main theoretical risks associated with the use of non-contrast MRI during pregnancy are due to the implication of strong static magnetic fields (1.5 T or 3 T commonly in clinical use), gradient fields, and radiofrequency pulses (RF) ^{[27][3][4][24]}.

The endogenous contrast produced by body components is not always enough to appropriately decipher MRI images. MRI, indeed, suffers from low sensitivity ^[22]. Contrast-enhanced MRI (CE-MRI) is a magnetic resonance modality in which exogenous substances are administered to increase the quality of images ^[34].

The most common MRI contrast agents (CA) are either complexes of paramagnetic ions, such as gadolinium Gd³⁺ and manganese Mn²⁺, or superparamagnetic iron oxide nanoparticles (SPIONs) ^[35][36]. Paramagnetic contrast agents usually shorten T1 relaxation time, resulting in the enhanced part being brighter (positive agents), while iron oxide particles shorten T2, producing a darker enhanced area (negative agents) ^[35]. Different SPIONs have been used in the past in MRI imaging of liver, spleen, and lymph nodes ^[37]; however, due to poor commercial interest, iron and manganese-based contrast agents are no longer commercially available ^[36][37][38].

When the administration of contrast to pregnant women is strictly required, gadolinium is the only recommended solution, because no animal or human fetal studies assess the safety of other agents during pregnancy ^[39].