How this calculation works
This calculator uses: Due Date = LMP + (280 − 28 + Cycle Length) days — Naegèle's Rule. Enter your values above to get an immediate result. Results are rounded for readability.
Using the pregnancy due date calculator
Start by entering values in the units shown. The result updates immediately, and the shareable link preserves your inputs without creating a separate indexable page. Use this as a clear estimate and check important assumptions before relying on it.
Common questions
How is the due date calculated?
The standard method is Naegèle's Rule: add 280 days (40 weeks) to the first day of your Last Menstrual Period (LMP). For cycles that aren't 28 days, the calculator adjusts the due date accordingly. An early ultrasound gives the most accurate date.
What week does pregnancy start?
Medically, pregnancy is counted from the first day of your LMP, not from conception. So you are 'officially' 2 weeks pregnant at the time of conception. A full-term pregnancy is 40 weeks from LMP.
What are the three trimesters?
First Trimester: Week 1-12 (organ development, morning sickness common). Second Trimester: Week 13-26 (most comfortable, baby movements felt). Third Trimester: Week 27-40 (rapid growth, preparation for birth). Full-term is 39-40 weeks.
Can the due date change during pregnancy?
Yes. An early ultrasound (before 14 weeks) gives the most accurate due date based on fetal size. If the ultrasound date differs significantly (±10 days) from the LMP-based date, doctors typically use the ultrasound date.
What is considered a normal delivery window?
Full-term delivery is between Week 37-42. Babies born before 37 weeks are premature. After 42 weeks is post-term. Most doctors induce labor at 41-42 weeks to avoid complications.