Hi i am new here and i was just wondering if I can have some advice please.

I have a healthy beautiful 4 year old daughter. I fell pregnant really easily with her and had a great trouble free pregnancy- but we are now trying for another baby and i have had 3 miscarriages all of which have been lost somewhere between 8 and 9 weeks. We had testing done on my last miscarriage and it was found that that baby had trisomy 13 so they had my husband and I tested to see if we had any genetic problems which would cause this. The tests showed that I have a balanced chromosomal abnormality and this is what was causing the miscarriages. We were told that any future pregnancies may
or may not have the same problem.
I am now pregnant again and so far have gotten to 11 weeks. I had a check up yesterday and the baby was measuring correctly for dates, had a heart rate of 158 and was showing lots of movements. All good news, but i am still worried.
My obstetrician has recommended that I have either a CVS or amnio to make sure the baby doesnt have trisomy 13 but I am way too scared to have either of these done due to the risks of miscarriage. It would just suck to have the test and miscarry because of it- and then find out that the baby was ok. He has said he understands my concerns and said that we will do the NT scan and if we get a low risk assessment with the combined screening it would be reasonable to wait until our 20 week scan to make sure there are no abnormalities but then said that if we get a high risk assessment it is important to do further testing.
If the baby was shown 100% to have trisomy 13 or 18 we would terminate due to the fact these conditions are almost guaranteed to result in death either in utero or soon after birth. My fear however, is miscarrying a perfectly healthy baby because of a test I chose to have done and i am just wondering how accurate a risk assessment is from the 1st trimester combined screening.

I do realise there are many instances where a high risk assessment has been given and the baby has been proven to be healthy, however I wonder if anyone knows how likely it is to have a low risk assessment and then find the baby has chromosomal abnormalities? Apparently trisomy 13 has quite obvious markers anatomically although I am not sure how early these can be detected. If anyone knows anything about these conditions and the 1st trimester screening- any information would be greatly appreciated as I cannot seem to find much information on the net.
Thanks for your help,
Shan