Like we mentioned earlier, due dates aren't set in stone.
Your OB/GYN may change your due date based on what they see. Reasons why your due date may shift:
If your baby is noticeably smaller or larger than "the average," then the doctor will be inclined to revise their estimations accordingly
Irregular periods can throw off a pregnancy's estimated length.
Abnormal fundal height (size of uterus).
Abnormal levels of proteins produced by your growing baby (Alpha-fetoprotein).