A molar pregnancy occurs when there is an extra set of paternal chromosomes in a fertilized egg. This error at the time of conception transforms normal placental tissue into a growing cystic mass 

Partial hydatidiform mole has a triploid karyotype in which two of the three chromosomal complements are paternal (with a majority of 70% being XXY)

• Incidence is 2-3 times higher than a complete mole and occurs in 1 of 700 conceptions.

Most partial moles miscarry before 10-12 weeks. Risk of progression to persistent gestational trophoblastic disease is 2-4% and the lungs are most common site of metastasis. Risk of second molar pregnancy is 1.7%

Differential diagnosis:

Considerations on early ultrasound scans include:

• fetal demise with placental hydropic change: beta HCG levels are invariably low in this case
• twin pregnancy with one normal twin and one complete hydatidiform mole: the normal twin usually has its own normal placenta
• placental mesenchymal dysplasia.