I guess they figure that's the most accurate starting point to work out the EDD. And I think alot of people wouldn't have a clue when they conceived LOL!

My OB asked me the date of my last period, I told him, but then also told him I was 100%sure of when we conceived! I O late in my cycle, so he didn't really believe me, but I assured him I was right! (I have known him for seven years, and feel quite comfy joking with him.) He said that we'd see at the u/s...........and we did! All three scans have proved I was right. And don't I like to remind him of that LOL!