5weeks 5days fetal heartbeat detected. brown constant spotting since period was due. chance of miscarriage?

Possible: It's possible, but the presence of a fetal heartbeat lessen those chances.

Sounds good: Congratulations. Spotting is usually irrelevant, but worth watching, heavier bleeding is more worrisome. Heartbeat at less than 6 weeks is also very reassuring. No one can tell you your chances, but statistically at age 29 - if 29 is your correct age - you have over 90% chance of continuing into the second trimester.