Here the docs say 12 weeks. One of the reasons is because of the statisticts Tkeys gave. The other reason is because at 12 weeks you can determine if you have a healthy fetus or not. Many tests are doen just before the 12th week and the results are within that week too.
That's why it's a gide line here, we only tell the workd that we're pg after the 12 weeks.

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