if you want to get technical, full term is after the baby has grown for 9.5 months. Full term is 40 weeks (10 months) but for the first 2 weeks of that, you have your period the first week, then another week before the sperm and egg join. That's why you are not considered 'overdue' until 42 weeks.
I've got my sources from a couple places, a good one is DiscoveryHealth.com.