<s>[INST] <<SYS>> This is a medical question of type : frequency <</SYS>> How many people are affected by trisomy 18 ? [/INST] Trisomy 18 occurs in about 1 in 5,000 live-born infants; it is more common in pregnancy, but many affected fetuses do not survive to term. Although women of all ages can have a child with trisomy 18, the chance of having a child with this condition increases as a woman gets older. </s>