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Triploidy

Triploidy is a rare lethal chromosomal (aneupliodic) abnormality caused by the presence of an entire extra chromosomal set. 

Epidemiology

It is considered the 3rd commonest fatal chromosomal anomaly 7. While it is thought to affect as much as 1 - 2 % of conceptions, the vast majority are thought to undergo spontaneous abortions in the early intrauterine period. 

Pathology

Genetics

The occurrence of triploidy is considered sporadic. A fetus with triploidy has 69 chromosomes. This can happen in three ways.

  • failure of division of meiosis I or II in spermatocyte : extra set of paternal chromosomes
  • failure of division of meiosis I or II in oocyte : extra set of maternal chromosomes
  • dispermy : double fertilisation of ovum with two sperms

In ~ 60 - 75% of cases, the extra set is paternally derived (diandric triploidy).

The reported chromosomal compositions are

  • 69XXY : ~ 60 %
  • 69XXX : ~ 37%
  • 69XYY : ~ 3 %

In some cases there can be diploid / triploid mosaicism.

Serological markers

The serological markers can vary depending of whether the extra set of chromosomes are maternally (digynic) of paternally (diandric) derived.

  • maternal beta HCG / alpha feto protein
    • maternally derived triploidy : markedly decreased
    • paternally derived triploidy : markedly increased
  • PAPP-A : pregnancy associated plasma protein A
    • maternally derived triploidy : mildly decreased
    • paternally derived triploidy : markedly decreased
Associations

Radiographic features

Antenatal ultrasound

Sonographic features of pregnancies complicated by fetal triploidy are not uniform and the diagnosis cannot be made by ultrasound alone

However common features include

Plain film

Not part of routine investigation and most fetuses are not liveborn. Reported features according to one study were 5

Complications

Maternal

Prognosis

Triploidy is incompatible with life and most fetuses abort in the 1st trimester. Extremely rare cases with survival few months into post natal life have been reported 6. Since most cases are sporadic, the risk of recurrence is usually not increased.

Differential diagnosis

Considerations on early ultrasound scans include

See also

  • partial mole : some consider this term to be synonymous with triploidy

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