How is Down Syndrome detected during early pregnancy?

Sad news came from my boyfriend today. His sister who's been pregnant between two to three months received news from her doctor that her baby has a high chance from getting Down Syndrome. They found out through a blood test. The only way to find out for sure is through an ultrasound. I'm really saddened by the news. Her first baby is perfectly healthy five year old. This past year she has trying to have a baby. Last time she was pregnant which was about five or six months ago, the doctor said that the baby just disintegrated or disappeared. Now when she finally is pregnant, there's bad news. I'm just curious, if anyone with any knowledge in the pregnancy field, know how is it that Down Syndrome can be detected through a blood test and ultrasound and how accurate is it?