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# Molar pregnancy (hydatidiform mole)

A molar pregnancy is an abnormal pregnancy in which the placenta grows as fluid-filled cysts after an abnormal fertilisation; it can persist as a tumour.

Updated 2026-10-29

## Structures involved

Tap one to see what it is and open it in 3D.

- [Body of uterus](https://durgical.com/en/anatomy/body-of-uterus/)
- [Endometrium](https://durgical.com/en/anatomy/endometrium/)
- [Myometrium](https://durgical.com/en/anatomy/myometrium/)
- [Ovaries](https://durgical.com/en/anatomy/ovary/)
- [Uterine artery](https://durgical.com/en/anatomy/uterine-artery/)

## In biochemistry

- [Dihydrofolate reductase (DHFR)](https://durgical.com/en/biochemistry/enzymes/dihydrofolate-reductase/)

## In histology

- [Fertilisation, implantation and the decidua](https://durgical.com/en/histology/fertilisation-implantation-and-the-decidua/)
- [Placenta](https://durgical.com/en/histology/placenta/)

## In embryology

- [Meiosis and nondisjunction: where trisomies come from](https://durgical.com/en/embryology/meiosis-and-nondisjunction-where-trisomies-come-from/)
- [Week 2: the bilaminar disc ('the week of twos')](https://durgical.com/en/embryology/week-2-the-bilaminar-disc/)
- [Trophoblast and hCG; gestational trophoblastic disease](https://durgical.com/en/embryology/trophoblast-and-hcg-gestational-trophoblastic-disease/)

## In physiology

- [The placenta and pregnancy hormones](https://durgical.com/en/physiology/the-placenta-and-pregnancy-hormones/)

## Why it happens

- [Genomic imprinting: Prader–Willi and Angelman](https://durgical.com/en/general-pathology/genomic-imprinting-prader-willi-and-angelman/)
- [Chromosome abnormalities: number and structure](https://durgical.com/en/general-pathology/chromosome-abnormalities-number-and-structure/)

## What it is

An abnormal pregnancy in which the placenta (trophoblast) grows as swollen, fluid-filled villi due to an abnormal fertilization. It is a form of gestational trophoblastic disease and can progress to a persistent tumour.

## Who gets it

An uncommon complication of pregnancy; texts report higher rates in some parts of Asia and Latin America. Most cases are now detected early by ultrasound in the first trimester.

## Causes

- Complete mole: an egg without maternal chromosomes fertilized by one or two sperm (diploid, all paternal); no fetus forms
- Partial mole: a normal egg fertilized by two sperm (triploid); fetal parts may be present
- The excess paternal genetic material drives trophoblast overgrowth

## Risk factors

- Maternal age under about 20 or over about 35–40
- Previous molar pregnancy
- Previous miscarriages
- Some dietary factors such as low carotene intake have been proposed

## Pathophysiology

With too much paternal genetic material, the trophoblast proliferates and the chorionic villi swell into grape-like vesicles. The abnormal tissue produces very high amounts of human chorionic gonadotropin (hCG), which can stimulate the [ovaries](https://durgical.com/en/anatomy/ovary/) (theca lutein cysts) and the thyroid. Some trophoblast can persist or invade the [myometrium](https://durgical.com/en/anatomy/myometrium/) after evacuation (gestational trophoblastic neoplasia).

## Classification

- Complete hydatidiform mole: 46,XX or 46,XY, all paternal, no fetal tissue, higher risk of neoplasia
- Partial hydatidiform mole: usually 69 chromosomes (triploid), fetal tissue may be present, lower risk
- Gestational trophoblastic neoplasia after a mole: invasive mole, choriocarcinoma and rarer forms

## Symptoms and signs

- Vaginal bleeding in the first trimester, sometimes passing grape-like tissue
- Uterus larger than expected for the dates (more often in complete mole)
- Severe nausea and vomiting (hyperemesis)
- Absent fetal heart in complete mole
- Less often: early preeclampsia (before 20 weeks), signs of hyperthyroidism, enlarged ovaries with theca lutein cysts
- Many cases are found on a routine early ultrasound without symptoms

## Red flags

- Heavy vaginal bleeding or haemodynamic instability
- Hypertension with proteinuria before 20 weeks
- Tachycardia, tremor or other signs of hyperthyroidism
- Breathlessness during or after evacuation (trophoblastic embolization)
- hCG that plateaus or rises after evacuation (neoplasia)

## Diagnosis

**Clinical**
Suspected from first-trimester bleeding, a large-for-dates uterus or hyperemesis; confirmed by histology of the evacuated tissue.

