Opinions on quad testing?

Caroline
I am almost 10 weeks. I need to decide by next week according to my midwife whether or not to have quad testing done. I've read that it is not very accurate and only calculates risk, nothing definitive. 
I am 99% sure I would not follow with amniocentesis as I feel it is quite invasive. And I worry that an indication of a higher risk may only serve to increasing my anxiety throughout the rest of this pregnancy, which is my first. 
I am 30 years old so not high risk according to age. But I did take a medication that could cause birth defects for many years, however I was off for 7+ months before conceiving. So I feel confident after speaking with my midwife and pharmacist that it was completely out of my system. And I am not even sure this would be indicated on such a test. 
Can the ultrasound at 18 weeks for sex determine abnormalities? Has anyone regretted having the quad testing done or vice versa? Thanks ahead of time! This is such a hard decision.