What causes trisomy 18 and trisomy 13?
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.
What types of problems do children with trisomy 18 and trisomy 13 typically have?
- thin and frail babies with a weak cry
- feeding problems
- small head size with the back of the head prominent
- ears are usually low-set on the head
- mouth and jaw unusually small
- shortened sternum (breastbone)
- about 90 percent of babies have heart defects
- clenched fists
- difficulty extending fingers
- contracted joints
- spina bifida, eye problems, cleft lip and palate and hearing loss may be present
- high blood pressure and kidney problems
- scoliosis (curvature of the spine)
- in males, the testes fail to descend into the scrotum
- low birthweight
- small head with a sloping forehead
- usually there are major structural problems with the brain including holoprosencephaly, when the brain does not divide properly
- close-set eyes
- underdeveloped nostrils
- eye problems
- ears low-set and unusually shaped
- scalp abnormalities that resemble ulcers
- birthmarks that are purplish-red
- extra fingers and toes (polydactyly)
- feet with prominent heels
- heart defects, kidney problems
- omphalocele, a condition in which some of the abdominal organs protrude through an opening in the abdominal muscle around the umbilical cord
- in males, testes may fail to descend into the scrotum
- females may have abnormally shaped uterus, or a bicornuate uterus
What is the risk of parents of a child with trisomy 18 or trisomy 13 having another child with trisomy 18 or 13?
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.