How accurate are early ultrasounds?

Question

Update:

I'm wondering about meassuring the size and estimated due date are they usually pretty spot on I know they can be off a few days or a week but are they usually when they say they are

Answer 1

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Accurate in what? Gender? Conception date? Actually detecting a live embryo?

ADD: They are usually very accurate at predicting a due date. But the baby always can be early, on time or overdue anyway.The size measurements are extremely accurate.