I had had 2 miscarriages before having our daughter then another miscarriage before going through much the same as you are - my 5th pregnancy - a son - was shown to be trisome 18 - they usually self-abort as the problems are so severe - I terminated the pregnancy, several years later I fell pregnant again - twins - lost one but have a son.
I had thought long & hard prior to falling pregnant (due to experiences of aquaintances) what I would do if the baby had serious health (physical/mental) issues & decided that I would not put myself, my family or the child through such an agonising soul destroying experience.
If this were a dog you would not make it live without quality of life, if it was you we were talking about what would you want for yourself - the life that you could have such as it would be/wouldn't be or heaven?
I have never stopped loving my son, I know he's in heaven & loved but as hard & as painful as it was & still is 13 years later I would not hesitate to do the same thing again in the same circumstances.
To be fair though I know of families who would totally disagree with me on every count - the decision must be yours & your husbands & should take into account emotional, financial, long term physical & emotional constraints on the entire family & the quality of life of your unborn child.
My prayers are with you all.