Right- I know the normal way to work it out is 40 weeks from first day of last period but I had very long cycles (35days ish) so I think I probably would need to add a week onto that date... The thing is, while we were trying I was taking OPK tests every day according to the tests I didn't actually ovulate until day 36 (!!) of my cycle then I got an absolutely clear positive pregnancy test just 10 days after that.