Heartbeat hard to see at 7 weeks?

Hi! I am 38 and have had 2 miscarriages in the past year and a half. This time around I have been on progesterone from the get go. My doctor told me my levels were looking good at my blood draws. Today we went for our first appointment and ultrasound. The doctor said we have a good gestational sac but she wasn't quite sure if she could see the heartbeat yet. So she sent us for a single draw to check my levels. Has anyone else had trouble seeing the heartbeat at 7 weeks and been ok?  I am just so nervous because of my past miscarriages.