Magnetic resonance imaging of the placenta and gravid uterus: a pictorial essay.

The radiologist must be familiar with normal gravid uterus and placental anatomy in order to recognize different pathologic conditions. The normal gravid uterus has an inverted pear-shape with the fundus and body greater in anteroposterior dimension than the lower uterine segment. The normal uterine contour is smooth with no focal bulges. The myometrium normally exhibits a tri-layer appearance on T2WI – the middle layer is intermediate signal and is surrounded on either side by thin low signal intensity inner and outer layers ( Figure 1 ). The utero-placental interface is normally seen as a thin, smooth, uninterrupted and gently curved hypointense interface on T2WI and SSFP. Normal flow-voids can be seen in the retro-placental region and in the placenta, especially near the cord insertion [ 11 ]. As the fetus grows and the normal myometrium thins later in pregnancy, the tri-layer appearance may no longer be apparent. The placenta normally has a discoid shape, measuring between 2–4 cm in thickness, and may be located anterior, posterior, lateral, or fundal. Placental tissue grows and can migrate to different positions throughout the first 15 weeks gestation as placental tissue seeks preferential areas of increased blood supply (trophpotropism). Abnormal thinning of the placenta can be seen with vascular and hematologic conditions while thickening may be due to hydrops, infection, maternal diabetes or anemia. Uterine leiomyomas and focal myometrial contractions ( Figure 2 ) are potential mimics of placental thickening, the latter of which are usually transient and may change or resolve completely during the course of the MR exam [ 16 ]. On T2WI and SSFP, the placenta is normally homogeneous intermediate signal, with gentle lobulations. The placenta has a fetal surface and a maternal surface. The fetal surface, also known as the chorionic plate, is the location of umbilical cord insertion. The maternal surface of the placenta contains placental cotyledons, which are visualized on T2W images separated by thin bands of T2 hypointense signal and become more prominent with increasing gestational age. As the pregnancy progresses and the normal uterus thins, the tri-layer appearance of the uterus may not be perceivable; however, the myometrium should not demonstrate focal bulges or areas of mural disruption [ 11 ]. Discoid placenta is the most common placental morphology ( Figure 3a ), followed by bilobed (two equal size adjacent placental lobes) and succenturiate lobes (an accessory lobe separate from the dominant placental lobe; Figure 3b – c ). Less frequently seen variants include circumvallate placenta (small chorionic plate relative to basal plate, creating a raised edge of tissue at the placental margins; Figure 3d ) and placenta membranacea (thin membranous structure circumferentially occupying the entire periphery of the chorion) [ 16 ]. The umbilical cord normally inserts centrally on the placenta ( Figure 4a ) but can be marginal (inserting less than 2 cm from placental edge; Figure 4b ) or velamentous (inserting on chorioamniotic membranes rather than directly on placental tissue; Figure 4c ). The latter scenario results in umbilical vessels which extend for a variable length between the amnion and chorion and are at risk for injury because they are unprotected by Wharton’s jelly [ 17 ]. Umbilical cord insertion is typically evaluated with second trimester US, but is also well seen on T2WI.

Chorionicity and amnionicity of twin gestations is most often determined with US, which has a high degree of specificity and sensitivity [ 18 ], but diagnostic findings can also be seen on MR. Dichorionic-diamniotic twins are characterized by separate amniotic sacs and placentas supplying each fetus. The twin peak sign represents a triangular projection of placental tissue seen in the first trimester and is pathognomonic for dichorionic-diamniotic twins, as is a thick (≥2 mm) inter-twin membrane which consists of the two layers of amnion and two layers of chorion separating the fetuses [ 19 ] ( Figure 5a ). Monochorionic-diamniotic twins have two amniotic sacs but share one placenta and have a thin inter-twin membrane [ 20 ]. Monochorionic-monoamniotic twins share one amnion and one placenta with no intervening membrane. Monochorionic twins may be complicated by twin-twin transfusion syndrome or twin embolization syndrome, and monoamniotic twins are at risk of cord entanglement [ 21 ] ( Figure 5b ).

Vasa previa represents fetal vessels crossing the internal cervical os. It is associated with low-lying placenta, placenta previa, variant placental morphology and twin gestations. The vessels are unprotected by Wharton’s jelly and are prone to injury during rupture of membranes and delivery, which can result in fetal hemorrhage and death [ 37 , 38 ]. The diagnosis of vasa previa is commonly made with Doppler US, demonstrating fetal vessels covering the internal os, but findings may also be seen on MR ( Figure 11 ). Flow voids overlying the internal os may be seen on T2WI, and time-of-flight sequences can demonstrate the direction of flow within the umbilical vessels [ 39 ].

Placental cysts are usually small and incidental findings, occur at the fetal surface of the placenta near the cord insertion, and are anechoic without internal Doppler flow on US [ 54 ]. They follow fluid signal on MR and do no demonstrate enhancement. Placental venous lakes represent prominent intervillous spaces, appear hypoechoic or anechoic on US, and are usually of little clinical significance when <2 cm, solitary or few in number [ 16 ]. Chorioangioma is the most common benign placental tumor, occurring in less than 1% of pregnancies. On US, chorioangioma appears as a solid well-circumscribed variable echogenicity mass with internal Doppler flow, often located near the cord insertion on the fetal side of the placenta. These lesions are usually asymptomatic and identified incidentally on US, however, multiple lesions or lesions larger than 5 cm have been associated with poor fetal outcomes [ 55 ]. MR is not usually performed to characterize chorioangiomas, but they show T2 hyperintense and T1 isointense signal, with areas of T1 hyperintense material representing hemorrhage [ 56 ].

