Calculating how far along with an irregular period

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Hi Everyone!

I started my period January 1st and it ended January 12th. This app had my most fertile days the few days following the end of my period.

My question is, if I am pregnant, do I calculate how far along I am the same way? I know normal calculations are based on you LMP and I was just wondering if that's still the case if your LMP was unusually long.

I haven't gotten a positive yet, I'm just very curious! Thank you!!