Pregnancy Due Date Calculator
Calculate your estimated due date (EDD) by entering your last menstrual period or conception date. See how many weeks pregnant you are and which trimester you are in.
This calculator provides general estimates for informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider.
How It Works
Naegele's Rule is the standard method for calculating an estimated due date (EDD). It was described by Franz Karl Naegele in 1812 and remains the basis for obstetric dating worldwide.
Standard calculation (28-day cycle):
EDD = LMP (last menstrual period) + 280 days (40 weeks)
Cycle length adjustment:
If your average cycle length differs from 28 days, the ovulation date — and therefore conception — shifts accordingly:
EDD = LMP + 280 days + (cycle length − 28) days
This adjustment works because ovulation occurs approximately 14 days before the next period. A longer cycle means later ovulation; a shorter cycle means earlier ovulation.
Conception date method:
If you know the conception date directly (e.g., from IVF), the EDD is simpler:
EDD = Conception date + 266 days (38 weeks)
Trimester boundaries:
- First trimester: Weeks 1–13 (conception through ~week 11)
- Second trimester: Weeks 14–26
- Third trimester: Week 27 to birth
Note: EDD is an estimate. Only about 5% of babies are born exactly on their due date. Most births occur within two weeks either side of the EDD. Gestational age is most accurately confirmed by ultrasound dating in the first trimester.
Frequently Asked Questions
How accurate is the due date calculator?
The calculated EDD is an estimate based on average cycle length and average pregnancy duration. Only 5% of babies are born on their exact due date. First-trimester ultrasound (crown-rump length measurement) is the gold standard for confirming gestational age and is more accurate than LMP dating alone.
What if my cycle length varies?
The cycle length adjustment in this calculator uses your average cycle length. If your cycles are highly irregular, LMP-based calculation is less reliable. Tracking ovulation (basal body temperature, ovulation predictor kits, or ultrasound follicle monitoring) provides a more accurate conception date.
When does each trimester start?
The first trimester runs from week 1 (LMP date) through week 13. The second trimester is weeks 14 through 26. The third trimester runs from week 27 until birth. These boundaries are conventional — different sources may use slightly different cutoffs (e.g., some define T2 as starting at week 14, others at week 12+1).
Is this calculator storing my data?
No. All calculations are performed in your browser using JavaScript. No personal health data is transmitted to or stored on any server.