Trisomy 13, also called Pätau syndrome, is a genetic defect in which chromosome 13 is present three times. This condition usually develops by chance, although there is also an inherited form.
Therefore, affected parents should have a genetic test. There are three types of trisomy 13 with varying degrees of symptoms. The effects on the fetus are extreme and usually associated with severe physical and mental disabilities.
Miscarriages are common and most surviving babies do not live beyond one year of age. There are numerous methods for diagnosis in utero, including nuchal fold measurement and the Harmony test.
Paetau syndrome is diagnosed in about one in 10,000 babies and is the third most common trisomy, along with trisomy 21 (Down syndrome) and trisomy 18 (Edwards syndrome).
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Trisomy Is A Genetic Defect
Normally, a person has 23 pairs of chromosomes. Chromosomes are the packages of genetic information made of DNA that contain the instructions the body uses to build a person.
Chromosomes come in 23 pairs, with most people having 46 total chromosomes. Trisomy 13 occurs when chromosome number 13 is present three times instead of the usual two, for a total of 47 chromosomes. This extra chromosome affects a baby’s development, which can lead to a number of medical problems. All babies who survive with trisomy 13 have significant, usually severe, intellectual disability.
Here’s How Trisomy 13 Occurs
Trisomy 13 occurs spontaneously and unexpectedly in most cases, but there are risk factors. The likelihood of having a baby with trisomy 13 increases with the age of the mother.
Trisomy 13 can also be inherited due to a family chromosome rearrangement called a translocation. However, trisomy 13 cannot be caused by a specific behavior of the parents and is not the result of environmental influences. Most cases of trisomy 13 are not inherited and result from random events in the formation of eggs and sperm in healthy parents even before conception. An error in cell division, called non-disjunction, results in a reproductive cell with an abnormal number of chromosomes. For example, an egg or sperm cell may receive an extra copy of chromosome 13. If one of these atypical reproductive cells contributes to a child’s genetic makeup, the child will have an extra chromosome 13 in every cell in the body.
Trisomy 13 Can Be Inherited
Trisomy 13 can also occur when part of chromosome 13 is attached (translocated) to another chromosome during egg or sperm formation or very early in fetal development. This is called translocation trisomy 13.
People with this type of trisomy 13 have chromosome 13 twice, plus an extra chromosome 13 that is bound to another chromosome. A person with a balanced translocation with chromosome 13 may have no symptoms themselves but an increased chance of passing extra material from chromosome 13 to their children. About 20% of cases of trisomy 13 are caused by translocation trisomy 13.
Genetic Testing For Parents With Trisomy 13
If a baby is affected by paetau syndrome caused by a chromosomal translocation, both parents must have their chromosomes analyzed. These genetic tests are not part of the decision-making process for the current pregnancy. They are done to help parents plan for future pregnancies. The test results will provide a more accurate assessment of the likelihood of trisomy 13 occurring in future pregnancies. Other family members may also be affected and should be tested.
Three Types Of Trisomy 13
Free Trisomy 13
Most cases of trisomy 13 result from three copies of chromosome 13 in each cell in the body instead of the usual two. The extra genetic material interferes with the normal course of development and causes the characteristic features of trisomy 13. About 95% of cases of trisomy 13 are of this type.
Mosaic Trisomy 13
A small percentage of people with trisomy 13 have an extra chromosome 13 in only some cells of the body. In these cases, it is called mosaic trisomy 13. The severity of the condition depends on the type and number of cells that have the extra chromosome. The physical characteristics of mosaic trisomy 13 are often milder than those of free trisomy 13. About 5% of cases of trisomy 13 belong to this type.
Partial Trisomy 13
In this case, only part of chromosome 13 is present three times in the cells. The type of symptoms depends on which section it is. Less than 1% of cases of trisomy 13 belong to this type.
The Diagnosis Of Trisomy 13
Pregnancies at increased risk for trisomy 13 can be identified by tests such as noninvasive prenatal testing (NIPT) and ultrasounds. An ultrasound alone to determine whether the baby is a girl or boy is not sufficient for this purpose.
