During pregnancy you hear terms like how "far along" you are or what your "gestational age" is. This simply means how long the baby has been growing. But, they can't make it quite that simple... Your gestational age (GA) is actually counted from the first day of your last period, which is roughly 2 weeks before conception. That means on the day your baby is conceived you are already "two weeks along."
That brings us to calculating due dates. The estimated date of delivery (EDD), or due date, is calculated to be at 40 weeks gestational age. It is interesting that this date is used, since the average pregnancy lasts about 40 weeks and 5 days when labor is not induced. This makes me think the original physician meant 40 weeks from the last day of the last period, rather than the first day of the last period. (1) Given that not everyone ovulates on day 14 an EDD is just an estimate. Ultrasounds can also be used to verify gestational age, though unless the ultrasound date is more than a week off from the original estimation the due date is rarely adjusted.
A term baby is said to be one that is born between 37 weeks and 42 weeks gestation. This is broken down into early term (37 to <38 weeks), full term (39 to 41 weeks) and late term (41 to <42 weeks). Babies born on or after 42 weeks are considered "post term". Premature babies are those born before 37 weeks gestation.
So many things in pregnancy are based around your due date. It's important to have an accurate due date, but it's also important to remember that it's simply an estimation and every pregnancy and baby is different.
For more information on due dates, check out the Evidence Based Birth article The Evidence on: Due Dates
Last updated 7/2023