Here they calculate your due date based on 40 weeks after the first day of your last period assuming you ovulated on the 15th day of your cycle. They don't adjust it unless you are measuring more or less than two weeks on scans. I ovulate late in my cycle, 20th day or more. Therefore I know when I get my DD my actual DD is more like 5 - 7 days later. I still go by the date they give me, but if they wanted to induce me at a week overdue or earlier I would put up a fight because I know my actual due date is more like a week over their due date and I want to give my body a chance to go into labor on it's own. I had my first at 40+4 and my 2nd at 40+2, so it hasn't been an issue yet. Hopefully it isn't an issue for baby #3!