What is Trisomy?
Genes are the blueprint for our bodies. Almost every cell in the body has a copy of the blueprint, stored inside a sac called the nucleus. Genes are beaded along chromosomes, which are tightly bundled strands of the chemical substance deoxyribonucleic acid (DNA). Humans usually have 23 pairs of chromosomes, with two sex chromosomes that decide gender and 44 chromosomes that dictate other factors, such as growth and function.
A chromosome disorder is caused by an alteration in the number or genetic structure of chromosomes. Trisomy ('three bodies') means the affected person has 47 chromosomes instead of 46. Down syndrome, Edward syndrome and Patau syndrome are the most common forms of trisomy. Children affected by trisomy usually have a range of birth defects, including delayed development and intellectual disabilities.
Trisomy 18 syndrome (Edwards syndrome) is a disorder of human chromosomes which occurs in approximately 1 in 7,000 live born infants. Trisomy refers to three copies of a chromosome instead of the normal two and in trisomy 18 there is a presence of an extra #18 chromosome.
Trisomy 13 syndrome (Patau syndrome) is a disorder of human chromosomes which occurs in approximately 1 in 10,000-25,000 live-born infants. Trisomy refers to three copies of a chromosome instead of the normal two and in Trisomy 13 there is the presence of an extra #13 chromosome.
Currently, more than 25% of SOFT's family membership consists of parents whose child does not (or did not) have full trisomy 18 or 13. Mosaicism, partial trisomy, translocations and other related chromosomal disorders are variable phenotypes of a full trisomy.
Diagnosis & Screening
Prenatal diagnosis of a fetus with a trisomy condition generally results in a discussion with the obstetrician about termination or continuation of the pregnancy. Genetic counseling is recommended for those with a positive diagnosis. Perinatal palliative care is available in most areas for expectant mothers who choose to continue a pregnancy with a life-limiting diagnosis. Some mothers report being pressured to terminate when diagnosed or encouraged to provide only comfort care for the live born infant. Some parents decline prenatal testing for personal reasons.
Traditional prenatal screening includes follow-up confirmation by invasive tests (e.g. chorionic villus sampling (CVS) or amniocentesis). Invasive tests have a less than 1% risk for the fetus. An explanation of traditional prenatal screening can be found here.
The recent development and marketing of a non-invasive prenatal diagnostic test (NIPD) uses DNA testing with a single blood draw from the mother. See a discussion about this new technology by Dr John C. Carey: http://trisomy.org/professional-viewpoint/