Here's what me and husband decided. For us at least, it would be a bit expensive. We're not very high risk I'm 23, my husband is 26. We have no factors especially increasing our risk. I've been taking folic acid since August so the risk of NT defects is less than an unplanned pregnancy. And the thing that really decided me against screening was thinking about what I would do with the info, if I would terminate. We seriously considered it, my husband was for that option because we feel overwhelmed at the idea of a Downs child. It's a scary thought. But, the idea of getting an abortion... I looked up the kinds of abortion and what would happen on Planned Parenthood. It would by then be too late for a medication abortion, and the process of a vacuum abortion just seemed too overwhelming. I decided I didn't want to terminate, so there was no point in screening, for us. For your situation, it sounds like you've decided you definitely don't want to terminate. So, reasons you might want to test: If you're in a high risk or very high risk group, or you're older or whatever other risk factors there are. If something is found, that may help your OB provide extra special treatment you might need. If its free or very inexpensive, also a good reason. The info may help your dr help you, and if it doesn't cost much you might as well IMO.
All of these applied to me - I was 23 when I got pregnant with DS2 and 24 when I had him. DH was 25. No prior family history. We were in a low risk category from the start. -- I had the screening done anyway and was given odds of 1:13 for DS2 to have Down's Syndrome. This is high risk! (anything 1:150 or less is high) Never would I have dreamed of this, and luckily DS2 was one of the 12:13 not to be affected, but what if?...
I couldn't even imagine not knowing at all and not being prepared if he did turn out to have Down's Syndrome. Now, odds are better I would have at least seen something at my A/S, but these things can be missed so easily (chances aren't high but that it happens at all is enough) and I needed to know 100% so we could prepare. Down's Syndrome brings a higher risk of heart defects (among other health problems), and there IS a history of heart problems from both my mom and my dad's side, so I couldn't risk not knowing.
It IS a very personal decision, and you do whatever works for your own family and your own situation, but this is why I chose to do what I did.