Q&A: I have irregular periods, but I want to know how far along I am in my pregnancy.

by Dr. Gerard M. DiLeo

How do you tell how far along pregnant you are when your periods are irregular?

ou can't. This requires ultrasound, probably two of them. The last menstrual period (LMP) is the famous landmark upon which to date a pregnancy. But it is only meaningful if you're "regular," because it assumes that you're also regularly ovulating. Irregular periods, on the other hand, mean irregular ovulations. You conceive with ovulation, not the last period, but we use it because it's an outward sign (most women cannot tell when they're ovulating). This outward sign is totally unreliable when it happens irregularly. The most accurate of ultrasounds for dating the gestational age are ones done before 20 weeks.

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