Hey mama! At 21 weeks, Braxton Hicks contractions are pretty common, yun yung practice contractions ng katawan mo na parang nag-harden ang tiyan for a few minutes, kaya normal lang yan usually, pero if it's staying hard for 3 hours straight, I'd really suggest checking in with your OB agad lang para sure everything's okay, kasi it's better to be safe especially pag first time mom ka pa. One of my mum friends had the same concern at that stage and doc assured her it was nothing, pero peace of mind talaga worth it sa pregnancy na!