If your dating from your last missed period then the growth would be approx 2 weeks behind.
This is because more women know when their last missed period was rather than the window of time in their cycle that they ovulated.
So, the first two weeks in the LMP dating is just a regular follicular phase (the cycle beginning with your period/flow then working toward developing and eventually releasing an egg).
Then conception occurs approximately 2 weeks from the start of the cycle provided that you ovulate approx cycle day 14. This is why (if they are dating LMP instead of ovulation date) you say you are 22 wks and the baby measures 20- which is fine. They will probably adjust your due date accordingly.
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