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What are trisomies?
Most people have 23 pairs of chromosomes in their cells. Trisomy means that a person has 3 of a certain chromosome pair. For example, if a baby is born with 3 No. 21 chromosomes, rather than the usual pair, the child would have trisomy 21. Trisomy 21 is also known as Down syndrome.
Other examples of trisomy include trisomy 18 and trisomy 13. Trisomy 18 means the child has 3 copies of the No. 18 chromosome. Trisomy 13 means the child has 3 copies of the No. 13 chromosome.
What are trisomy 13 and trisomy 18?
Trisomy 13 and trisomy 18 are genetic disorders. They include a combination of birth defects. This includes severe learning problems and health problems that touch nearly every organ in the body.
Most babies born with trisomy 13 or 18 die by age 1. But some babies with these disorders do survive the first year of life. It’s hard to predict how long a child with these disorders might live. There are a few reports of babies with trisomy 13 or 18 surviving to their teens, but this is unusual.
What causes trisomy 13 and trisomy 18?
When a baby is conceived, the egg cell of the mother and the sperm cell of the father start out with the usual number of 46 chromosomes. The egg and sperm cells both divide, so that the chromosome number is half of 46. The egg and the sperm cells then have 23 chromosomes each. When a sperm with 23 chromosomes fertilizes an egg with 23 chromosomes, the baby will have a complete set of 46 chromosomes. Half are from the father and half are from the mother.
Sometimes an error occurs when the 46 chromosomes are being divided in half. An egg or sperm cell keeps both copies of the No. 13 or No. 18 chromosome instead of just one copy. If this egg or sperm is fertilized, then the baby will have 3 copies of the No. 13 or No. 18 chromosome. If the baby has 3 copies of the No. 13 chromosome, the condition is called trisomy 13. If the baby has 3 copies of the No. 18 chromosome, the condition is called trisomy 18. The features of these disorders are caused by the extra copy of chromosome No. 13 or No. 18 being in every cell in the body.
Sometimes the extra No. 13 or No. 18 chromosome or part of it is attached to another chromosome in the egg or sperm. This is called a translocation. This is the only form of trisomy 13 o4 18 that may be inherited from a parent. Some parents have a rearrangement called a balanced translocation, in which the No. 13 or No. 18 chromosome is attached to another chromosome, but it does not affect their own health.
Another form called “mosaic trisomy 13 or 18" may happen when an error in cell division happens after the egg is fertilized. This is rare. People with this syndrome have some cells with an extra chromosome No. 13 or No. 18, and other cells have the usual number.
What types of problems do children with trisomy 18 typically have?
Babies with trisomy 18 appear thin and frail. They fail to thrive and have problems feeding. Trisomy 18 causes a small head size, with the back of the head (occiput) most noticeable. Ears are usually low-set on the head. The mouth and jaw are unusually small. The child has a shortened breastbone (sternum).
At birth, these babies are small for their age, even when delivered full-term. They have a weak cry. They don’t respond well to sound. Most babies with trisomy 18 have heart defects. They clench their fists in a certain way and have problems extending their fingers fully. They usually have joint contractures. This means their arms and legs are in a bent position, rather than relaxed. Their feet have a curved shape and may be called "rocker bottom."
Babies with trisomy 18 may also have spina bifida, eye problems, cleft lip and palate, and hearing loss. They also often have feeding problems, slow growth, seizures, high blood pressure, kidney problems, and curvature of the spine (scoliosis). In boy babies, the testes sometimes don’t descend into the scrotum.
Most babies with trisomy 18 have problems that affect all parts of the body in some way. Most children with trisomy 18 will have most of the health problems described. Heart problems, feeding problems, and problems with infections are the things that most often contribute to the death of these children.
What types of problems do children with trisomy 13 typically have?
Babies with trisomy 13 often have a low birthweight, even when born full-term. They have a small head, with a sloping forehead. They usually have major structural problems with the brain. Often the front of the brain doesn’t divide properly (holoprosencephaly). This can cause changes in how the baby’s face develops. The baby’s eyes are close set or the nose or nostrils aren’t well developed. Babies with trisomy 13 often have cleft lip and cleft palate.
Babies with trisomy 13 often have eye problems. Their ears are low-set with an unusual shape. Sometimes babies with trisomy 13 can have scalp problems (cutis aplasia) that look like sores. They can also have purplish-red birthmarks. The color is caused by tiny blood vessels close to the skin (hemangiomas).
Many babies with trisomy 13 have extra fingers and toes (polydactyly). Their feet may have prominent heels. In many cases, babies with trisomy 13 have health problems present at birth. These include heart defects and kidney problems. They may also have an omphalocele. This is a condition in which some of the abdominal organs stick out through an opening in the muscles near the umbilical cord.
In boy babies, the testes sometimes don’t descend into the scrotum. Girl babies may have an abnormally shaped uterus (bicornuate uterus).
How are trisomy 13 and trisomy 18 diagnosed?
Doctors can sometimes diagnose a baby with trisomy 13 or 18 with just a physical exam. To confirm the physical findings, the doctor can take a blood sample. This is looked at in the lab to find the extra No. 13 or No. 18 chromosome.
Chromosomal problems such as trisomy 13 or 18 can often be diagnosed before birth. This is done by looking at cells in the amniotic fluid or from the placenta. This can also be done by looking at the amount of fetal DNA in the mother's blood (noninvasive prenatal screening).
Fetal ultrasound during pregnancy can also show the possibility of trisomy 13 or 18. But ultrasound is not 100% accurate. Many babies with trisomy 13 or 18 may look the same on ultrasound as those without Down syndrome.
The chromosome analysis done on a blood sample or on cells from the amniotic fluid or placenta is very accurate.
What is the risk for parents of a child with trisomy 13 or trisomy 18 of having another child with this disorder?
If you are under age 35, the chance of having another baby with trisomy 18 or 13 depends on several things. The risk of having a baby with trisomy 18 or 13 goes up slightly each year as you get older.
After the baby is born, the healthcare provider will usually take a blood sample to do a chromosome analysis (karyotype). This confirms the physical findings of trisomy 18 or 13. It also finds the chromosome problem that caused the disorder. This information is important in figuring out the risk in future pregnancies. Translocation and mosaic trisomy 18 and 13 have different risks for future pregnancies.
Your healthcare provider may refer you to a geneticist or genetic counselor. This expert can explain the results of chromosome tests in detail. He or she can talk about risks for future pregnancies and what tests are available to diagnose chromosome problems before a baby is born.
Can trisomy 13 or trisomy 18 be cured or prevented?
There is no cure for trisomy 13 or 18. Doctors are not certain how to prevent the chromosome error that causes these disorders. To date, there is no reason to believe that a parent could have done anything to cause or prevent the birth of a baby with trisomy 13 or 18.
What else should I know about trisomy 13 and trisomy 18?
Because many babies born with trisomy 18 and trisomy 13 may not live beyond the first few days or weeks of life, parents may have to face the fact that the baby may never go home from the hospital.
Many times, parents are frightened and overwhelmed by the diagnosis of trisomy 18 and 13. They face difficult decisions about how to care for their baby.
Many resources available to help you during this time. These include early intervention services, hospice care, social workers, the hospital chaplain or clergyman, and genetic counselors. Turning to families who have or have had a baby with trisomy 18 or trisomy 13 can provide help and support because they have had many of the same questions and emotions.