Placenta

Gestational trophoblastic disease

Molar pregnancies

Partial hydatidiform mole



Topic Completed: 30 August 2021

Minor changes: 30 August 2021

Copyright: 2003-2021, PathologyOutlines.com, Inc.

PubMed Search: Incomplete mole[TI] OR partial mole[TI]

Aurelia Busca, M.D., Ph.D.
Carlos Parra-Herran, M.D.
Page views in 2020: 37,723
Page views in 2021 to date: 34,389
Cite this page: Busca A, Parra-Herran C. Partial hydatidiform mole. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/placentaincompletemole.html. Accessed October 15th, 2021.
Definition / general
  • Gestational trophoblastic disease with abnormal chorionic villi (hydatidiform mole) characterized by a spectrum of villous populations ranging from normal to substantially enlarged, with irregular contours and mild, focal trophoblastic hyperplasia
Essential features
  • Hydatidiform mole resulting from fertilization of a normal egg by 2 spermatozoa or by 1 spermatozoa followed by duplication of the paternal chromosomal content
  • 2 discrete populations of villi (enlarged, hydropic villi and small, fibrotic villi), with the larger villi showing irregular scalloped borders and cistern formation
  • Typically has a diandric triploid genotype
  • Main differential diagnosis includes complete mole (p57-, diandric diploid genotype) and hydropic abortus (p57+, biparental diploid genotype)
  • Definitive diagnosis requires molecular testing for confirmation (essential diagnostic criterion in the 2020 WHO classification of female genital tumors)
Terminology
  • Partial molar pregnancy, partial / incomplete mole
ICD coding
  • ICD-O: 9103/0 - partial hydatidiform mole
  • ICD-11: JA02.1 & XH5325 - incomplete or partial hydatidiform mole & partial hydatidiform mole
Epidemiology
Sites
Pathophysiology
  • Most result from fertilization of a normal egg by 2 spermatozoa (dispermic, 99%)
  • Rarely from fertilization by 1 spermatozoa followed by duplication of the paternal chromosomal content (monospermic, 1%) (Mod Pathol 2014;27:238)
    • As a result, partial moles have a triploid genome, diandric and monogynic
  • Rare tetraploid partial moles have been reported, with 3 sets of paternal chromosomes and 1 set of maternal origin (Mod Pathol 2021;34:961, Int J Gynecol Pathol 2012;31:73)
Etiology
Diagrams / tables

Images hosted on other servers:

Algorithmic approach to diagnosis

Clinical features
Diagnosis
  • Combination of clinical, laboratory and imaging data raises the possibility of molar pregnancy
  • Poor interobserver agreement in differentiating between complete mole, incomplete mole and hydropic abortus when using histology alone, with discordance predominantly seen in partial mole versus hydropic abortus (Am J Surg Pathol 2012;36:443, Am J Surg Pathol 2012;36:1747, Am J Surg Pathol 2005;29:942)
  • Algorithmic approach, in conjunction with p57 immunohistochemistry, has been proposed by using morphological assessment to triage cases for genetic analysis
    • p57 negative cases with molar morphology would be diagnosed as complete hydatidiform moles without genotyping and all p57+ cases would be subjected to genotyping (Mod Pathol 2014;27:238)
  • Definitive diagnosis of partial mole requires molecular genotyping
Laboratory
Radiology description
  • Ultrasound but the sensitivity is low in the first trimester: routine pre-evacuation ultrasound examination identifies < 50% of hydatidiform moles before 14 weeks, with lower detection rate for partial moles (29%) compared with complete moles (79%) (Ultrasound Obstet Gynecol 2006;27:56)
  • Partial moles often present with minor cystic changes of the placenta or normal sonographic findings (J Clin Ultrasound 2017;45:72)
Radiology images

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Partial molar pregnancy

Prognostic factors
Case reports
Treatment
Gross description
Gross images

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Partial hydatidiform mole

Microscopic (histologic) description
  • Heterogeneity in villous size with 2 discrete populations (large, hydropic villi and small, fibrotic villi)
  • Enlarged villi are irregularly shaped with scalloped borders and secondary trophoblastic pseudoinclusions
  • Cistern formation can be seen in enlarged villi
  • Mild circumferential trophoblastic hyperplasia
  • Fetal parts and nucleated red blood cells can be present
  • Presence of at least 3 of the above histologic features of partial mole correlates with triploidy on FISH and flow cytometry analysis (Hum Pathol 2000;31:914, Int J Gynecol Pathol 2001;20:315)
Microscopic (histologic) images

Contributed by Aurelia Busca, M.D., Ph.D.
2 populations of villi 2 populations of villi

