The most common prenatally diagnosed aneuploidies are the trisomies of chromosome 21 (Down syndrome), 18 (Edwards syndrome), 13 (Patau syndrome), and Turner syndrome. Other aneuploidies also detected by cytogenetic studies include abnormalities in the number of sex chromosomes (besides Turner syndrome), triploidy, deletions, and translocations. The incidence of aneuploidies increases according to maternal age, except in cases of sex chromosome aneuploidies. In general, the prevalence of aneuploidies is stable in relation to maternal and gestational age. The approximate frequencies are 0.5% in women at 35 years, 1.2% at 37 years, 6% at 43 years, and 15% at 45 years.

In many countries, fetal aneuploidy screening programs are applied and invasive procedures are usually offered to pregnant women over 35 years of age (WHO definition of advanced maternal age). Above 35 years, the incidence of abnormal fetal karyotype exceeds the risk of fetal loss inherent to invasive diagnostic procedures.

In the last few years, even after the spread of diagnostic methods, the number of fetuses born with syndromes, especially Down syndrome, has not decreased. This is because 70% of the children with aneuploidies are born from a low-risk pregnancies group (under 35 years).

The ultrasound findings can determine an increased risk for chromosomal abnormalities in a patient who would not have an indication for fetal cytogenetic study based on age only; moreover, women with advanced maternal age who decline to perform invasive tests may have their risk decreased after screening. This helps when they choose not to obtain a certain diagnosis by undergoing an invasive procedure.