Baby's heart rate ranges

trombonecheesecake2008

Hi everybody!  I just have a quick question.  This morning I went in for my 20 week anatomy ultrasound.  Everything looked great and I got to find out that I'm having a little baby GIRL!! I'm so excited but now I'm a little worried.  Her heart rate was 167 during the ultrasound (she was moving around some) and then this evening (about five hours after the ultrasound) I had a prenatal visit and her heart rate was 145.  My midwife didn't seem too concerned and said that she might just be resting.  Does anybody know if I should worry about this?  I have no idea what the normal day to day range is for babies during pregnancy.