Works both ways, too. 52 weeks - 40 weeks is 12 weeks. Which is about 3 months, plus 1 day since 52*7=364. So close, even without the rounding. I imagine that since the 9 months take up a greater proportion of the slight rounding, where 280 days is 9.333 30-day months, then the remaining 12 weeks would be slightly under, at 2.667 30-day months.

I think then we can also just reason that 9 months is a colloquialism that's close enough on average to have remained worth using. Especially when a date of conception often isn't really known. Babies don't have some turkey timer that pops up exactly at 40 weeks. Plenty of kids are born "early" or "late" based on a guess of a date most times.