false positive for blood test that tests for downs/spina bifida etc

beck8008

I can't for the life of me remember what this dang blood test is called but it basically checks for 4 different things, downs syndrome, spina bifida and 2 others.  I just had the test done on Monday and won't receive results until next week. 

I'm a little nervous though- the doc said that about 1/3 of women get a false positive with this test (meaning you could get bad news even when there isn't anything wrong)  If that's the case, i'll have to go back in and they'll do a really in deapth ultrasound to make sure everything is ok. 

 has anyone had a false positive with this test??  seemed kinda funny that I had never heard it was such a high occurrance...