Coexistent complete mole with a concomitant live fetus is a rare occurrence, with a rate of about 1 in 22,000–100,000 pregnancies [1]. Typically first identified as an incidental finding on Ultrasound (US), a coexistent complete mole and live pregnancy is a dizygotic twin pregnancy wherein the normal fetus will have a normal placenta [2]. Complete moles contain 46 paternal chromosomes and do not have any fetal parts, whereas partial moles (the main differential diagnostic consideration) contain a triploid set of chromosomes resulting from two sperms that fertilize a haploid oocyte [3].

One of the methods to differentiate between complete and partial moles is the serum human chorionic gonadotropin (hCG) levels. In normal singleton pregnancies without moles, serum hCG levels typically peak in the 9th and 10th weeks of gestation and decline steadily thereafter [4]. In contrast, complete or partial hydatidiform moles are usually diagnosed when an abnormally high serum hCG is detected.

US, the initial study of choice for hydatidiform mole, typically shows a multi-cystic mass without fetal parts and without central color Doppler flow in the cystic component [5]. Doppler typically demonstrates high velocity and low impedance flow in the solid parts of the molar mass. However, the US detection rate of a coexistent mole with a concomitant live fetus is only about 68% [6]. Magnetic resonance imaging (MRI) is a useful alternative to US because MRI depicts the molar mass as separate from the live fetus with its gestational sac and normal placenta. Moreover, the mole’s depth of myometrial invasion on T2-weighted MRI as indicated by heterogeneous signal at an indistinct endo-myometrial boundary is important for planning separation of the molar mass from the uterus [2]. Magnetic resonance imaging also can be useful when US is equivocal or technically limited, such as for obese patients.

In this case, a multicystic complete molar pregnancy was not associated with a markedly increased hCG, a live twin had a separate sac and placenta, and the obstetrical US was equivocal. This case highlights the utility of MRI for diagnosis and optimized conservative management of a complete mole and concomitant live twin.

A 35-year-old woman, P5 G4 SAB1, presented with vaginal bleeding at 10 weeks gestational age estimated by date of last menstrual period. No clinical evidence of a twin gestation was found (i.e., no abnormal elevation in hCG, hyperemesis, abnormally large uterus). Transvaginal US showed a live fetus and findings compatible with a 1.0 1.4 cm fundal subchorionic hemorrhage. There was no sonographic evidence of a twin fetus, fetal parts, or empty twin sac.

Obstetrical US at 22 weeks and 5 days gestational age for fetal survey revealed a single male fetus in vertex position with a posterior placenta free of the cervical os. Growth, fetal survey, and a biophysical profile were normal. An anterior mural based multi-cystic lenticular shaped uterine mass measuring 11.8 cm long × 4.2 cm AP × 12.8 cm W was completely separate from the posterior placenta in the gestational sac of the live fetus. Color Doppler evaluation showed prominent vascularity within the mass (Figure 1A and Figure 1B).

Magnetic resonance imaging at 26 weeks and 6 days gestational age showed that only one gestational sac was visible containing the live fetus, which was similar to the US findings. The multi-cystic mass measuring 13.1 cm L × 4.7 cm AP × 12.8 cm W was located between the myometrium and the amniotic sac (Figure 2A and Figure 2B). No evidence of hemorrhage was seen. The ovaries were normal, without evidence of theca lutein cysts.

Because the serum beta hCG at 27 weeks 5 days was 60,239 mIU/mL (within normal limits for a singleton gestation at those dates [4]), a partial mole was suspected (Figure 3). Also considered in the differential diagnosis, but thought unlikely, was placental mesenchymal dysplasia, which may be associated with vaginal bleeding, increased vascularity in the mass, fetal intrauterine growth retardation, and sometimes with Beckwith-Wiedemann syndrome [7]. In contrast to molar pregnancy, mesenchymal dysplasia occurs within the placenta of the live fetus, not separately from it [8],[9].

The patient spontaneously delivered a live infant at 35 weeks and 4 days gestational age. The placenta and chorioamniotic membrane of the live fetus appeared normal on gross examination. Diagnosis of complete hydatidiform mole was confirmed by cytopathology without identifiable fetal tissue in the mass. The mass had chorionic villi demonstrating diffuse hydatidiform swelling [10] (Figure 2C, Figure 2D, Figure 2E).

Gestational trophoblastic disease includes a broad spectrum of findings ranging from complete and partial hydatidiform mole to placental site trophoblastic tumor, chorioadenoma destruens, and choriocarcinoma. Complete hydatidiform mole with a coexisting live fetus is one of the rarest presentations of trophoblastic disease [10]. It is even more unusual with a normal hCG for gestational age.

Complete or partial hydatidiform moles are premalignant conditions that require mandatory evacuation of the uterus [1]. Management with a concomitant live fetus is even more complicated, as there are increased risks of vaginal bleeding, prematurity, intrauterine fetal demise, preeclampsia, hyperthyroidism, ovarian theca lutein cysts, uterine rupture, and malignant degeneration [3],[11],[12],[13].

Patients with molar pregnancies also have an increased risk of gestational trophoblastic disease in subsequent pregnancies. Patients with a complete mole and twin, as in this case, have a higher risk of gestational trophoblastic neoplasia as compared to patients with a single complete hydatidiform moles [14]. After evacuation of hydatidiform moles, serial measurements of hCG for several months after delivery is recommended to surveil for the development of gestational trophoblastic neoplasia [15].

With complete molar pregnancies, serum beta hCG usually exceeds 100,000 mIU/mL. However, with partial moles, serum beta hCG can be within the normal range. Therefore, patients with partial moles typically show less severe symptoms than with complete moles [16]. Since the present rare case had normal for gestational age hCG levels, the patient lacked symptoms that would be expected with a complete hydatidiform mole (i.e., no evidence of hyperemesis gravidarum, preeclampsia, nor hyperthyroidism [15]).

Ultrasound imaging in this case showed an indeterminate uterine mass separate from the placenta of the normal twin, but there was doubt about the etiology of this mass. The differential diagnosis included a residual blighted twin with hydropic degeneration, partial molar pregnancy, placental mesenchymal dysplasia in a succenturiate placenta (very rare) [5], or a subchorionic hemorrhage. Magnetic resonance imaging narrowed down the differential diagnosis by excluding hematoma and placental mesenchymal dysplasia. Partial hydatidiform mole and complete mole with live twin were the main considerations. Magnetic resonance imaging demonstrated lack of molar infiltration into the endometrium and a separate live twin that led to the conservative decision to continue the pregnancy [17].

This rare case highlights the utility of MRI after equivocal US for diagnosis and optimized conservative management of a hydatidiform complete mole with concomitant live twin. The clinical presentation was complicated by the normal maternal serum hCG, a rare finding that explains the lack of signs and symptoms usually expected with a complete mole. As MRI provided the necessary detailed information to arrive a fitting differential diagnosis, conservative management resulted in a live born healthy fetus in this case. Obstetrical MRI can be essential in the workup of selected cases with an indeterminate uterine masses in pregnancy like this one.