Going back to maths ...

13 weeks = 3 calendar months
39 weeks = 9 calendar months

40 weeks pregnant is 9 calendar months plus 1 week

Pregnancy is said to be 280 days (though actually 266 days from conception approx.)Most of the tickers /counter boxes go by 280 days and count in weeks/days to go. Months is too confusing.

OR 280 divided by 30 days is 9 months (give or take a few days for months that have 31 days - which is 7 months of year)

EG. Last Saturday 25th Feb - I was 17wks and 4 days = exactly 4 calendar months of pregnancy (from 25th October.

At 16 weeks (some say this is 4 months) - I was not yet 4 months (as there are more than 28 days or 4 wks in a month)

Dr's calculate due date by adding 9 calendar months plus 1 week to your LMP.

So you are actually 1 month pregnant after 30-31 days - 4 weeks and 2/3 days from last period. { unless February conception and not a leap year in which case you would be 1 month - 4 weeks later}

2 months at 8 weeks 5 days etc

3 months at 13 weeks or so and so on.