Personally I wouldnt worry too much about measuring 4 days behind. Plenty of women do and they catch up. Also remember their 'dates' are just based on averages, just as every child is different surely every fetus in the womb will develop at slightly different rates. I hate it when doctors immediately jump to the worst possible outcome! WHen I was TTC and I'd come off the pill my periods were all over the place. My doctor took bloods on the wrong day of my cycle and then told me I was in premature menopause! When I argued and said my blood test had been done incorrectly and they did it again - normal. But, I still had weeks of worrying I was infertile for no good reason. Honestly I think sometimes they cant see the wood for the trees!