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 

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

• Ratio of transverse to anteroposterior dimension of gestational sac greater than 1.5 
• Fetus shows growth restriction in 80%
• Fetal nuchal translucency thickened in 59% of cases 

Incidence is 2-3 times higher than a complete mole and occurs in 1 of 700 conceptions. Incidence in Southeast Asia is 7 to 10 times higher than in Europe or Australia

Positive stain for p57 and PHLDA2 (complete moles have no immunoreactivity) 

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%