The due date calculators are generally accurate within 7 days. Typically, most carriers do not regulate the expiration dates in both directions, if the difference between the two possible dates of less than 7 days. It is also true of the early ultrasound.
There are generally two ways to measure pregnancy, LMP (last menstrual period) and gestational age. Most due date calculators go LMP, which calculates a pregnancy of 40 weeks or 10 months (4 weeks) per month.
LMP means that ovulation has taken place in its second week of gestation (day 14). Gestational starts from the date of ovulation resulting in pregnancy of 38 weeks. So if you know the date of ovulation, you can work out where either LMP or gestational.
Ultrasound technology can give you a strange time because they can not use any of them, they sometimes take the speed of development and change the date based on where their child is away. The doctor may even move the due date on this basis, even if you do not absolutely know when the baby was conceived!
Children are so many possible dimensions change during pregnancy, and so most of them are of a certain size at some point in pregnancy. Later during pregnancy is more individual variation among infants, and so is less accurate estimate of the size and date. However, for example, almost all children are 2 inches to 13 weeks (LMP), and each a little 'bigger than themselves, should be tacked on an extra day and an ultrasound evaluation.
The solution is to calculate the due date then, fewer complications, but be ready to go early evening set. Many people have put their heart into adulthood and are ready to date and nothing happens. To be quite annoying.