Triploidy is a rare lethal chromosomal (aneuploidic) 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. 

Associations

• neural tube defects

  • spina bifida

Pathology

Markers

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

• maternal beta HCG and alpha-fetoprotein (AFP):

  • 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

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.

Radiographic features

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 restriction (IUGR) 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 radiograph

Not part of a 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 

Treatment and prognosis

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

Complications

• maternal: development of pre-eclampsia

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