First, let us agree that a woman is either pregnant or she is not I suppose a third state of uncertainty is possible, but that is more of a mental as opposed to a physical state.
Is the phrasing "20 weeks pregnant" legitimate English? It is used quite frequently, but a reading without context would seem to lead one to believe that it means there are degrees of being pregnant that can be measured in weeks. I googled it and came up with many examples of this phrasing being used in mainstream media and websites.
I understand the general intent of this phrasing, but is it correct English? Should it be promoted and used in publications that care about correct use of the English language? There are alternatives that are more clear, such as "she completed the 20th week of her pregnancy" or "You are in week 21 of your pregnancy."
If it is accepted language (regardless of being "correct"), what does it mean?
Does it mean a woman has completed her 20th week of pregnancy?
Or does it mean that she is currently in her 20th week, or in other words, she is currently on a gestational day somewhere from 141–147?