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\title{Vesicular mole in a twin pregnancy. (Mini-commentary on BJOG-19-1590.R1)}



\author[1]{Amarnath Bhide}%
\affil[1]{St George's University Hospitals NHS Foundation Trust}%


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  \date{March 15, 2025}


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Mini-commentary on BJOG- 19-1590.R1: Obstetric outcomes of twin
pregnancies presenting with a complete hydatidiform mole and coexistent
normal fetus: A systematic review and meta-analysis

\textbf{Vesicular mole in a twin pregnancy}

Amarnath Bhide

Fetal Medicine Unit

St George's University Hospitals NHS Foundation Trust London

Email: abhide@sgul.ac.uk

The authors describe the problems associated with a twin pregnancy
combining a complete vesicular mole and an apparently normal fetus (BJOG
2020 xxxx). The problem is uncommon and the diagnosis may prove
challenging. The ultrasound features of this condition can be very
similar to those of placental mesenchymal dysplasia (PMD) or a partial
vesicular mole.

PMD is an under-recognised entity. The most common sonographic features
include enlarged (50\%) and cystic (80\%) placenta in the first half of
pregnancy and dilated chorionic vessels later on (Nayeri et al,
Ultrasound Obstet Gynecol, 2013). Elevated maternal serum hCG and AFP
levels are common. Association with imprinting disorders (particularly
Beckwith-Wiedemann syndrome) in the fetus has been reported. Therefore,
careful search for features of this syndrome (enlarged cystic kidneys,
exomphalos, glossoptosis) is warranted. Trophoblastic proliferation and
stromal inclusions are not seen in PMD, and the risk of gestational
trophoblastic neoplasia is minimal. Paternal uniparental disomy of two
key genes has been reported in PMD (Robinson et al, Hum Reprod 2007). In
complete hydatidiform mole the whole genome is affected with uniparental
disomy, explaining the similar ultrasound features. Partial vesicular
mole is much less common. The placenta is triploid as well as the fetus.
Cases with a normal fetus result from confined placental mosaicism where
the placenta is triploid but the fetus diploid (Kawasaki et al, JOGR,
2016).

After making a diagnosis of complete vesicular mole and an apparently
normal fetus in a twin pregnancy using ultrasound, invasive prenatal
diagnosis (amniocentesis) should be considered. Molecular diagnosis of
Beckwith Wiedemann syndrome on the amniotic fluid sample should be
offered. However, the parents should be informed that the sensitivity of
molecular diagnosis is only 80\%. From the current review, we learn that
the obstetric outlook is guarded for a twin pregnancy consisting of a
complete vesicular mole and an apparently normal fetus. It is not any
less problematic for the other possibilities though. Even if invasive
prenatal diagnosis results are apparently normal in cases of PMD or a
partial vesicular mole with apparently normal fetus, the risks of growth
restriction, stillbirth and pre-eclampsia remain high. We learn that
gestational trophoblastic neoplasia (GTN) develops in about a third of
all twin pregnancies combining a complete vesicular mole and an
apparently normal fetus, even with voluntary pregnancy termination. This
underscores the need of a close surveillance regardless of the outcome
of the pregnancy.

Another peculiar problem that we need to keep in mind is the possibility
of a falsely negative test for beta hCG in this condition. hCG molecules
can completely saturate the binding sites of the sandwich
radio-immunoassay, the typical assay used for estimation of maternal
serum or urinary beta hCG and falsely lead to a negative result. This
phenomenon, `The hook effect', has been reported with levels in excess
of 1 000 000 IU/L (Tabas et al, N Engl J Med 2003). Appropriate prior
dilution (at least 1:10) of the specimen would avoid this problem.
Therefore, it is important to alert the laboratory about the possible
diagnosis.

\textbf{No disclosures:} A completed disclosure of interest form is
available to view online as supporting information.

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