Completely agree with VSS, ask the scanner, they all like different times and some are flexible and others not. The one I used to use on my previous farm liked to scan 2 months before due date - that would be about 87 days after tup went in, wouldn't it. She was completely inflexible in every respect but incredibly accurate, so we kept having her every year and putting up with the brusqueness. In the end we got used to her (and she us) - but still woe betide you if you didn't have enough people and dog power to keep the sheep flowing! (You were allowed one breakdown an hour, she'd have a cigarette and her assistant would get some more hot water if needed.)