Congratulations on your pregnancy!!
I have a BIL with downs syndrome and although he has some learning difficulties he is a very happy and healthy man. He is now well into his 40's with no long term health issues caused by his condition. He has a young mental age (think mid teens for understanding and capability levels) but that hasn't stopped him.
He has made thousands for charity, lives a full life, has been to college and works part time while also volunteering for another charity.
All cases are different but they can have very fulfilled lives and there is much more support than before.
They just have slightly different lives.
I also work in mental health and adult support and have experienced several levels of the condition. All have their pros and cons but all are individuals who excel in their own ways,whether it be artistically, musically, even memory ( I know a guy that can tell you every album and date of his preferred band and details I could never remember)
This is your baby and I'm sure you will love them regardless. Its scary but they are still a little person with personality and quirks. Its scary as right now it's all just information and imaginative.
Best of luck :)