Cystic formation in the confined area of the placenta along with a normal appearing texture in the other parts, should raise concern whether it is a twin pregnancy with one normal fetus and complete molar pregnancy in co-twin, partial molar pregnancy or placental mesenchymal dysplasia. Although in most of these pregnancies, the diagnosis can be straight forward according to ultrasound imaging features, in some borderline cases reaching definite diagnosis can be challenging, especially in early stages of pregnancy.
The observation of bipartite placenta with cystic areas beside a normal fetus may raise critical concerns regarding the pregnancy outcome. The main differential diagnoses are: 1) dichorionic twin with a normal fetus in one sac and complete mole in another, 2) dichorionic twin with a normal fetus accompanied by partial mole of the missed fetus, 3) mesenchymal dysplasia, and 4) very rare event of a normal fetus and confined placental partial hydatidiform mole.
The main critical diagnosis is the presence of complete molar pregnancy because of catastrophic consequences. The karyotype in complete hydatidiform mole is 46,XX, but all the chromosomes are from the paternal origin. In the clinical setting, instead of a genetic study, the diagnosis is established by observing the pathognomonic appearance in ultrasound imaging and also pathological examination and gross visualization of the molar tissue. However, in cases such as pregnancies with normal fetus and a cystic pattern of placenta, a way to reach definite diagnosis should be granted.
The prevalence of complete hydatidiform mole is 1%- 6% which is higher than partial mole (8). On the other hand, partial hydatidiform moles come from dispermic fertilization of a normal haploid oocyte that generally generates a triploid set of chromosomes. In cases with partial hydatidiform moles, the triploid fetus can develop but it is anomalous and is non-viable. In our case we had a normal diploid fetus with triploid hydropic placenta in one side and normal appearing placenta in the other side.
There are some case reports with partial hydatidiform molar pregnancy with diploid fetus (6, 12). There are three types of molar pregnancies with a normal live fetus (9). Molar hydatidiform should be considered in pregnancies with high level of serum β-hCG serially (10). In partial molar pregnancies usually ultrasound imaging shows honeycomb-like echo in the placenta; the borders between the normal placental tissue and the honeycomb echo are not clear, and most fetuses are dead or malformed (11). In a case report by Hossain et al, the fetus could not continue to live on for more than 21 weeks and the pregnancy ended up with intra-uterine fetal death (12). Still, there have been cases, e.g. Zeng et al, in which a live baby was delivered (13). In pregnancies with partial hydatidiform moles, only a few villous vesicular changes occur; the cellular proliferation is bolded, 90% of the fetal chromosome karyotypes are triploid, and most pregnancies end with an abortion and fetal death.
To understand the unique genetic pattern in our case and the underlying reason for diploid fetus accompanied by triploid hydropic placenta, we should consider that the characteristic of triploidy is existence of three (3n) instead of two (2n) haploid chromosome sets on the cellular level. There are two types of triploidy depending on the parental origin; "diandric triploidy" or "diandry" where the extra chromosome set comes from the paternal origin, and "digynic triploidy" or "digyny" where the extra chromosome set comes from the mother (19, 20). Mosaic triploidy happens less frequently than complete triploidy. The mechanisms suggested for mosaic triploidy include fusion of one normal zygote with one triploid that results in a chimeric fetus; delayed fertilization of a zygote with a second sperm; and reincorporation of the second polar body into the fertilized egg (21).
All the molecular markers used in the QF-PCR assay on both amniotic fluid and chorionic villus samples showed similar size indicating that fetus and molar placenta had originated from the same zygote and the extra chromosome set may come from incorporation of the second polar body into the fertilized egg in very early stages of postzygotic mitotic cell divisions (Fig. 3).