I had this when pregnant with the twins. I was trying to emigrate at the time and my GP banned me from thinking about the move for more than an hour a day. He said if I had a thought about something I had to action to write it down and deal with it at my allotted time. It really did make a difference. Obviously this was after checking that I was no anaemic, low blood pressure, etc.

It's definitely worth mentioning to your midwife, but in the absence of a physical ailment is there anything worrying you at the moment that might be stressing you out a little?