I don't know what Trisomy Is but my prayers are with you and your family!
I am so sorry to hear that. No one can tell you what to do but please know that you are definitely in my prayers! :)
There is a support community out there for Trisomy 18 & 13 parents called SOFT.
http://trisomy.org/
We also found it very helpful on informing us in more detail about Trisomy 13. They have literature to read and when you join wonderful members from around the country will reach out to you to listen to your story and offer some wisdom from their own experiences.
My wife and I are carrying our son to term and letting God's will be done for our son Gabriel. He too has trisomy 13 and is due to arrive in December. We have two other daughters and since experiencing the feeling of holding that little life in your arms after birth we could never do anything to cause intentional harm to our child to selfishly expedite our own grieving process. It is a sad, painful journey - but we have chosen to enjoy every moment we are given together before our son goes to heaven. Our faith teaches us that our lives on earth are short, but our time together in heaven will last for eternity.
Nobody can tell you what is the right thing for you to do because we have not lived your life or know your beliefs. Let me recommend a book that was recommended to us and extremely helpful, A Gift of Time: Continuing Your Pregnancy When Your Baby's Life Is Expected to Be Brief.
http://www.amazon.com/Gift-Time-Continuing-Pregnancy-Expected/dp/0801897629
I wish you the best and pray that you have the love and support to help you through the difficult days ahead.