One prenatal test for genetic changes is nuchal fold measurement. Determining the thickness of the nuchal fold during nuchal fold measurement can help assess the risk for Pätau syndrome or Down syndrome and other genetic defects.
The doctor uses abdominal ultrasound for nuchal fold measurement. All unborn babies have some fluid in the neck. A baby with trisomy 13 or other genetic disorders has more fluid than normal.
A neck fold measurement will reveal a thicker neck fold. The diagnosis can be confirmed prenatally with 99% accuracy by placental puncture (chorionic villus sampling) or amniocentesis from about SSW 14 to SSW 18.
It can also be confirmed shortly after birth by blood tests such as the Harmony Test.
The Harmony test is a blood test. It analyzes cell-free DNA in maternal blood and gives a strong indication of whether the fetus is at high or low risk for trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), or trisomy 13 (Pätau syndrome).
The Harmony test identifies 92% of fetuses with trisomy 13. However, if the Harmony test shows that there is a high risk that the fetus has trisomy 13, it does not mean that the fetus is definitely affected.
A placenta puncture, amniocentesis, or nuchal fold measurement should also be performed to get a definite result. If the Harmony test shows that there is a small risk that the fetus has trisomy 13, it is unlikely that it is present.
Trisomy 13 And Birth
Because of the complex nature of trisomy 13, babies often face medical challenges during birth and the newborn period. Each baby’s situation is different, and parents receive support to prepare for their baby’s birth and to create a care plan to follow after birth.
This plan may include medical interventions or be limited to comfort measures, depending on the baby’s health and the parents’ wishes.
Effects Of Trisomy 13 On The Baby
Babies with trisomy 13 can have several life-threatening medical problems at birth and throughout life. In addition to birth defects and cognitive impairment, many babies with trisomy 13 die before birth.
Of those who are born alive, few survive the first year of life. It is not known why many of these babies die before birth or in the first year of life. Growth in the uterus is often restricted, resulting in low birth weight, and eight out of ten babies are born with severe heart defects. The brain often does not divide into two halves, which is called holoprosencephaly.
- Absence of one or both eyes (anophthalmia)
- Unusually small eyes (microphthalmia)
- A reduced distance between the eyes (hypotelorism)
- A cleft lip and/or palate
- Problems with the development of the nasal passages
Other abnormalities of the face and head include:
- Ear deformities and deafness
- Smaller head size (microcephaly)
- Missing skin on the scalp (cutis aplasia)
- Raised red birthmarks (capillary hemangiomas)
Pätau syndrome can cause other problems as well:
- Abnormal cysts in the kidneys
- An abdominal wall defect in which the abdomen does not fully develop in the uterus, so that the intestines are outside the body
- Spinal cord anomalies
There may also be abnormalities of the hands and feet, such as extra fingers or toes (polydactyly) and flat feet.
Trisomy 13 Treatment Options
Although there is no cure for trisomy 13, there are medical treatments that can be done after birth to improve the quality and length of life for these babies.
Doctors usually focus on minimizing discomfort and making sure the baby is able to breastfeed. In addition, postpartum treatment may be available for some of the birth defects caused by trisomy 13. Babies with trisomy 13 have different outcomes. For some, the medical challenges they face will be so great that they will die shortly after birth, while a small number of families can celebrate their baby’s first birthdays and beyond. These children have significant cognitive impairments and other medical problems that require extensive care throughout their lives.
Trisomy 16 Genetic Defect
A similar genetic abnormality to trisomy 13 is trisomy 16, although there are significant differences in the effects. Trisomy 16 results from an extra chromosome 16, and diagnosis is possible via a nuchal fold measurement and the Harmony test. If the diagnosis is positive, there will most likely be a miscarriage in the first trimester.
If mosaic trisomy 16 is present, the fetus may survive with near normal developmental outcomes, this depends on the presence and extent of malformations. The prevalence of trisomy 16 varies by sex and is more common in male fetuses, with a predominance of female fetuses in mosaic trisomy.
Therefore, an ultrasound is definitely advisable whether it is a girl or a boy. Free trisomy 16 is incompatible with life.