Molar pregnancy

Hydatidiform moles (HM) are one of the most common but benign form of gestational trophoblastic disease. 

Epidemiology

It is a common complication of gestation, estimated to occur in one of every 1,000-2,000 pregnancies ³. These moles can occur in a pregnant woman of any age, but the rate of occurrence is higher in pregnant women in their teens or between the ages of 40-50 years. There is a relatively increased prevalence in Asia (for example compared with Europe).

Pathology

Subtypes

A hydatidiform mole can either be complete or partial. The absence or presence of a fetus or embryo is used to distinguish complete moles from partial moles. Complete moles are associated with the absence of a fetus and partial moles usually occur with an abnormal fetus or may even be associated with fetal demise.

Rarely, moles co-exist with a normal pregnancy (co-existant molar pregnancy), in which a normal fetus and placenta are seen separate from the molar gestation.

Chromosomal composition

Ninety percent of complete hydatidiform moles have a 46XX diploid chromosomal pattern. All the chromosomes are derived from the sperma single sperm in 90% or less likely two sperms, suggesting fertilization of a single egg that has lost its chromosomes.

With partial moles, the karyotype is usually triploid (69XXY): the result of fertilization of a normal egg by two sperm, one bearing a 23X chromosomal pattern and the other a 23Y chromosomal pattern.

Location

Complete hydatidiform moles usually occupy the uterine cavity and are rarely located in fallopian tubes or ovaries.

The chorionic villi are converted into a mass of clear vesicles that resemble a cluster of grapes.

Markers

In the classic case of molar pregnancy, quantitative analysis of beta-HCG shows hormone levels in both blood and urine greatly exceeding those produced in a normal pregnancy at the same stage. 

Radiographic features

Ultrasound

Complete hydatidiform mole

• enlarged uterus
• classic sonographic appearance is that of a solid collection of echoes with numerous small (3-10 mm ⁶) anechoic spaces (snowstorm or granular appearance).
• the term snowstorm sign usually dates back to earlier ultrasoundequipment with inferior resolution.
• the molar tissue demonstrates the bunch of grapes sign which represents hydropic swelling of trophoblastic villi.
• variable appearance
• normal interface between abnormal trophoblastic tissue and myometrium.
• no identifiable fetal tissue or  gestational sac is seen. 
• colour Doppler interrogation may show high velocity, low impedance flow.

Partial mole

• placenta is enlarged and contains areas of multiple, diffuse anechoic lesions
• a fetus with severe structural abnormalities or growth restriction, oligohydramnios or a deformed gestational sac may be noted.
• colour Doppler interrogation may show high velocity, low impedance flow.

Other features may include

• ovarian theca lutein cysts: may be seen bilaterally in 25-60% of cases

CT

A CT scan usually demonstrates a normal-sized uterus with areas of low attenuation, an enlarged inhomogeneous uterus with a central area of low attenuation, or hypo-attenuating foci surrounded by highly enhanced areas in the myometrium.

MRI

MRI shows heterogenous endometrial thickening with T2 hyperintense areas. MRI is indicated in aggressive gestational trophoblastic disease to look for myometrial invasion and pathologically dilated endometrial, myometrial, or parametrial vessels.

Treatment and prognosis

• progression to invasive mole: ~15% 
• progression to gestational choriocarcinoma: ~7%

See also

• molar pregnancy