Triploidy is a rare lethal chromosomal (aneupliodic) abnormality caused by the presence of an entire extra chromosomal set.
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.
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.
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
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
- body asymmetry with relative macrocephaly and an elevated head : abdominal circumference ratio (HC >> AC)
- fetal hydrocephalus
- placental changes
- syndactyly : tends to affect the 3rd and 4th digits of the hands
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
- development of pre-eclampsia
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.
Considerations on early ultrasound scans include
- partial mole : some consider this term to be synonymous with triploidy
- 1. Del valle GO, Izquierdo LA, Joffe GM et-al. Prenatal diagnosis of triploidy. A case report. J Reprod Med. 1992;37 (4): 360-2. - Pubmed citation
- 2. Ranzini AC, Sharma S, Soriano C et-al. Early diagnosis of triploidy. Ultrasound Obstet Gynecol. 1997;10 (6): 443-4. doi:10.1046/j.1469-0705.1997.10060443.x - Pubmed citation
- 3. Crane JP, Beaver HA, Cheung SW. Antenatal ultrasound findings in fetal triploidy syndrome. J Ultrasound Med. 1985;4 (10): 519-24. J Ultrasound Med (abstract) - Pubmed citation
- 4. Pircon RA, Porto M, Towers CV et-al. Ultrasound findings in pregnancies complicated by fetal triploidy. J Ultrasound Med. 1989;8 (9): 507-11. J Ultrasound Med (abstract) - Pubmed citation
- 5. Silverthorn KG, Houston CS, Newman DE et-al. Radiographic findings in liveborn triploidy. Pediatr Radiol. 1989;19 (4): 237-41. - Pubmed citation
- 6. Bianchi DW, Crombleholme TM, D'Alton ME. Fetology, diagnosis & management of the fetal patient. McGraw-Hill Professional. (2000) ISBN:0838525709. Read it at Google Books - Find it at Amazon
- 7. Entezami M, Albig M, Knoll U et-al. Ultrasound Diagnosis of Fetal Anomalies. Thieme. (2003) ISBN:1588902129. Read it at Google Books - Find it at Amazon
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