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^rd 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

• neural tube defects
  • spina bifida

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

• second trimester-onset (early) fetal intrauterine growth retardation with a reduced growth potential pattern of anthropometric growth
  • 69 XXX : tends to have early-asymmetrical IUGR
  • 69 XXY : tends to have symmetrical IUGR
• body asymmetry with relative macrocephaly and an elevated head : abdominal circumference ratio (HC >> AC)
• fetal hydrocephalus
• oligohydramnios
• placental changes
  • abnormally large placenta (placentomegaly) and / or hydropic placenta : in cases of paternal origin
  • small placenta : in maternally derived cases
• syndactyly : tends to affect the 3^rd and 4^th digits of the hands

Plain film

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

• harlequin orbits
• small anterior fontanelle
• gracile ribs
• diaphyseal overtubulation of long bones
• upswept clavicles 
• antimongoloid pelvis 

Complications

Maternal

• development of pre-eclampsia

Prognosis

Triploidy is incompatible with life and most fetuses abort in the 1^st 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

• fetal demise with placental hydropic change : beta HCG levels are invariably low in this case

See also

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