October and November due date confusion

Hello beautiful mommies! I follow both the October 2018 and November 2018 groups because I have such an early November due date (November 7th). I am currently 19 weeks and 2 days along and noticed some mommies in the October page posting that they’re over +20 weeks along? So now I feel a bit confused on that part. If it’s medically necessary that you deliver early and want to keep that private then you don’t have to disclose that! I respect all the mommies out here 😊 I’m just genuinely curious 🤷🏼‍♀️