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Don't mix these up for NEET PG: complete vs partial hydatidiform mole

ObstetricsPublished 31 October 2026 · Recallab

A high-yield NEET PG pair. Both moles are paternal excess; what separates them is whether a maternal set, and so a fetus, is present. Once that is clear, the villi, the hormone and the risk all follow.

Recallab Scroll short for NEET PG: fetal red cells distinguish the moles, 40 seconds.

Complete vs partial hydatidiform mole: side by side

	Complete mole	Partial mole
Genetics	Diploid, usually 46,XX, entirely paternal	Triploid, 69, two paternal sets and one maternal
Fetal tissue	Absent within the mole	Present
Villi	All swollen and avascular	Only some swollen
hCG	Very high	Mildly raised
Persistent disease	15 to 20 per cent	1 to 5 per cent
The clue	No fetal vessels	Nucleated fetal red cells in villous capillaries

A complete mole forms when an empty ovum is fertilised and the paternal genome duplicates, so there is no maternal material and no embryo. A partial mole forms when two sperm fertilise a normal ovum; the maternal set lets fetal parts and vessels develop. Both need serial hCG follow-up after evacuation.

The one trap

The trap is reaching for central cisterns. They occur in both moles, so they separate neither. The feature that settles it is nucleated fetal red cells in villous capillaries, which only a fetus makes.

The second trap is a normal fetus beside a complete mole. That may be a coexisting normal twin, with its own placenta, not fetal tissue within the mole.

Try it: say it once before the questions: complete is all father and no fetus; partial has a mother, a fetus and fetal red cells.

The deciding clue

Recallab example 1 · two fields of villi

Two uterine evacuations from women with a bulky uterus and early bleeding are drawn in the figure as histology fields at one magnification. Which single feature in the figure settles which field is a partial mole?

Look first: find the field with two sizes of villus, then look inside the small ones.

Two histology fields of chorionic villi at one magnification. Field 1: every villus is swollen to a similar large size with a pale central space, trophoblast thickened all the way round and no vessels. Field 2: two sizes of villus mixed together; the swollen ones have scalloped outlines with trapped trophoblast and a rim thickened on one side, and the small ones contain open capillaries holding round cells with a nucleus. — Recallab question 57571. Two fields of evacuated villi at one magnification.

AUniform villous swelling
BScalloped villous outlines
CCentral cisterns
DNucleated fetal red cells

Show the answer and explanation

Answer: D. Nucleated fetal red cells

D. Nucleated fetal red cells. The capillaries in field 2’s small villi hold red cells with nuclei, and only a fetus makes those, so field 2 is a partial mole. A complete mole arises from paternal chromosomes alone, so no embryo forms: every villus is swollen, avascular and ringed by trophoblast all round, as in field 1. A partial mole is triploid, with an embryo that lives long enough to make vessels, so only some villi swell and two populations sit side by side.

Why the other options lose: uniform villous swelling (A), with trophoblast all round, is field 1, the complete mole. Scalloped outlines and trapped trophoblast (B) support a partial mole but do not prove a fetus. Central cisterns (C) form in both moles and separate neither.

Recallab question 57571 · Obstetrics › Trophoblastic Disease · reference: Williams Obstetrics

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Where the mother shows

Recallab example 2 · a stain that needs the mother

Two uterine evacuation specimens, both showing swollen villi, have each been stained for a protein that a cell can make only from the copy of its gene inherited from the mother. The result is drawn in the figure. What is specimen 2?

Look first: check the maternal lining first: it must stain. Then ask whether the villi do.

Two evacuation specimens of swollen villi, each with a strip of maternal uterine lining along one edge and an immunostain panel beside it. In specimen 1, brown nuclei appear in the maternal lining and in the villous cells. In specimen 2, brown nuclei appear in the maternal lining only, and every villous nucleus stays blue; its villi are larger and more uniformly swollen. — Recallab question 58520. Two evacuation specimens, each with a nuclear immunostain beside it.

AComplete hydatidiform mole
BChoriocarcinoma
CPartial hydatidiform mole
DHydropic abortion

Show the answer and explanation

Answer: A. Complete hydatidiform mole

A. Complete hydatidiform mole. The stained protein, p57, is made only from the maternal copy of its gene. Specimen 2’s villous cells stay unstained because a complete mole has no maternal chromosomes, while the mother’s own lining in the same section stains, proving the stain worked. Specimen 1 stains in both places, so its villi do have a maternal contribution.

Why the other options lose: choriocarcinoma (B) forms no villi, and villi are drawn here. A partial mole (C) carries a maternal set, so its villi stain. A hydropic abortion (D) is a non-molar conception with a maternal set, so its villi stain too.

Recallab question 58520 · Pathology › Female Genital Tract · reference: Robbins, Cotran & Kumar Pathologic Basis of Disease

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A fetus beside a mole

Recallab example 3 · two scans at thirteen weeks

Two women at 13 weeks have very high serum hCG and cystic placental tissue on scan. Their scans are drawn in the figure. Which pregnancy may be allowed to continue after counselling?

Look first: count the placentas, then ask where the fetus sits.

Two ultrasound line drawings at thirteen weeks. Scan 1: two separate placental areas with a clear line between them; a normally shaped fetus in its own sac on a smooth placenta, and separately a large mass of countless tiny cysts with no fetus. Scan 2: one enlarged placenta with scattered larger cystic spaces among normal tissue and one small, thin fetus attached to it. — Recallab question 57877. Two scans at thirteen weeks with cystic placental tissue.

ABoth
BNeither
CScan 1
DScan 2

Show the answer and explanation

Answer: C. Scan 1

C. Scan 1. It has two separate placentas, one normal with its fetus and one entirely molar: a complete mole beside a normal twin, with no fetal tissue in the mole. The twin usually has a normal karyotype, which invasive testing confirms, and about four in ten such pregnancies that continue end in a live birth. Continuing does not appear to raise the later risk of trophoblastic neoplasia; pre-eclampsia, bleeding and thyrotoxicosis are watched for, and hCG is followed afterwards.

Why the other options lose: scan 2 is a partial mole, one placenta with focal cysts and a small fetus, whose fetus is usually triploid and rarely survives, so both (A) and scan 2 (D) lose. Neither (B) is wrong because a complete mole beside a normal twin may continue safely.

Recallab question 57877 · Obstetrics › Trophoblastic Disease · reference: Williams Obstetrics

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Drawn from questions #57571, #58520, #57877, #50578, #50875 in the Recallab bank, each with its own verified answer and current-edition reference. Last reviewed 31 October 2026.