Among chromosomal number and structural abnormalities, the most common condition is Down syndrome, caused by the presence of a third copy of chromosome 21 (trisomy 21). Detailed ultrasound is used to look for markers that may suggest such conditions.
However, detailed ultrasound is not a definitive diagnostic method for chromosomal disorders. Results from first-trimester (combined) or second-trimester (triple) screening tests can increase or decrease the estimated risk.
The gold standard for diagnosing chromosomal abnormalities remains the analysis of fetal chromosomes obtained through chorionic villus sampling (CVS) or amniocentesis, where the number and structure of chromosomes are examined.
