How due dates work
A due date is estimated as 280 days (40 weeks) from the first day of your last menstrual period (LMP). Only about 1 in 20 babies arrive exactly on the due date — it's a guide, not a deadline.
Trimesters
- First trimester: weeks 1–12
- Second trimester: weeks 13–26
- Third trimester: weeks 27–40
Frequently asked questions
Is this a substitute for a doctor?
No — always confirm with your healthcare provider, who may adjust the date by ultrasound.
What if my cycle isn't 28 days?
The LMP method assumes a 28-day cycle; your provider can fine-tune it.