My daughter had a spot on her heart and I had placenta previa with her. They needed one to check her heart and one to check the placenta to make sure it had moved (thankfully it did) and her heart was ok! But it was a scary time. Don't let the need for the test stress you. There are too many possibilities and you will warp your mind trying to figure out "what-ifs".

How far are you?

Also low fluids or measuring either behind or ahead. They gave me an extra one with my son because he only had a two vessel umbilical cord instead of three. There's lots of things that could make them want to check up on baby.

I'm sure that there's a lot of different reasons but the first three that come to my head are pain, bleeding or lack of movement