When does 2nd trimester start?

[Sulkypatsy]

Looking this up there's different answers everywhere. The guide book from my maternity hospital says the start of week thirteen, some internet sources say start of week fourteen.

trimesters are one third of a pregnancy but pregnancies are of slightly variable length and no one can predict exactly how long each will be. so for most cases it isn't exactly known and doesn't usually matter. if you have been advised to not do something until you are in the 2nd trimester then err on the safe side.

Thanks, for me it's more about the feeling of having reached a milestone, it's not a big deal really I'm just curious