partial molar pregnancy

**Genetics/Pathophysiology** - Triploidy (69,XXY, 69,XXX, or 69,XYY): one haploid maternal set (23) + two paternal sets (46). Usually from dispermy (two sperm fertilizing one ovum); less often from a diploid sperm. - Contrast with complete mole: 46,XX (most common, from duplication of a single sperm's haploid genome into an empty egg, so entirely paternal) or 46,XY; no fetal tissue. - Because a maternal genome is present, an embryo/fetus (often with growth restriction and multiple anomalies) or amniotic membranes/fetal RBCs can be seen. Paternal genes drive trophoblastic overgrowth (genomic imprinting). - Villi: only some are swollen (focal hydropic change), with focal (not diffuse) trophoblastic hyperplasia. Villi are p57 (KIP2) positive, because a maternal allele is present. Complete moles are p57 negative.

**Clinical presentation** - Often presents as missed or incomplete abortion in the first or early second trimester, with vaginal bleeding. - Uterus is small or normal for gestational age (complete moles are often large for dates). - β-hCG is elevated but usually less so than in complete mole (often <100,000 mIU/mL). - Complications such as hyperemesis, early preeclampsia (<20 weeks), hyperthyroidism, and theca lutein cysts are less common than in complete mole.

**Diagnosis** - Ultrasound: focal cystic spaces in the placenta ("Swiss cheese"), with a fetus or fetal parts. The classic "snowstorm" or "cluster of grapes" appearance is more typical of complete mole. - Often diagnosed by pathology after evacuation. Karyotype/flow cytometry and p57 immunostaining help differentiate partial from complete mole.

**Management** - Suction dilation and curettage (D&C), the treatment of choice. - Serial β-hCG monitoring until undetectable, typically weekly, then monthly for about 3–6 months. - Reliable contraception during follow-up (avoid pregnancy so a new rise in hCG can be interpreted). Hormonal contraception is acceptable. - Rh immune globulin if Rh-negative. - Persistent or rising hCG suggests gestational trophoblastic neoplasia (invasive mole). Work up with imaging (chest X-ray/CT for lung metastases) and treat with methotrexate or dactinomycin. Choriocarcinoma is rarer after partial mole.

**Malignant potential** - Partial mole: ~1–5% persistent GTN, and choriocarcinoma is very rare. - Complete mole: ~15–20% persistent GTN, and ~2–3% choriocarcinoma.

**High-yield comparison (Partial vs Complete)** - Karyotype: 69,XXY (triploid) vs 46,XX (paternal). - Fetal parts: present vs absent. - Villous swelling: focal vs diffuse. - Trophoblastic hyperplasia: focal vs diffuse. - p57: positive vs negative. - β-hCG: mildly elevated vs markedly elevated. - Uterine size: small or normal vs large for dates. - Risk of choriocarcinoma: very low vs higher.

**Board pearls** - A triploid karyotype with a fetus and a "Swiss cheese" placenta points to a partial mole. - Choriocarcinoma is the malignant counterpart. It is hematogenous, spreads to lung, and has no villi on histology. - Post-evacuation, the rising hCG pattern is the key sign of persistent disease.