I wouldn't let them know too soon, considering, they may find somewhere else to go, leaving you high and dry with no income. If you do need the income through to the end of pregnancy, I think that one month notice would be more than enough to let them find another provider, not to mention by the end of your pregnancy you will be ready to be done, if that is what you want to do. I say...look out for yourself and keep quiet until the end. One month is plenty of notice! and good luck with everything!!!