Trisomy 18 and 13

The term trisomy describes the presence of three chromosomes instead of the usual pair of chromosomes. For example, trisomy 21, or Down syndrome, occurs when a baby has three #21 chromosomes. Other examples are trisomy 18 and trisomy 13, fatal genetic birth disorders.

• Trisomy 18 occurs in about one out of every 6,000 to 8,000 live births, and trisomy 13 occurs in about one out of every 8,000 to 12,000 live births.
• It's characterized by severe cognitive disability and health problems involving nearly every organ system of the body.
• Babies with the disorders usually die by age 1, but there have been a few cases in which children survive into their teens.
• Trisomy 18 is also called "Edwards syndrome" and trisomy 13 is also called "Patau syndrome," after the physicians who first described the disorders.

The chance of having another baby with trisomy 18 or 13 is no more than 1 percent, although the risk increases slightly as the mother ages. The genetic test to diagnose trisomy 18 or 13 in a baby can determine the underlying chromosomal abnormality, which is important in determining a mother's risk in future pregnancies. Your doctor may refer you to a genetic physician or genetic counselor who can explain the results of chromosomal tests in detail, including recurrence risks.

Trisomy 18 and 13 are usually caused by spontaneous genetic mutations that occur at the time of fertilization. Normally, each egg and sperm cell contains 23 chromosomes. The union of these cells creates 23 pairs, or 46 total chromosomes, half from the mother and half from the father. Sometimes, an error occurs when an egg or sperm cell is forming, causing it to have an extra chromosome #18 or #13.

• When this cell contributes the extra chromosome 18 to the embryo, trisomy 18 results.
• When this cell contributes the extra chromosome 13 to the embryo, trisomy 13 results.

The extra chromosome 18 or 13 can come from either the mother's egg cell or the father's sperm cell.

In some instances, the extra chromosome 18 or 13 is attached to another chromosome in the egg or sperm. This is called translocation and is the only form of trisomy 18 or 13 that can be inherited. Sometimes, a parent can carry a "balanced" rearrangement in which chromosome 18 or 13 is attached to another chromosome. However, since the parent does not have any extra or missing chromosome material, they are said to have a "balanced translocation" and they are usually normal and healthy.

Chromosomal abnormalities such as trisomy 18 and 13 can be diagnosed before birth by analyzing cells in the amniotic fluid (amniocentesis) or from the placenta.

• A fetal ultrasound during pregnancy can also give information about the possibility of trisomy 18 or 13, but ultrasound is not 100 percent accurate because some babies with trisomy 18 and 13 may look the same on ultrasound as those without the syndrome.
• A chromosome analysis, whether performed on a blood sample, cells from the amniotic fluid, or placenta, is over 99.9 percent accurate.

Since trisomy 18 and trisomy 13 each have a unique group of characteristics, a physician may be able to determine simply by physical examination whether a baby has trisomy 18 or 13. To confirm the physical findings, a small blood sample can be taken and the chromosomes can be analyzed to determine the presence of an extra 18 or 13 chromosome.

There is no cure for trisomies 18 and 13 at this time. Treatment is based on support and managing symptoms. In fact, many babies born with these disorders may not live beyond the first few days or weeks of life. It is important to discuss whether to use life support measures or other medical procedures.

Decisions surrounding the care of infants with trisomy 18 and 13 are difficult and personal. Your doctors at Boston Children's can connect you with resources available to provide support and help your during this time, including early intervention services, social workers, the Department of Spiritual Care (chaplaincy), and genetic counselors.

Other families who have or have had a baby with trisomy 18 or trisomy 13 are particularly helpful and supportive, since they have experienced many of the same questions and emotions. We can help connect you to families who have faced similar situations.