triploidy

Triploidy is a chromosomal abnormality in which a fertilized egg contains three complete sets of chromosomes rather than the usual two. In humans, this results in a total of 69 chromosomes in somatic cells (3n). Triploidy is generally incompatible with normal development, and most conceptions end in early pregnancy loss. A small number of pregnancies progress further but result in infants with severe anomalies who rarely survive beyond the neonatal period.

Causes of triploidy include fertilization of an oocyte by two sperm (dispermy) or the formation of a diploid gamete, leading to three chromosome sets in the embryo. Both paternal and maternal origins occur. The parental origin influences the placental and fetal phenotype: diandric triploidy (two paternal genomes) is often associated with an enlarged, molar-appearing placenta, whereas digynic triploidy (two maternal genomes) tends to have a smaller placenta and markedly restricted fetal growth.

The triploid karyotype is typically reported as 69,XXX, 69,XXY, or 69,XYY, with variations such as mosaic triploidy in which some cells are triploid and others are diploid. Prenatal detection may arise from ultrasound findings suggesting abnormal growth or molar changes, with confirmation by chromosomal analysis from chorionic villus sampling or amniocentesis, and postnatal testing if a pregnancy continues to birth.

Prognosis is poor; most pregnancies terminate spontaneously or are terminated, and live-born infants face extensive medical complications and limited survival. Triploidy is a relatively rare cause of congenital chromosomal disorders but is a recognized mechanism of severe early pregnancy loss.