The additional chromosome usually occurs before conception. The effects of the extra copy vary greatly, depending on the extent of the extra copy, genetic history, and chance. Edwards syndrome occurs in all human populations, but is more prevalent in female offspring.A healthy egg and/or sperm cell contains individual chromosomes, each of which contributes to the 23 pairs of chromosomes needed to form a normal cell with a typical human karyotype of 46 chromosomes. Numerical errors can arise at either of the two meiotic divisions and cause the failure of a chromosome to segregate into the daughter cells (nondisjunction). This results in an extra chromosome, making the haploid number 24 rather than 23. Fertilization of eggs or insemination by sperm that contain an extra chromosome results in trisomy, or three copies of a chromosome rather than two.Trisomy 18 (47,XX,+18) is caused by a meiotic nondisjunction event. With nondisjunction, a gamete (i.e., a sperm or egg cell) is produced with an extra copy of chromosome 18; the gamete thus has 24 chromosomes. When combined with a normal gamete from the other parent, the embryo has 47 chromosomes, with three copies of chromosome 18.A small percentage of cases occur when only some of the bodys cells have an extra copy of chromosome 18, resulting in a mixed population of cells with a differing number of chromosomes. Such cases are sometimes called mosaic Edwards syndrome. Very rarely, a piece of chromosome 18 becomes attached to another chromosome (translocated) before or after conception.
Affected individuals have two copies of chromosome 18 plus extra material from chromosome 18 attached to another chromosome. With a translocation, a person has a partial trisomy for chromosome 18, and the abnormalities are often less severe than for the typical Edwards syndrome. Diagnosis
Ultrasound can increase suspicion for the condition, which can be confirmed by CVS or amniocentesis.Levels of PAPP-A, AFP, uE3, free β-hCG, all of which are generally decreased during pregnancy. Prognosis
About 95% of pregnancies that are affected do not result in a live birth. Major causes of death include apnea and heart abnormalities. It is impossible to predict an exact prognosis during pregnancy or the neonatal period. Half of the live infants do not survive beyond the first week of life. The median lifespan is five to 15 days. About 8–12% of infants survive longer than 1 year. One percent of children live to age 10. However, these estimates may be pessimistic; a retrospective Canadian study of 254 children with trisomy 18 demonstrated ten-year survival of 9.8%, and another found that 68.6% of children with surgical intervention survived infancy. Epidemiology
Edwards syndrome occurs in about one in 5,000 live births, but more conceptions are affected by the syndrome because the majority of those diagnosed with the condition prenatally will not survive to birth. Although women in their 20s and early 30s may conceive babies with Edwards syndrome, the risk of conceiving a child with it increases with a womans age. The average maternal age for conceiving a child with this disorder is 32+1⁄2.