Triploidy is a rare lethal chromosomal (aneupliodic) abnormality caused by the presence of an entire extra chromosomal set.
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.
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.
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
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 restriction (IUGR) with a reduced growth potential pattern of anthropometric growth
- body asymmetry with relative macrocephaly and an elevated head: abdominal circumference ratio (HC >> AC)
- fetal hydrocephalus
- placental changes:
- syndactyly: tends to affect the 3 and 4 digits of the hands
Not part of routine investigation and most fetuses are not liveborn. Reported features according to one study were :
- harlequin orbits
- small anterior fontanelle
- gracile ribs
- diaphyseal overtubulation of long bones
- upswept clavicles
- antimongoloid pelvis
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.
- maternal: development of pre-eclampsia
Considerations on early ultrasound scans include:
- partial mole: some consider this term to be synonymous with triploidy