Hello, and congrats on your pregnancy.
Risks and being a forties mum (I'm 40) ... OK, depending on the protocol your hospital uses, various factors will affect your risk of Down's.
Age.
Nuchal scan (fluid at the back of baby's neck measured in mm).
Bloods: hCG, PAPP-A, and maybe estriol and others??
Your age, nuchal measurement and the gestational age of your baby, and blood results are then used to work out your overall risk.
Depending on your nuchal and blood results, your initial 'background age risk' can go up or down.
These measurements help to measure risk, but none on its own will give a definite result.
More definite results can be obtained by invasive testing like CVS or amniocentesis These almost always give a definitive result (with a few exceptions) BUT they both carry a risk of miscarriage (often quoted as amnio 1%, and CVS sometimes 1-2% - risks can vary so I would ask your own medical team for specific advice).
You might also read about things like missing nasal bones, short femurs and heart defects. These are all issues that can be related to Down's but not necessarily, and they are not always reliable as markers - seek medical advice before relying on these.
Good luck for you, your baby and your DH. I hope you have a good scan and that everything is fine - and that you enjoy your pregnancy.