INSIGHT LADIES😳😕

ok so my last period was December 24-28 2017. I have a 31 day cycle. I received my BFP on January 26th . But One app tells me I'm due on September 29th and another tell me October 3rd. I don't have a dr. appointment for awhile . But if any of you have any Insight. on a due date please help a sista out!!