Hi Saggy, just popping in to say congratulations. I'm mum to a fab 3 yo with ds who had some complicated health problems inc cardiac issues. It was tough but 3 years on and he's a bundle of energy and perfectly healthy. If i can help at all, shout up 
Entering into this new world of having a child with disabilities can be daunting but do you know what, it's ok. Our little man has changed our family for the better and he is the centre of our universe. Expect lots of appointments in the early days, take all the support you can get and most importantly, enjoy being a mum.
Also not sure where you're based but there are some wonderful local support groups out there who would welcome you with open arms. Best of luck with the rest of your pregnancy and please don't worry, things have a funny way of turning out ok when you have Down's syndrome in your life 