triploidy

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

Epidemiology

It is considered the 3 commonest fatal chromosomal anomaly . 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 the division of meiosis I or II in spermatocyte: extra set of paternal chromosomes
  • failure of the division of meiosis I or II in oocyte: extra set of maternal chromosomes
  • dispermy: double fertilisation of the 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 on whether the extra set of chromosomes are maternally (digynic) of paternally (diandric) derived.

  • maternal beta HCG and alpha fetoprotein:
    • 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 radiograph

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

Treatment and prognosis

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

Complications

Differential diagnosis

Considerations on early ultrasound scans include:

See also

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