You may have a shorter or longer cycle than 28 days and you may have ovulated any time in between! But for the sake of a date, everyone's pregnancy is calculated based on an average. Therefore when you miss your first period, you’re already 4 weeks pregnant. Okay, got it, but how many weeks am I, you ask…. To find out how many weeks you are, use our pregnancy calculator. It will give you an estimated due date and estimated date of conception. It will also tell you how many weeks pregnant you are, and how long you have to go until you reach the end of full term pregnancy at 42 weeks.