Uterine rupture is a potentially catastrophic complication of pregnancy, defined as disruption of the uterine wall, with or without expulsion of the fetus [ 26 ]. Most uterine ruptures occur during labor. Risk factors include previous cesarean section (especially vertical/classical), trauma, uterine over distension, placenta percreta and choriocarcinoma [ 27 ]. US and MRI findings include a frank defect in the uterine wall, intrauterine and free peritoneal blood, and fetal or placental tissue located outside of the uterus [ 28 ] ( Figure 7 ).

Placenta previa occurs when placental implantation in the lower uterine segment results in partial or complete covering of the internal cervical os. Cesearean delivery is typically recommended to minimize the risk of maternal hemorrhage during labor. The inferior edge of the placenta is normally located at least 2 cm from the internal cervical os after a gestational age of 15 weeks. Low-lying placenta is characterized by the lower placental margin within 2 cm of the internal os ( Figure 10a ). Marginal previa is diagnosed if the placenta extends to the edge of the internal os but does not cover it ( Figure 10b ). In partial placenta previa, the placenta partially covers the internal cervical os ( Figure 10c ) and in complete previa, the entire internal cervical os is covered by the placenta ( Figure 10d ). Due to normal trophotropism and migration of the placenta during the first trimester, a diagnosis of placenta previa should not be made until after 15 weeks gestation. Suspected cases of placenta previa seen prior to 15 weeks should be re-evaluated with US to confirm placental position prior to delivery [ 35 ]. Transvaginal US is the gold standard for imaging of placenta previa, but this condition can also be accurately diagnosed with MR. Sagittal MR images optimally display the relationship between the placenta and internal os to evaluate for placenta previa. An important pitfall is an over-distended urinary bladder which can compress the lower uterine segment and result in a false positive diagnosis of placenta previa [ 36 ].

Retained products of conception (RPOC) is usually suspected clinically when examination reveals an incomplete placenta after delivery, miscarriage or termination of pregnancy. US findings of RPOC are non-specific but include heterogeneous echogenicity material in the endometrial canal which may have associated Doppler flow [ 47 ]. On MR, findings of RPOC include an endometrial mass of heterogeneous T2 signal, areas of T1 hyperintense blood products and post contrast enhancement [ 48 ] ( Figure 15 ). While the imaging appearance is non-specific and overlaps with that of GTD, the clinical scenario is often different, and β-hCG is usually only mildly elevated or normal. An additional differential diagnostic consideration is uterine arteriovenous malformation (AVM), which may develop following dilation and curettage, however uterine AVM shows characteristic flow voids, enhancing serpentine vessels on MR angiography and may demonstrate areas of susceptibility artifact [ 49 ].

Metastases to the placenta are very rare, believed to occur via hematogenous dissemination with deposition in the intervillous space of the placenta. Melanoma is the most common primary malignancy to metastasize to the placenta, with less frequent primary sites including lung, breast, gastric, and gynecologic neoplasms [ 57 ]. The appearance has not been well described in the radiology literature but could be included in the differential diagnosis of multiple solid placental lesions in the appropriate clinical context.

MRI is being utilized with increasing frequency and has many advantages compared with sonography for evaluating the placenta and gravid uterus. MR provides valuable information about the fetus, uterus, placenta and a detailed assessment of maternal pelvic anatomy. Radiologists should be aware of the spectrum of normal placental variants, as well as benign and life-threatening conditions affecting the placenta and gravid uterus to optimize patient care and minimize fetal and maternal perinatal complications.

Gestational trophoblastic disease (GTD) is a category which includes partial and complete hydatidiform moles, invasive mole and choriocarcinoma. Patients usually present with first-trimester bleeding, large-for-date uterine size and hyperemesis. Laboratory analysis will show markedly elevated β-human chorionic gonadotropin (β-hCG) [ 9 ]. Complete molar pregnancy is the most common form of GTD and occurs following fertilization of an empty ovum, resulting in proliferation of trophoblastic tissue and a complex multicystic mass. Sonographically, the typical appearance is a large heterogeneous endometrial mass with numerous variably-sized cysts and no apparent fetal parts, producing a cluster of grapes appearance [ 45 ]. Partial hydatidiform mole is less common and results from fertilization of a normal ovum by two sperm, resulting in a triploid genotype. On US, the appearance can resemble that of a complete mole, but unlike complete moles, fetal tissue is also present. MR is not performed to distinguish between partial or complete moles, but may be obtained to determine if there is an invasive component, as evidenced by molar tissue extending into the myometrium ( Figure 14 ). Invasive mole may occasionally demonstrate deep invasion with extension through the uterus into the peritoneum [ 9 ]. A highly vascular process, GTD demonstrates avid enhancement, heterogeneous signal on T2WI, and may have areas of hemorrhage and cystic change [ 46 ]. In contradistinction to invasive moles, which are locally aggressive but not malignant, choriocarcinomas are malignant neoplasms and frequently metastasize to the lungs. MR is useful in determining depth of invasion of both invasive mole and choriocarcinoma, but cannot reliably distinguish between these two entities [ 9 ].

Ultrasound (US) is the primary imaging modality used for antepartum fetal imaging because it can detect fetal and placental abnormalities, lacks ionizing radiation, has widespread availability, and can be repeated over the course of the pregnancy with minimal risk to the patient and fetus [ 1 ]. Magnetic resonance (MR) imaging offers many advantages to US, including superior soft tissue contrast resolution as well as advanced fetal imaging techniques such as arterial spin labeling. MR is being performed with increasing frequency in the prenatal setting to evaluate fetal and maternal pathologies. The purpose of this pictorial essay is to describe the MR imaging techniques and findings of various conditions which affect the placenta and gravid uterus.