Monday, May 25, 2009
Most people are born with 23 pair of chromosomes. Sometimes due to genetics, maternal age, or simply a fluke, an extra chromosome is added. The most common trisomy is Trisomy 21 or Down Syndrome. I was introduced to Trisomy 18, or Edward's Syndrome, thirteen years ago. Trisomy 21, 18, and 13 are the only known viable trisomies at this time. All others end in miscarriages usually early in the pregnancy.
There are two screening tests which cannot diagnose chromosome disorders but can be used to indicate whether further testing should be done. One is AFP or the triple screen test. It's a blood test performed typically between the 15th and 17th weeks of pregnancy. AFP tests can give false positives causing families unneeded stress and worry. The other screen test is a Level II ultrasound which can be used to look for certain markers known in Trisomy 18 babies. Some of the markers sonographers look for are heart malformations, a small mouth and jaw, external ear variations, clenched fist with overlapping fingers, underdeveloped or altered thumbs, short breastbone, club feet, spina bifeda and kidney size (one may be smaller than the other). If only one marker is found the chances are good that the baby is normal and healthy. If two or more markers are discovered futher testing would be suggested.
If the screen tests indicate that there is a possiblity the baby could have Trisomy 18 a diagnostic test can be performed. The CVS or (Chorionic Villi Sampling) which takes a piece of chorionic villi for analysis is performed in the first trimester if there are indications of a chromosomal anomoly. An amniocentesis uses a long needle to draw out amniotic fluid which surrounds the baby. Results usually come back in ten days. This is considered an evasive procedure with a 1 in 200 chance of causing a miscarriage.
Sometimes Trisomy 18 remains undetected during the pregnancy. Once the baby is born, however, there are several physical features which will indicate the disorder. The baby will be very small despite being carried to full term, she may have a small head, small wide-set eyes, a small lower jaw, cleft lip and palate, vision and hearing problems, and the facial features will be close together making her head appear to be larger than it is. Club feet, under developed fingernails, an unusual shaped chest and overlapping fingers in clenched fists may also be apparent.
Most T18 babies have heart defects such as ventricular septal defect, atrial septal defect, or patent ductus arteriosis. They will suffer from mental retardation, delayed development, brain anomolies, seizures, and digestive and urinary tract malformations. They will be fed through a tube, have infections treated, and possible surgeries if the parents choose to do so.
Trisomy 18 occurs in 1 out of every 3,000 live born infants, or 1 out of 5,000, or 6,000, or 8,000 depending on which site you visit.
5% are translocation (piece of chromosome breaks off and sticks to another chromosome) 95% are full trisomies (affects every cell of the body)
95% die as fetuses or embryos
50% don't survive the first week after their birth
10% live past their first birthday
90% are girls
90% have heart defects
2-3% of families will have another baby with a trisomy
"While we tend to focus on trisomy 21 because it is the most common trisomy, it is important to remember that trisomy is a common occurrence in pregnancy. There is no known cause or cure."