hi amna
Sorry to hear you are stressing, this is such a difficult thing to have to deal with when you just want to get on and enjoy being pregnant! I had a nuchal test at 12 weeks (scan and blood tests), and with those, they give you a number like 1 in 2500, 1 in 300, which is the risk of there being something wrong with the baby - it is a screening test, so cannot say definitely one way or the other, only give you the odds. For us, we would only have had a CVS or amnio (the diagnostic tests that give a definite answer), if the risk was higher than about 1 in 100 - otherwise the risk of miscarriage would have been higher than the risk of abnormality.
High risk is classed as anything higher than about 1 in 200 I think (varies from area to area, but is the level at which they offer a diagnostic test). But it is very, very important to remember, even if they come back and say you have a 1 in 30 risk (for example), that means the odds are the baby is healthy - 29 out of 30 babies with that result will be absolutely fine. Not helpful if you sadly have the one that isn't, but helps to keep it in proportion. The decision whether to have the diagnostic test is a very personal one - many people won't because of the risk of miscarriage, or because they would never consider ending the pregnancy whatever the result. Others do because they would rather know all the facts. It is a very personal decision, one only you and your partner can make.
Did the midwife give you the booklet on screening tests - it should have been in the pack you got when you booked in, that should explain everything in more detail for you. Hope that helps, and try not to worry