If i am 23yo, why would my doctor tell me i have a high risk of having a down syndrome baby?

Affected relative: Younger (under 16) and "older" (arbitrarily over age 35) have higher risks for a child with down syndrome--about 1% increasing to ~2% for women over 40. Mid-twenties to early thirties is a risk of 1 in 2000 or less. If you had a prior child with down syndrome, or someone in the family had a child with translocation down syndrome, your risks could be higher.

Down's syndrome: Women at risk are usually much older than you. Ask your doctor if there is an indication of a chromosomal abnormality, such as having an extra piece of a 21 chromosome attached to another chromosome or a family history of young women in your family giving birth to children with down syndrome. Your doctor should be able to explain why he feels your are at risk.

Chromosome analysis : obtained by amniocentesis or Chorionic Villous Sampling is diagnostic for Trisomy 21, other trisomies & monosomies. It is offered if the calculated statistical risk of DS from screening by blood tests & fetal ultrasounds + maternal age at EDD & family history reaches a cut-off point, often 1:270. This calculated risk method has an 80% detection rate & a 5% false-positive rate.