I am sorry to hear you are worrying.
I wanted to tell you that your test couldn't have come back with a figure of a 1/20 chance your baby will be 'normal', as the test gives you a risk of having a baby with Down Syndrome, not the other way around. So, your test has come back with a 1/20 chance your baby has Down Syndrom, and a 19/20 chance your baby doesn't, or a 95% chance that your baby doesn't. The triple screen (also called MSS) has a high false positive rate, and has a sensitivity of abour 70-80% (meaning it only picks up that percentage of babies with DS). It is a lousy test.
Only an amnio can tell you if your baby has a genetic disability, and even then is only 99.9% accurate (cells sampled can be maternal, or the the baby could have a mosaic form of a genetic disability and the cells sampled may not have the genetic disability in them). The current published rates of miscarriage associated with amnios is 1/100 or 1/200.
With these tests you have to decide if you would terminate the pregnancy because of a genetic disability. Otherwise, statistically, there is no benefit to knowing ahead of time. The kind of surgeries that infants with DS may need are not emergent, and many women report being depressed and anxious the whole pregnancy. There is a lot of research out there that supports this. Of course, there will always be someone for whom the advance warning of a problem is helpful to the baby, but statistically these are infrequent.And finding out for almost certain, by amnio, has such a huge risk.
However, if you would terminate, you'd need to know fast ... you are approaching the limit of termination in most places by 20 weeks.
You mentioned that there may be a 'need' for an amnio. It's not a need -- it's totally up to you to decide if you want it.
I hope you can come to some peace about what you want to do in going forward, Mama.
(Just curious, did you ask for the test, was it offered or recommended? I see so many women coming from physicians and other midwives that have done the test without having anyone walk them through their options in the decision, to test or not test, and what each decision may mean. They may not have even really consented to what the test actually was and could mean, it was just 'routine'. I mean, if women want it, they are responsible for the information that can come out of it, but if they didn't really know what it was, what a burden that information can be. I see you struggling with the outcome of this test, and I am just hoping you had *wanted* the test done and understood the whole thing well. Still a difficult thing in either case, and I am sorry for the worry it is causing you, Mama.)
"My midwife called me this week to tell me the triple screen blood test came back positive for at risk for Downs Syndrome. I have a one in twenty chance of the baby being normal"