I'm definitely proof that dating on LMP alone is just stupid. The month before conceiving Finlay I didn't ovulate. I had been doing opks and we'd decided to stop as it kept giving the same CD date for ovulation, so that cycle we didnt do them. I eventually got my BFP on CD56, having had blood tests etc. to track where I was in the cycle. My LMP put me ahead by a good few weeks, but in reality my body skipped a month. We would have been way out using my LMP!