What she meant is that if your periods are irregular, you might not have ovulated at a predictable time, so the standard dating methods might not apply.
Pregnancy dating by LMP implies that you have regular, 28 day cycles.
Your doctor will date the pregnancy by LMP and the size of the uterus. Sometimes early ultrasounds are used to help if there is uncertainty about the dates. Most women also receive an ultrasound at around 16-18 weeks to furthur get the dates pinned down as accurately as possible.
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