Then, when she was 15 weeks pregnant, results from genetic testing confirmed what an earlier blood test had indicated: Her child had Trisomy 18, a genetic condition that affects how a child’s body develops and grow.
5 to 10 percent die within the first year. So it's not fatal for 90-95 percent, but most people will not know this.
It is fatal. The 5-10% who die in the first year are the 5-10% who live long enough to make it to term. Based on our research during my wife’s pregnancy with a trisomy 18 baby, I think we found that only a handful ever made it past 1 year, and none made it more than a few years after that. The complications from this condition are severe.
From the Wiki:
About 95% of pregnancies that are affected do not result in a live birth.[13] Major causes of death include apnea and heart abnormalities. It is impossible to predict an exact prognosis during pregnancy or the neonatal period.[13] Half of the live infants do not survive beyond the first week of life.[17] The median lifespan is five to 15 days.[18][19] About 8–12% of infants survive longer than 1 year.[20][21][better source needed] One percent of children live to age 10.[13] 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.[21]