I had 4 healthy, term babies then miscarried 2, and one more healthy term baby. With each of my six pregnancies I made different decisions. With my first they only had quad screen and I did it, I didn't think much about it and my doc recommended it. With my second they routinely offered quad to all mothers, first trimester screening was reserved for already high risk (which I wasn't), I declined the quad, I just didn't feel it was necessary at that time. With my third, first trimester was now routinely offered, I declined. With my 4th pregnancy I just felt something was "off" from the moment I found out. I opted for the 1st tri screening, all looked fine at 12 weeks, but had to go back at 14 for a follow up and my baby died that day during the US visit. I doubt the screening would have found anything, the autopsy and genetics after delivery were both normal. But something in me told me to test that time. I also lost my 5th, but at 11 1/2 weeks, just before deciding about screening, I don't know what my decision would have been. My 6th, I really felt at peace during the pregnancy and felt in my heart that what will be will be, I declined the 1st tri screening, he is a healthy 11 mo old.
My midwife felt that you should only do the screening if you would go "all the way" with any recommended follow up testing like an amnio. Otherwise a positive screening result will just lead to high stress the remainder of the pregnancy. And you seem to understand this, but just wanted to clarify that the screening doesn't diagnose any problems, it simply means your baby is more likely than the average pregnancy to have a problem based on the presence of some suspicious markers. And the screening is admittedly "generous" with putting people in that higher risk category, it makes it less likely that it would miss someone who's baby really does have a problem but had a negative screening.
Just give yourself some time to really think it through, there are pros and cons to all prenatal testing, including ultrasound. There are reasons to know other than termination (which was never an option for me either), like being at the best hospital to address possible post delivery complications, thorough testing and monitoring to lead to the best possible outcome. Heart defects are mcuh more common in babies with Downs Syndrome, Omphalocele (where abdominal organs develop outside the abdomen) is best delivered by c-section, there are some experimental , but promising, in utero surgeries for spina bifida. Then again, as you know, there is a high positive rate on the screen that leads to stress, and invasive testing only to have a healthy baby. The decision is a personal one, take time to reflect on it and follow your heart.