Molar pregnancy, or hydatidiform mole, is the most common form of gestational trophoblastic disease (GTD). It's an abnormal growth of placental tissue, and it can turn up on what was booked as a routine dating scan or a "rule out miscarriage" exam. Seeing it is what sends the patient for serial hCG follow-up after evacuation, because a small share of moles go on to become persistent or invasive disease.

Complete vs. Partial Mole

The two types differ in genetics, in appearance, and in what you'll see on the screen.

Feature	Complete mole	Partial mole
Genetics	Diploid, all paternal DNA	Usually triploid (69 chromosomes)
Fetus or embryo	None	Often present, usually abnormal or growth-restricted
Placenta	Diffusely abnormal, vesicular	Focal cystic change in an enlarged placenta
hCG	Often markedly elevated for dates	Normal to mildly elevated
Theca lutein cysts	More common	Uncommon
Risk of persistent disease (GTN)	Higher	Lower

The Complete Mole: "Snowstorm" Uterus

The textbook second-trimester appearance is an enlarged uterus filled with echogenic, heterogeneous tissue scattered with countless tiny cystic spaces. These are the hydropic chorionic villi. There is no gestational sac, no embryo, and no cardiac activity.

Complete hydatidiform mole: the classic "snowstorm" appearance, with heterogeneous echogenic uterine contents and no fetal partsImage: Mikael Häggström, Public Domain, via Wikimedia Commons

Look for:

Uterus large for dates
Echogenic, heterogeneous intracavitary mass with many small anechoic cysts (the vesicular, or "cluster of grapes," pattern)
No fetal pole, yolk sac, or cardiac activity
Bilateral, enlarged, multiloculated ovarian cysts (theca lutein cysts), driven by very high hCG
Variable color Doppler flow within the mass, but the myometrium should look intact

Molar pregnancy: heterogeneous, vesicular uterine contents with caliper measurementImage: Mme Mim, CC BY-SA 4.0, via Wikimedia Commons

Why Early Moles Get Missed

Most moles are now scanned in the first trimester, often before 10 weeks, and at that stage the classic snowstorm often hasn't developed yet. An early complete mole can look like:

A missed miscarriage or anembryonic pregnancy
A thickened, slightly cystic, or "bubbly" endometrium
An irregular, collapsed sac with echogenic surroundings

That's why many moles are diagnosed by pathology after evacuation for presumed miscarriage, and why the hCG level matters so much. Uterine contents that don't fit the clinical picture, with an hCG far higher than expected for dates, should always raise the question. You don't have to make the call. Describe what you see clearly so the interpreting physician can.

The Partial Mole

A partial mole is subtler, because a fetus is usually present.

Look for:

An embryo or fetus, often growth-restricted or with structural anomalies (triploidy features)
An enlarged placenta with focal cystic spaces ("Swiss cheese" placenta)
A gestational sac whose transverse diameter is increased relative to its AP diameter
Oligohydramnios in some cases

Document the fetal biometry, any anomalies, and the placental appearance together. The combination, not any single feature, is what points toward a partial mole.

Important Mimics

Mimic	How it differs
Incomplete miscarriage / retained products	Usually vascular tissue at the endometrium; hCG falling, not elevated
Hydropic degeneration of the placenta	Follows fetal demise; hCG not markedly elevated
Placental mesenchymal dysplasia	Cystic placenta with a normal, often healthy fetus; normal karyotype
Complete mole with a coexisting normal twin	Separate normal placenta and fetus beside a molar component
Degenerating fibroid	Arises from the myometrium, not the cavity; hCG negative

The coexisting twin scenario is worth special attention. A normal fetus beside a cystic placental mass isn't automatically a partial mole. Map each placenta separately and describe their relationship.

What to Document

Uterine size and whether it's large for dates
Intracavitary contents: echogenicity, cystic spaces, measurements in three planes
Presence or absence of a gestational sac, yolk sac, embryo, and cardiac activity
Placental appearance if a fetus is present
Both ovaries, with size and any multiloculated cysts
Myometrial interface: is it clearly defined, or is there concern for invasion?
Free fluid in the pelvis

Example wording to suggest to the interpreting physician:

"Heterogeneous, vesicular-appearing intrauterine contents without an identifiable gestational sac or embryo. Bilateral multiloculated ovarian cysts. Findings may represent gestational trophoblastic disease; correlation with quantitative hCG recommended."

Red Flags

Myometrial invasion: loss of the clear interface between the mass and the myometrium, with disorganized, high-velocity, low-resistance flow extending into the wall, raises concern for invasive mole or gestational trophoblastic neoplasia
Large theca lutein cysts carry a risk of torsion. Check ovarian flow if the patient has acute pain.
Very high hCG with symptoms such as severe hyperemesis, early-onset hypertension (before 20 weeks), or thyroid symptoms. Make sure the ordering provider knows promptly.

The Patient in the Room

These patients have often been told they're pregnant, and they may be watching the screen closely. Seeing no baby while you take extra images is frightening. Stay within scope: explain that you're collecting all the images the radiologist needs, and that their provider will go over the results. Don't speculate, and don't say "nothing's there." Use the scripts in our article on what to say when the patient asks what you see.

Bottom Line

The classic snowstorm uterus is easy to recognize, but most moles you see won't look classic. Think of GTD whenever first-trimester contents don't fit a normal or failing pregnancy, especially with a markedly elevated hCG. Document the uterus, contents, embryo, placenta, and ovaries systematically, and let the hCG correlation and pathology finish the story.

SonoBuddy is a reference tool, not a diagnostic authority. Final interpretation rests with the interpreting physician.

Molar Pregnancy on Ultrasound: Complete vs. Partial Mole