It's perfectly normal for this stage of pregnancy. I have had 2 u/s so far, one in my 7th week, and one in my 9th week. At 7 weeks the heart rate was 136, and just 2 weeks later it was 173! If you are worried at all, call and speak with your doc. I'm sure he/she will tell you that your baby's heartbeat is fine too though!

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~Amanda~ Mommy to my girls: Madison "Kate" (3/21/01) Aubrey Claire (12/18/02) and Lucy Elise (8/17/04) and wife to Matt 9/21/01
Aubrey, Tasha (a cousin) Katey, and Lucy July 4th 2008