If you tell them now be prepared for them to leave for September with the idea they want to be established and no sense in staying longer with you if it isn't going to be long term. Could you actually return to work for January if this happened?

If you wait to tell them in January you will have a better idea of where they are at and your exact dates. Things happen and they know they are just as likely to get pregnant and mess with the schedule too. I wouldn't lie to them but at the same time I probably would keep it to myself till Jan.