2 populations of villi

Edematous hydropic villi with pseudoinclusions

Edematous hydropic villi with pseudoinclusions

Hydropic villi with cistern formation

Hydropic villi with cistern formation

p57 nuclear staining p57 nuclear staining

p57 nuclear staining

Virtual slides

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Partial hydatidiform mole

Partial hydatidiform mole, p57

Positive stains
  • p57
    • Nuclear expression in at least 10% of villous stromal cells and cytotrophoblasts is considered positive
    • Requires internal positive control in maternal decidua and syncytiotrophoblast
    • Cannot distinguish partial mole from nonmolar specimens as both have maternal DNA (Hum Pathol 2005;36:180)
    • Rare cases of incomplete moles may lack p57 expression because of loss of the maternal copy of chromosome 11; genotypic analysis would confirm diandric triploidy (Mod Pathol 2021;34:961, Am J Surg Pathol 2011;35:1586)
Flow cytometry description
  • Cannot distinguish between maternal and paternal origin of genetic material
  • Triploid karyotype by flow cytometry is consistent with partial hydatidiform mole in conjunction with morphological features
  • Would require additional DNA testing to establish if triploidy is diandric versus digynic (Iran J Reprod Med 2015;13:269)
Molecular / cytogenetics description
  • Genotyping:
    • PCR amplification of short, tandem and repeat (STR) loci
    • Testing of both maternal decidua and villous tissue can identify diandric triploid genotype (Int J Gynecol Pathol 2011;30:101)
    • In a study of 158 partial moles, 155 were diandric triploid (83 cases XXY, 59 cases XXX, 13 cases XYY) and 3 were triandric tetraploid (2 cases XXYY and 1 case XXXY) (Mod Pathol 2014;27:238)
  • Fluorescence in situ hybridization (FISH):
    • Uses fluorescently labeled DNA probes targeting regions of various chromosomes, including X and Y
    • Can be used to determine sex chromosome and copy number
    • Cannot differentiate between maternal and paternal origins of genetic material (Prenat Diagn 2005;25:314, Hum Pathol 2000;31:914)
Videos

PathCast presentation on molar pregnancies and gestational trophoblastic

Sample pathology report
  • Uterus, curetting:
    • Partial hydatidiform mole (see comment)
    • Comment: Sections show immature chorionic villi with hydropic changes and cistern formation. There is focal noncircumferential trophoblast proliferation, without significant trophoblast atypia. No fetal parts are identified. By immunohistochemistry, there is retained p57 expression in villous stromal cells and cytotrophoblasts
    • Molecular testing shows the presence of diandric triploidy. The findings are consistent with partial hydatidiform mole
    • Note: If molecular testing results are unavailable at signout, issue a preliminary report followed by an addendum when available.
Differential diagnosis
  • Complete mole:
    • p57- and androgenetic diploidy by molecular testing
    • Early complete mole (less than 12 weeks):
      • No embryonic development
      • May have morphological overlap with incomplete mole, as molar features are not well developed (villi are smaller, cisterns and cavitation not well developed, some degree of trophoblastic proliferation and atypia)
    • Well developed complete mole:
      • No embryonic development
      • 1 population of villi with extensive hydrops, more trophoblastic hyperplasia and atypia
  • Hydropic spontaneous abortion:
    • Only mildly enlarged and edematous villi, occasional cistern formation
    • Tissue is usually less voluminous than partial mole
    • Biparental diploidy by molecular testing
  • Beckwith-Wiedemann syndrome:
    • Placenta often has gross abnormalities, including edema with enlarged villi and centrally cavitated stem villi
    • Scalloping and syncytiotrophoblastic hyperplasia should be absent
    • Diploid (Obstet Gynecol 1994;83:813)
  • Mosaic / chimeric conceptions:
    • Can exhibit a spectrum of morphologic alterations, some of which can suggest a hydatidiform mole
    • p57 is useful in identifying androgenetic tissue (which is p57-)
    • FISH and genotyping are required for full characterization (Int J Gynecol Pathol 2013;32:199)
  • Nonmolar placentas with cytogenetic abnormalities, such as trisomies:
    • Can show abnormal villi suggestive of an incomplete mole
    • Distinction on morphologic grounds alone is difficult and poorly reproducible
    • Would require genetic testing to identify the cytogenetic abnormality (J Mol Diagn 2010;12:525)
  • Twin gestation with complete mole and coexisting fetus:
    • Rare
    • Typically shows 2 villous populations, which may be mistaken for partial mole
    • Molar villi show typical features for complete mole: atypia, cavitation, marked trophoblastic proliferation
    • Fetus has a normal karyotype and is normally developed
    • p57 will be absent in the molar villi only (J Matern Fetal Neonatal Med 2007;20:175, J Ultrason 2017;17:299)
  • Placental mesenchymal dysplasia:
    • Rare and still poorly understood disorder
    • In 33% of cases, Beckwith-Wiedemann syndrome (associated with androgenetic and biparental mosaicism)
    • More commonly recognized in mature placentas; occasionally identified in early gestation POCs
    • Placentomegaly with changes in stem villi, including massive hydrops, cystic dilation and vesicle formation
    • Abnormally thick villous vessels with fibromuscular hypertrophy
    • Terminal villi appear normal
    • No trophoblastic hyperplasia or atypia
    • Placenta and fetus have a normal karyotype
    • Peculiar p57 pattern described in some cases, with retained staining in cytotrophoblast but loss in the villous stroma
    • References: Arch Pathol Lab Med 2014;138:1247, Surg Pathol Clin 2013;6:127
Board review style question #1
Partial hydatidiform mole Partial hydatidiform mole


Products of conception in a 30 year old patient. Which of the following statements is true for the entity depicted in the photos?

  1. It has exclusively paternal genetic material
  2. It has increased risk of persistent gestational trophoblastic disease and choriocarcinoma
  3. p57 immunohistochemistry can be used to differentiate it from complete hydatidiform mole
  4. p57 immunohistochemistry can be used to differentiate it from hydropic abortion
Board review style answer #1
C. p57 immunohistochemistry can be used to differentiate it from complete hydatidiform mole

Comment Here

Reference: Partial hydatidiform mole
Board review style question #2
What is the typical genetic profile of incomplete mole?

  1. Biparental diploidiy
  2. Diandric triploidy
  3. Digynic triploidy
  4. Triandric tetraploid
Board review style answer #2
B. Diandric triploidy

Comment Here

Reference: Partial hydatidiform mole
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