What's with the 3+2 or 5+3 thing?

[Stinkyminkymoo]

dumb Q no doubt (spot the newbie!), but what does 3+2 or 5+3 mean? Have seen it on a couple of posts but can't be bothered to work it out.

:) cheers

3 weeks plus 2 days or 5 weeks plus 3 days. Used in the early days of pregnancy when every day makes a difference.