It can be a viable condition if the trisomic component affects only part of the cells of the body (mosaicism) or in cases of partial trisomy of the short arm (trisomy 9p) in which cells have a normal set of two entire chromosomes 9 plus part of a third copy of the short arm ("p") of the chromosome.

Symptoms vary but usually result in dysmorphisms in the skull, nervous system problems, and developmental delay.

An infant with complete trisomy 9 surviving 20 days after birth showed clinical features including a small face, wide fontanelle, prominent occiput, micrognathia, low set ears, upslanting palpebral fissures, high-arched palate, short sternum, overlapping fingers, limited hip abduction, rocker bottom feet, heart murmurs and a webbed neck.

[2] Trisomy 9 can be detected prenatally with chorionic villus sampling and cordocentesis, and can be suggested by obstetric ultrasonography.

[citation needed] Because trisomy 9 may appear with mosaicism, it is suggested that doctors take samples from multiple tissues when karyotyping for diagnosis.