Is this accurate?

[OrangePeppaPig]

I always just assumed you were just nine months pregnant when the baby was due.

40 weeks is what's considered the length of pregnancy so its just over 9 months

Although I always thought 2nd trimester started from week 13

Yes - weeks 36 - 40 is your 9th month of pregnancy. You complete 9 full months of pregnancy at the end of week 40.

It's the same as how 0 - 12 months is a baby's first year but they have their first birthday at the end of their first 12 months