I know mine's usually around 32 days, so with my first pregnancy
, it was off by
nearly a week
Once you have an ultrasound tho, the doctor will base your due date
off of how big the baby is measuring. However, this can still be 'off' if your baby hasn't grown properly or is growing a lot!
Lastly, since most babies don't bother to come on their expected due dates, I wouldn't worry too much about pinning down a certain day.