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Obstetrics » Obstetrics 1st Trimester
Partial Molar Gestation
Author(s): Chaitali Shah, FRCR | Pamela T. Johnson, MD | Phyllis Glanc, MD, FRCPC
ArticleOther contents by this Author

Molar gestation is a complication of pregnancy and occurs due to an abnormal fertilization process.

Gestational Trophoblastic Neoplasia [GTN]

GTN is a spectrum of entities characterized by abnormal proliferation of pregnancy-related trophoblasts with variable malignant potential.  A wide spectrum of entities ranging from:
1. Non-invasive hydatidiform mole [complete or partial],
2. Invasive mole,
3. Choriocarcinoma,
4. Placental site trophoblastic tumor [PSST] and
5. Epithelioid trophoblastic tumor [a newer entity].

Partial molar gestation occurs commonly and is yet an underdiagnosed entity as majority of the patients present with missed abortion. Histological analysis of the aborted tissue reveals the hydropic chorionic villi and hence the diagnosis.


1. A partial mole results when two sperms fertilize a single ovum and results in development of certain or all fetal parts. A partial mole predominantly has a triploid karyotype of 69XXX or 69 XXY or 69 XYY; however, a diploid karyotype may also exist.
2. A complete molar pregnancy occurs when a sperm fertilizes an empty ovum, resulting in the development of only placental parts. A complete mole is completely paternal in origin, with a karyotype of usually 46 XX.

Clinical Presentation of non-invasive partial molar gestation

1. Symptoms of a missed or incomplete abortion [vaginal bleeding].
2. Abnormally high levels of beta-HCG or
3. Usually asymptomatic, but may present with hyperemesis gravidarum or pre-eclampsia.

Ultrasound features

Partial moles are often indistinguishable from complete moles on ultrasound. However, demonstration of fetal parts favors the diagnosis of a partial mole. Naumoff, et al described the following appearances:
• Enlarged and thickened placenta relative to the size of the uterus.
• Cystic spaces within the placenta.
• An alive or dead, well formed but growth retarded fetus.
• An empty gestational sac [anembryonic appearance] or a sac that contains ill-defined fetal echoes.

Caption : Transvaginal scan of the uterus.

Description : An enlarged uterus with multiple cystic areas within endometrium/placenta is noted. A rounded 11 mm structure thought to be the yolk sac is identified [on the right side]. This is abnormal as the normal yolk sac diameter should be < 6 mm at 10 weeks of gestation. This was a non-invasive partial molar pregnancy, with hydropic changes in an enlarged placenta.

Please click here to view the source of this image.

Differential Diagnosis:  Hydropic placental degeneration, typically associated with an abnormal or failing first trimester pregnancy, may appear similar to a partial molar pregnancy on ultrasound.

Studies have concluded that it is not always possible to make a diagnosis of early molar pregnancy by ultrasonography and therefore, histological examination of the aborted or evacuated specimens remains important and DNA analysis should be carried out for the final diagnosis, if histology is inconclusive. Genetic marker analysis using polymerase chain reaction is rapid and accurate in identifying and classifying complete and partial moles. A partial mole has about a 3% chance of recurrence, while a complete mole has about a 15% chance.

Serum quantitative beta HCG levels provide important information for deciding on the likelihood of a molar pregnancy. These levels are usually very high for the given gestational period, although early stages may have normal levels. Failure of these levels to return to a normal value, post treatment, is a prognostic indicator of retained molar tissue. Therefore, all patients with molar gestation must be followed with up with serial ultrasounds and serum HCG levels, until normal scans and no detectable HCG levels are recorded. Patients are often counseled to avoid pregnancy for at least one year due the risk of persistent trophoblastic neoplasia.

The case demonstrated here showed an enlarged yolk sac of 11 mm [normal diameter should be less than 6 mm at 10 weeks of gestational age]. Although, there is still not enough supporting evidence in the literature, articles have been published that discuss the possibility of a large yolk sac serving as a clue to the presence of gestational trophoblastic disease. Nevertheless, an enlarged and yolk sac is known to be associated with abnormal pregnancy and a poor outcome. These cases therefore need to be followed up closely.


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