Most women and many pregnancy books will refer to pregnancy by months. Seems to make sense—3 trimesters, 3 months per trimester. Then why do doctors measure pregnancy in weeks? The answer lies in how a pregnancy is dated and how it develops.
So how do doctors determine your due date? A due date is based on the first day of a woman's last menstrual period. This is a relatively reliable and obvious date that most women remember with accuracy. In many instances, an early first-trimester ultrasound can be ordered to confirm dating or to obtain a due date if the last menstrual period is uncertain or unknown.
Obstetricians use weeks instead of months because not all months are created equal. Simply put, most months contain four and a half weeks. If a woman says she is 6 months pregnant, she could be anywhere between 24 and 27 weeks along. This matters if there are pregnancy complications that may require intervention.
It's important to know exactly how far along a pregnancy is because if medical issues develop and earlier delivery needs to be considered, interventions for the mother and the baby's well-being will differ based on how many weeks of development have already occurred.