How is a due date calculated?
The standard method, Naegele's rule, adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). Pregnancy is counted from the LMP, not conception, which is why you're already considered "2 weeks pregnant" at the moment of conception.
The three trimesters
- First trimester: weeks 1–12
- Second trimester: weeks 13–26
- Third trimester: weeks 27–40
What if my cycle isn't 28 days?
A longer or shorter cycle shifts ovulation, so we adjust the due date by the difference from a 28-day cycle.
Is the due date exact?
No. It's a best estimate — most babies are born within two weeks either side of it. Always follow your healthcare provider's dating.