(See also Overview of Chromosomal Disorders.)
Chromosomes are structures within cells that contain DNA and many genes. A gene is a segment of deoxyribonucleic acid (DNA) and contains the code for a specific protein that functions in one or more types of cells in the body (see Genes and Chromosomes for a discussion about genetics). Genes contain instructions that determine how the body is supposed to function.
An extra chromosome, making three of the same chromosome (instead of the normal two), is called trisomy. Children who have trisomy 13 have a third chromosome 13. Trisomy 13 occurs in about 1 out of 10,000 live births. The extra chromosome usually comes from the mother. Mothers who are over age 35 are at increased risk of having a child with trisomy 13.
In the womb, affected fetuses are typically not very active. The amount of amniotic fluid may be too much or too little.
At birth, newborns tend to be small. The brain typically does not develop properly, and these children have many facial abnormalities such as cleft lip and cleft palate, small eyes, defects of the iris (the colored area of the eye), and underdeveloped retinas (the clear, light-sensitive structures at the back of the eyes). The ears are abnormally shaped and usually low-set. Defects in the scalp and openings in the skin are common. Loose folds of skin often are present over the back of the neck. Newborns commonly have a single crease in the palm, extra fingers and toes (polydactyly), and poorly developed fingernails.
About 80% of newborns have severe heart defects. A defect called ventricular septal defect, in which there is an abnormal opening between the right and left ventricles, is common. Undescended testes and an abnormal scrotum occur in boys. An abnormally formed uterus occurs in girls. Newborns frequently have prolonged periods of no breathing (apnea). Hearing loss is common and is often suspected but can be difficult to confirm because these children are severely intellectually disabled.
Before birth, trisomy 13 may be suspected based on findings detected during an ultrasound of the fetus. Doctors also can do a test to find deoxyribonucleic acid (DNA) from the fetus in the mother's blood and use this DNA to determine an increased risk of trisomy 13. This test is called noninvasive prenatal screening (NIPS) or cell-free fetal DNA testing. If doctors suspect trisomy 13 based on these tests, they often confirm the diagnosis using chorionic villus sampling, amniocentesis, or both.
After birth, the infant's physical appearance may suggest the diagnosis of trisomy 13. To confirm the diagnosis, the infant's chromosomes are analyzed using a blood test.