**Tests**
Quantitative serum hCG, often markedly elevated; blood count, blood group and Rh, [kidney](https://durgical.com/en/anatomy/kidney/), [liver](https://durgical.com/en/anatomy/liver/) and thyroid function; histopathology (and sometimes ploidy or p57 staining) to separate complete from partial mole.

**Imaging**
Pelvic ultrasound: a complete mole shows a heterogeneous vesicular mass without a fetus ("snowstorm" pattern); partial mole shows a thickened cystic placenta, sometimes with an abnormal fetus. [Chest](https://durgical.com/en/anatomy/region/chest/) X-ray if neoplasia is suspected.

## Differential diagnosis

- Miscarriage (incomplete or missed)
- Ectopic pregnancy
- Multiple pregnancy
- Hyperemesis gravidarum of a normal pregnancy
- Uterine fibroids with pregnancy

## Goal of treatment

Remove all molar tissue safely, follow hCG to normal to detect persistent disease early, and preserve fertility when desired.

## Treatment

**Surgical**
Suction evacuation of the uterus, regardless of uterine size, with oxytocic drugs given only after evacuation begins if bleeding is heavy; hysterectomy is an option for women who do not want more children.

**Medical**
Anti-D immunoglobulin for Rh-negative women; treatment of hyperthyroidism, preeclampsia and anaemia when present.

**Follow-up**
Serial hCG measurements until normal and then for a defined period by protocol, with reliable hormonal contraception during follow-up to avoid confusion with a new pregnancy.

**Medical**
Chemotherapy (single-agent antimetabolite or multi-agent regimens, by risk score) for gestational trophoblastic neoplasia.

## Complications

- Haemorrhage and anaemia
- Uterine perforation during evacuation
- Gestational trophoblastic neoplasia: invasive mole, choriocarcinoma
- Hyperthyroidism and, rarely, thyroid storm
- Early preeclampsia
- Respiratory distress from trophoblastic embolization
- Theca lutein cysts with torsion or rupture

## Prognosis

Excellent with evacuation and complete hCG follow-up; even trophoblastic neoplasia is highly curable with chemotherapy. The risk of another mole in a future pregnancy is higher than average, so early ultrasound is advised in later pregnancies.

## Test yourself

Think of the answer, then tap the question to see it.

## What is the karyotype of a complete hydatidiform mole?

Usually 46,XX (sometimes 46,XY), with all chromosomes of paternal origin and no fetal tissue.

## How does a partial mole differ genetically from a complete mole?

A partial mole is usually triploid (69 chromosomes, two paternal sets and one maternal) and may contain fetal parts.

## Which tumour marker is used to monitor a molar pregnancy?

Serum human chorionic gonadotropin (hCG), followed serially until it normalizes.

## What is the classic ultrasound appearance of a complete mole?

A heterogeneous vesicular intrauterine mass without a fetus, described as a snowstorm pattern.

## Why can a molar pregnancy cause hyperthyroidism?

Very high hCG shares a subunit with TSH and stimulates the thyroid TSH receptor.

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## Related conditions

- [Miscarriage (early pregnancy loss)](https://durgical.com/en/conditions/miscarriage/)
- [Ectopic pregnancy](https://durgical.com/en/conditions/ectopic-pregnancy/)
- [Preeclampsia](https://durgical.com/en/conditions/preeclampsia/)
- [Ovarian cyst](https://durgical.com/en/conditions/ovarian-cyst/)
- [Uterine fibroids (leiomyomas)](https://durgical.com/en/conditions/uterine-fibroids/)

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Hydatidiform Mole](https://www.ncbi.nlm.nih.gov/books/NBK459155/) (StatPearls (NCBI Bookshelf), 2024).

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Source: https://durgical.com/en/conditions/molar-pregnancy/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/conditions/molar-pregnancy/