heart rate while exercising

My doctor told me to keep my heart rate under 140 bmp while exercising.  I'll check my Fitbit during exercise and sometimes it will climb to 142 and once I notice that I slow down so it doesn't get any higher. Problem is that when I go to log my exercise in the app my heart rate will sometimes show in the 150 range right when I start my workout. I know that Fitbit is not 100% accurate but I'm a bit worried. This is baby #4 after three losses in a row. Any input is appreciated