I think something you just said there is important too. We all know that term is classed up to 42 weeks, and I see many examples all the time of women who consistenly birth at 42 weeks or around the same time. Obviously this isnt always the case and other factors can come in, but I think those babies stay in there for a reason, usually the reason of not being ready - one mother told me she did EVERYTHING you can think of for natural induction, but nothing worked and all three/four babies were born in the 42nd-43rd week, of course both mum and baby were healthy. We all have different length cycles etc and I think thats why its so unfair to use a generic calculating system or mathematics to give a 'due date' because they can be so innaccurate, and mums just get so anxious when the date comes and no baby, when only 3% of babies happen to magically appear on their due date. There's a greater chance - 20% - that your baby will come in the two weeks after yor due date.