Calculating how far along you are if you have irregular cycles?

Hey guys. I'm hoping some of you will be able to help me. I've always had really irregular periods, which has made it super difficult to pinpoint the length of my cycle. My husband and I are now pregnant and I'm trying to calculate how far along I am. All the sites and pregnancy calculators online have you start from the first day of your last period  as well as enter the length of your cycle to calculate this. But with irregular cycles, it ranges so I can't get an accurate answer. Any insight you ladies can offer??  :)