system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do doctors detect Down Syndrome in pregnancy?
gpt
It's a two-way process. There are two markers in blood plus the measurement of the thickness of the neck in a given week of the pregnancy. 

The doctor checks then in a look-up table the risk of Down Syndrome given the age of the mother, the blood test results, and the ultrasound measurements. 

If the risk is very high, the only way to know for sure is an amniocentesis, but in that case there is a risk of losing the pregnancy, so it's not routine.

There have been some advances in research that have proven that the baby's blood mixes with the blood of the mother, and the trisomy could be detected in a blood sample from the mother, but this is still under research.