Due Date?!

While I'm very excited that we are expecting, and that it didn't take to long after getting my IUD removed to do so, I'm not sure of my due date. Things have changed in the medical world since I had my child 5years ago. I will not be sent for any blood work to check the levels and due date is based solely on last day of menstral cycle. I looked at a couple websites to figure it out as well as this app but the dates differ. I have an apt in 5 weeks but I'd like to have a close answer to this question before then. Does anyone have advice/tips for narrowing it down??