Aneuploidy refers to genetic mutations that result in the presence of more or less chromosomes than normal. Human cells normally have 23 pairs of chromosomes for a total of 46 chromosomes. Chromosomal abnormalities- extra or missing chromosomes- are common causes of congenital anomalies.
Monosomy is a form of aneuploidy characterized by the presence of only one chromosome from a pair. In the case of Turner syndrome, also known as monosomy X, a female baby is missing one copy of the X chromosome.
Trisomy is a form of aneuploidy characterized by the presence of an additional copy of a chromosome, resulting in three copies of that chromosome in the cell. In the case of a diagnosis of trisomy 18, the baby has three copies of chromosome 18 rather than the normal two copies.
Aneuploidy can be the result of an entire extra or missing copy of a chromosome, but it can also be the result of a duplication, in which part of a chromosome is copied; a deletion, in which part of a chromosome is removed; an inversion, in which a section of a chromosome is flipped around; or a translocation, in which a piece of one chromosome attaches to another chromosome.
Aneuploidy can be full, in which every cell in the body contains the mutation; mosaic, in which the mutation is present in only some, but not all, of the cells in the body; or partial, in which a chromosomal translocation has taken place.
Screening for aneuploidy can occur during both the first and second trimesters of pregnancy. In the first trimester, typically between weeks 10 and 13, two screening tests are performed to assess the risk of aneuploidy and other anomalies. This first trimester screening test includes a maternal serum screening, which tests the levels of two different substances in the mother’s blood, and an ultrasound exam, in which the nuchal fold at the back of the baby’s neck is measured.
In the second trimester, typically between weeks 15 and 22, more screening tests are done to rule out anomalies like aneuploidy and other structural abnormalities. The second trimester test includes a maternal serum screening, which tests the levels of four different substances in the mother’s blood, and an ultrasound exam, referred to as the anatomy scan, which checks for anomalies in the brain, spine, face, abdomen, heart, organ systems, and limbs.
A large majority of pregnancies affected by aneuploidy end in miscarriage. The most common aneuploidies seen in live births are trisomy 21 (Down syndrome), which is not fatal, and trisomy 18 and trisomy 13, which are both typically fatal in the perinatal period, though the prognosis may vary. The risk of conceiving a baby with aneuploidy increases with maternal age, which is referred to as maternal age-related risk.