Trisomy 18 and 13
What are trisomies?
The term trisomy is used to describe the presence of three chromosomes, rather than
the usual pair of chromosomes. For example, if a baby is born with three #21 chromosomes,
rather than the usual pair, the baby would be said to have "trisomy 21." Trisomy 21
is also known as Down syndrome. Other examples of trisomy include trisomy 18 and trisomy
13. Again, trisomy 18 or trisomy 13 simply means there are three copies of the #18
chromosome (or of the #13 chromosome) present in each cell of the body, rather than
the usual pair.
What are trisomy 18 and trisomy 13?
Trisomy 18 and trisomy 13 are genetic disorders that include a combination of birth
defects. This includes severe intellectual disability, as well as health problems
involving nearly every organ system in the body. Unfortunately, most babies born with
trisomy 18 or 13 die by age 1. It is important to note that some babies with trisomy
18 or 13 do survive the first year of life. Therefore, these disorders are not universally
fatal. In the absence of any immediate life-threatening problems, accurate predictions
of life expectancy are difficult to make. There are a few reports of babies with trisomy
18 or 13 surviving to their teens, however, this is unusual.
Trisomy 18 is also called "Edwards syndrome," named after the physician who first
described the disorder. Trisomy 13 is called "Patau syndrome," in honor of the physician
who first described it.
What causes trisomy 18 and trisomy 13?
Usually, each egg and sperm cell contains 23 chromosomes (half the normal number i
body cells). The union of these cells creates 23 matched pairs, or 46 total chromosomes
at the time of fertilization. In this manner, a person receives exactly half of their
genetic material from each biological parent. Sometimes, an error occurs when an egg
or sperm cell is forming, causing it to have an extra chromosome #18 or #13 inside.
If this egg or sperm cell contributes that extra chromosome 18 to the embryo, then trisomy
18 results. If this egg or sperm cell contributes that extra chromosome 13 to the
embryo, then trisomy 13 results. The extra chromosome 18 or 13 can come from either
the mother or father. The features of trisomy 18 and trisomy 13 result from having
this extra copy of chromosome 18 or 13 in each of the body's cells.
Occasionally, the extra chromosome 18 or 13 is attached to another chromosome in the
egg or sperm; this is called a translocation. This is the only form of trisomy 18
or 13 that can be inherited from a parent. Sometimes, a parent can carry a "balanced"
rearrangement, where 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. Rarely,
mosaic trisomy 18 or 13 may occur when the error in cell division occurs after fertilization.
These affected persons have some cells with an extra chromosome 18 or 13 and others
with the normal 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)
prominent. Ears are usually low-set on the head. The mouth and jaw are unusually small,
and there is a shortened sternum (breastbone).
At birth, these babies are small for their age, even when delivered full-term, and
have a weak cry. Their response to sound is decreased and there is often a history
of infrequent fetal activity during the pregnancy. Most babies with trisomy 18 have
heart defects. They clench their fists in a characteristic manner and fully extending
their fingers is difficult. Joint contractures—where the arms and legs are in a bent
position, rather than relaxed—are usually present. The feet may be referred to as
"rocker bottom," due to their curved shape.
Babies with trisomy 18 may also have spina bifida, eye problems, cleft lip and palate,
and hearing loss. It is also common to see feeding problems, slow growth, seizures,
high blood pressure, kidney problems, and scoliosis (curvature of the spine). In males,
the testes sometimes fail to descend into the scrotum.
Most babies with trisomy 18 have problems that affect all parts of the body in some
way. The majority of children with trisomy 18 will have most, but not all, of the
health problems mentioned here. Heart problems, feeding difficulties, and an increased
susceptibility to infection are factors which, 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. Usually, there are major structural problems
with the brain that are diagnosed shortly after birth. Often, the front of the brain
does not divide properly, resulting in a condition called holoprosencephaly. This
can cause changes in the development of the baby's face, where the eyes are close
set, or the nose or nostrils are underdeveloped. Cleft lip and cleft palate are common
in babies with trisomy 13.
Eye problems are common and the ears are low-set and unusual in shape. Sometimes,
babies with trisomy 13 can have scalp abnormalities (cutis aplasia) which resemble
ulcers. They can also have birthmarks that are purplish-red in color; the color is
due to tiny blood vessels close to the skin (hemangiomas).
Many babies with trisomy 13 have extra fingers and toes (polydactyly). The feet may
have prominent heels. In many cases, there are other health problems present at birth.
These include heart defects, kidney problems, and/or an omphalocele (a condition in
which some of the abdominal organs protrude through an opening in the abdominal muscles
in the area of the umbilical cord). In males, the testes sometimes fail to descend
into the scrotum. Females may have an abnormally shaped uterus, called a bicornuate
How are trisomy 18 and trisomy 13 diagnosed?
Since trisomy 18 and trisomy 13 each have a unique group of characteristics, a physician
may be able to determine whether a baby has trisomy 18 or 13 simply by physical examination.
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.
Chromosomal abnormalities such as trisomy 18 and 13 can be diagnosed before birth
by analyzing cells in the amniotic fluid or from the placenta. Fetal ultrasound during
pregnancy can also give information about the possibility of trisomy 18 or 13. Ultrasound
is not 100 percent accurate, since 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 very accurate.
What is the risk for parents of a child with trisomy 18 or trisomy 13 having another
child with trisomy 18 or 13?
In general, for women under 35 years of age, the chance of having another baby with
trisomy 18 or 13 depends on several factors. The risk to having a baby with trisomy
18 or 13 does increase slightly with each added year of maternal age.
After birth, the physician usually takes a blood sample from a baby suspected of having
trisomy 18 or 13, to perform a chromosomal analysis (called a karyotype). This confirms
the physical findings of trisomy 18 or 13 and determines the underlying chromosomal
abnormality. This information is important in determining the risk in future pregnancies.
(Translocation and mosaic trisomy 18 and 13 have different recurrence risks for future
pregnancies). The physician may refer parents to a genetic physician or genetic counselor
who can explain the results of chromosomal tests in detail. This includes what the
recurrence risks may be in another pregnancy and what tests are available to diagnose
chromosome problems before a baby is born.
Can trisomy 18 or trisomy 13 be cured or prevented?
There is no cure for trisomy 18 or trisomy 13. We are not certain how to prevent the
chromosomal error that causes trisomy 18 and trisomy 13. To date, there is no scientific
evidence that a parent could have done anything to cause or prevent the birth of their
baby with trisomy 18 or 13.
What else should I know about trisomy 18 and trisomy 13?
Because many babies born with trisomy 18 and trisomy 13 may not live beyond the first
few days or weeks of life, it is possible that 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 all of the information associated
with trisomy 18 and 13. Decisions surrounding the care of an infant with trisomy 18
and 13 are difficult and personal. There are many resources available to parents to
help them during this time, including early intervention services, hospice care, social
workers, the hospital chaplain or clergyman, and genetic counselors. 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.