SONOWORLD : Partial molar pregnancy
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Obstetrics » Obstetrics 1st Trimester
Partial molar pregnancy
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Presentation A 39 year old woman, 11 weeks pregnant, presents with vaginal bleeding. The patient had very high levels of beta-HCG and a pelvic ultrasound was performed.
Caption: Transvaginal scan of the uterus.
Description: Enlarged uterus, multiple cystic areas within endometrium/placenta. 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.
Caption: Sagittal view of the uterus.
Description: The possible yolk sac and the cystic spaces in the endometrial complex are again noted in this sagittal image.
Caption: Transverse scan of the uterus.
Description: The abnormal endometrial echo complex is noted again with visualization of numerous cystic spaces.
Caption: Color Doppler image.
Description: No abnormal vascularity is noted. The myometrium shows normal flow. The abnormally enlarged yolk sac is noted again in this image.
Caption: Power Doppler image of the uterus.
Description: The hypoechoic spaces are truly cystic and show no flow. No abnormal vascularity is noted.
Differential Diagnosis Molar gestation [more likely to be a partial mole than a complete mole, as there is evidence of possible fetal remnants].

Final Diagnosis Non-invasive partial molar pregnancy, with hydropic changes in an enlarged placenta.

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

Types of molar gestation – 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.

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.

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 tumor levels, until normal scans and no detectable tumor levels are recorded.

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.

Case References
  1. Repiska V, et al. DNA analysis in gestational trophoblastic disease. Ceska Gynekol. 2003 Nov; 68(6):442-8.
  2. Woo JS, et al. Sonographic appearances of the partial hydatidiform mole. J Ultrasound Med. 1983 Jun; 2(6):261-4.
  3. Woo JS, et al. Partial hydatidiform mole: USG features. Aust N Z J Obstet Gynaecol. 1983 May; 23(2):103-7.
  4. Naumoff P, et al. Ultrasonography of partial hydatidiform mole. Radiology. 1981 Aug; 140(2):467-70.
  5. Szulman AE, Surti U. The clinicopathologic profile of the partial hydatidiform mole. Obstet Gynecol. 1982 May; 59(5):597-602.
  6. Fine C, et al.  Obstet Gynecol. 1989 Mar; 73(3 Pt 1):414-8.
  7. Jeffers MD, et al.  Int J Gynecol Pathol. 1993 Oct;12(4):315-23.
  8. Gurel SA, et al. Eur J OGRB. 2000 Jul; 91(1):91-3.
  9. Zalel Y, et al. J Clin Ultrasound. 1994 Oct; 22(8):519-21.
  10. Lindsay DJ, et al. Radiology. 1992 Apr; 183(1):115-8.
Follow Up This patient underwent curettage and had a confirmed partial molar gestation. Genetic analysis revealed diploid tissue [which is rare, partial moles almost always reveal triploidy].
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