Today is D-Day. I hate due dates! They're full of lies!
The idea of a due date comes first, from the Bible's account of human gestation from conception to birth lasting about 10 lunar months and second, our miscalculation of a lunar month being 28 days, and third, the unscientific thought that all women have a 28 days menstrual cycle and ovulate on day 14 of that cycle. A lunar month is actually 29.53 days, meaning about 296 days or 42 weeks and a couple days. And remember this is 42+ weeks AFTER conception. Doctors begin counting your 40 weeks at the first day of your last menstrual cycle, about 2 weeks before you're even pregnant. They are giving you a expiration date about 4 weeks too early! All that said, the average first time mom gives birth after about 288 days. The average mom having her second or subsequent child gives birth after 283 days, both longer than 266-280 days a doctor assigns you. But when it all comes down to it, babies come when they're ready and that shouldn't be unnecessarily interfered with. And slapping a specific date on it completely unhelpful and asinine. :P