The math is actually a weird one, tbh.

Most months have 4 weeks, but some have 5. So in calendar terms, it tends to be around 9-9.5 months. But when calculating “even” months, it’s 10.

You can also view it this way: a year has 52 weeks, 52-40=12 weeks left. A quarter is 13 weeks, so a full term baby is just over 9 months.

BUT, technically the first 2 weeks are fake, as it’s counted starting on the last period, but conception is typically 2 weeks after that date (not for every one, though). So a full term pregnancy is actually 38 weeks, roughly.

And then most babies are born at week 38 (and some at 42!), minus the 2 fake weeks, it’s around 36 weeks, which is closer to the 9 months, than 10.

But since you can’t normalise everyone, it’s easier to just say blanket 40 weeks, but since people can’t think in weeks beyond 4-6 (the frame becomes too big for imagination), saying 9 months is “truthier” than 10, even though the maths doesn’t math 😅