What to put for LMP when you haven't had one

[vegisaurus]

I have recently found out I am pregnant, and need to fill out a self referral for the midwives. I came off the pill in November and haven't actually had a period yet. What do I put in the LMP box in this scenario? (presumably not the actual date of my last period 4 years ago!).
I could take my estimated ovulation date and minus 2 weeks but I am not 100% sure of the exact date so don't want to just make it up!

I’d estimate that way, and then explain when you have your booking in appointment. Once you hopefully get to your 12 week scan they’ll do a more accurate dating

Just put it as it was a month ago and this would be your first missed period. It doesn't matter that much. A midwife will be in touch and you can tell her then. It just means if you're further along than that you'll have to wait a little while for your 12 week scan