## Karyotype of Complete Molar Pregnancy **Key Point:** Complete molar pregnancies are **diploid (46,XX or 46,XY) with 100% paternal origin of chromosomes**. This is called androgenesis or androgenetic diploidy. ### Mechanism of Formation **Mnemonic:** **EMPTY EGG RULE** — A complete molar pregnancy results from fertilization of an **empty ovum** (no maternal DNA) by one or two sperm. 1. Empty egg (no maternal chromosomes) is fertilized 2. Single sperm duplicates its own DNA → 46,XX (90% of cases) 3. OR two sperm fertilize the empty egg → 46,XY (10% of cases) ### Karyotype Distribution in Complete Molar Pregnancy | Karyotype | Frequency | Origin | Mechanism | |-----------|-----------|--------|----------| | 46,XX | 90% | Paternal (single sperm) | Sperm duplication | | 46,XY | 10% | Paternal (two sperm) | Dispermy | | **Total** | **100%** | **All paternal** | **Androgenesis** | **High-Yield:** The complete absence of maternal DNA is the defining feature that distinguishes complete from partial molar pregnancy. ### Contrast: Partial Molar Pregnancy Partial molar pregnancies are **triploid (69,XXY or 69,XXX)** with **two sets of paternal chromosomes and one maternal set**. This results from fertilization of a normal haploid egg by two sperm. ### Clinical Significance of Paternal-Only Karyotype - **Imprinting abnormality:** Absence of maternal imprinting leads to uncontrolled trophoblastic proliferation - **No fetal development:** Paternal genes alone cannot support normal embryogenesis; only trophoblastic tissue develops - **Very high hCG:** Uncontrolled trophoblastic growth produces markedly elevated β-hCG (often >100,000 mIU/mL) - **Higher malignant potential:** 15–20% risk of persistent GTD or choriocarcinoma [cite:Robbins & Cotran 10e Ch 22] 
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