Hydatidiform mole: complete vs partial
- Lesion
A molar pregnancy is a placenta with too many paternal chromosomes: the villi swell like grapes, hCG soars, and a few cases become cancer.
What it is
A hydatidiform mole is an abnormal pregnancy in which chorionic villi swell and the trophoblast proliferates because of an excess of paternal genes. A complete mole is diploid and entirely paternal, usually from an empty ovum fertilised by one sperm that duplicates; it has no embryo, all villi are swollen and it carries a real risk of persistent disease and choriocarcinoma. A partial mole is triploid, with one maternal and two paternal sets, has fetal tissue and a much lower risk. The p57 immunostain, made only from the maternal gene, separates them.
Structure
- Complete mole: the uterus is filled with delicate thin-walled translucent grape-like vesicles; microscopically all villi are oedematous with central cisterns and surrounded by atypical cytotrophoblast and syncytiotrophoblast.
- Partial mole: a mixture of normal-sized and swollen villi with irregular scalloped outlines, focal trophoblast hyperplasia, and fetal tissue or nucleated fetal red cells.
- Invasive mole: molar villi within the myometrium or vessels, sometimes causing perforation and haemorrhage.
Step by step
- An ovum that has lost its nucleus is fertilised by one sperm that duplicates, or by two sperm: a complete mole with only paternal genes.
- Or a normal ovum is fertilised by two sperm: a triploid partial mole.
- An excess of paternally expressed (imprinted) genes drives trophoblast growth and villous swelling.
- The proliferating trophoblast secretes large amounts of hCG.
- After evacuation, trophoblast that remains can persist, invade the myometrium or become choriocarcinoma.
Complete vs partial hydatidiform mole
| Feature | Complete mole | Partial mole |
|---|---|---|
| Karyotype | Diploid, all paternal (46,XX or 46,XY) | Triploid, two paternal and one maternal set |
| Embryo or fetal parts | Absent | Present |
| Villous swelling | All villi | Some villi |
| Trophoblast proliferation | Diffuse, circumferential | Focal, slight |
| p57 immunostain | Negative | Positive |
| Serum hCG | Very high | Often near normal for the stage |
| Risk of persistent disease | High; choriocarcinoma possible | Low; choriocarcinoma rare |
Markers
- p57 (CDKN1C)
- Serum hCG
- Molecular genotyping (short tandem repeats)
Clinical notes
- Presents with vaginal bleeding in early pregnancy, a uterus large for dates, hyperemesis, early preeclampsia, hyperthyroidism (hCG stimulates the TSH receptor) and theca lutein cysts.
- Ultrasound of a complete mole shows a snowstorm pattern; most are now diagnosed in the first trimester.
- Treatment is suction evacuation followed by serial hCG until it is undetectable; a plateau or rise means persistent disease.
Test yourself
Think of the answer, then tap the question to see it.
What is the karyotype of a complete mole and where do its chromosomes come from?
Usually 46,XX, entirely paternal, from an empty ovum fertilised by a sperm that duplicates its DNA (or by two sperm).
Why is p57 negative in a complete mole?
p57 is expressed only from the maternal allele; a complete mole has no maternal genome, so it does not stain.
Which mole has fetal parts and a low risk of choriocarcinoma?
The partial mole, which is triploid.
How is a patient followed after evacuation of a mole?
With serial serum hCG until it is undetectable; a plateau or rise indicates persistent gestational trophoblastic disease.
Elsewhere in DurgiPedia
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Hydatidiform Mole (StatPearls (NCBI Bookshelf), 2026); Gestational Trophoblastic Disease Treatment (PDQ®)–Health Professional Version (National Cancer Institute, 2026).
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Representative model, for educational purposes only.