How Pregnancy Weeks and Due Dates Are Calculated
Pregnancy weeks are calculated from the first day of your last menstrual period (LMP) using Naegele's Rule, the standard method used by obstetricians worldwide. The estimated due date (EDD) is LMP + 280 days (40 weeks).
Naegele's Rule Formula
Due date = LMP date − 3 months + 7 days + 1 year. For example, if your LMP was January 1, your due date is October 8. Equivalently, simply add 280 days to your LMP date.
Prenatal Milestone Schedule
1st Trimester (weeks 1–13): First ultrasound, NT scan, blood work. 2nd Trimester (weeks 14–27): Anatomy scan, gestational diabetes test. 3rd Trimester (weeks 28–40): More frequent check-ups, GBS test, birth preparation.
Important Disclaimer
This calculator assumes a standard 28-day cycle. If your cycle is irregular, an ultrasound scan provides a more accurate gestational age estimate. Always consult your obstetrician for medical guidance and to confirm your due date.
Frequently Asked Questions
What if my cycle isn't 28 days?
Naegele's Rule is based on a 28-day cycle. If your cycle is longer or shorter, ovulation timing differs, which can shift the actual due date. Your OB will use an early ultrasound to confirm the gestational age more precisely.
Can I use this calculator before a positive pregnancy test?
You can use it to estimate, but always confirm pregnancy with a home test or healthcare provider first. The calculator uses your LMP date and today's date — no test result is needed as input.