Due Date Calculator
Estimate your baby's due date from your last period or conception date, adjusted for your typical cycle length.
Due Date Calculator — input your values
What does this health calculator do?
This due date calculator estimates your baby's due date from either the first day of your last menstrual period or your conception date. It starts with the standard pregnancy length — 280 days from the LMP or 266 days from conception — then adjusts for cycles that are shorter or longer than 28 days.
Beyond the due date, it produces a timeline of key pregnancy milestones, including the 12-week end of the first trimester, the 20-week anatomy scan and the start of the third trimester, each mapped to a calendar date.
Due dates are estimates used for planning and screening timing, not a promise of delivery day. Always confirm your dates with a healthcare provider.
Formula and variables
From the conception date, use 266 days instead of 280, since conception typically occurs around day 14 of a 28-day cycle.
| Symbol | Meaning |
|---|---|
| LMP | First day of the last menstrual period |
| cycle | Typical cycle length in days (21–35) |
Worked example
LMP of April 15, 2026, with a typical 26-day cycle.
- Cycle adjustment: 26 − 28 = −2 days.
- Pregnancy length: 280 + (−2) = 278 days.
- Due date: April 15, 2026 + 278 days = January 18, 2027.
Answer: Estimated due date: January 18, 2027.
Tips and common mistakes
Tips
- Know your typical cycle length — a 26-day cycle moves the due date two days earlier than the standard 28-day assumption.
- An early ultrasound between 8 and 12 weeks usually provides the most accurate dating.
- Full term is weeks 37 to 42, so plan flexible leave and travel windows around the estimate.
- If you tracked ovulation, the conception-based calculation can be more reliable than the LMP-based one.
Common mistakes to avoid
- Using the date your period ended rather than the first day it started.
- Forgetting that shorter cycles pull the due date earlier and longer cycles push it later.
- Treating the due date as an exact delivery date — most births happen within two weeks of the estimate.
Due Date Calculator — frequently asked questions
How accurate is the due date calculator?
LMP-based dating is most accurate with regular 28-day cycles. About 4% of babies arrive on their exact due date; a normal range is weeks 37 to 42.
Should I calculate from my LMP or conception date?
LMP is the clinical standard because conception dates are often unknown. If you tracked ovulation precisely, the conception-based result may be more accurate for you.
Why does my cycle length change the due date?
Ovulation typically happens 14 days before the next period, so a 26-day cycle shifts ovulation — and conception — earlier, moving the due date up by two days.
When is the anatomy scan?
The anatomy scan is usually scheduled between 18 and 22 weeks, with 20 weeks as the typical midpoint. The calculator lists this window's center date.
Can my due date change during pregnancy?
Yes — providers often revise the due date after an early ultrasound if the measurements suggest the LMP estimate is off.
What if my periods are irregular?
The cycle adjustment assumes consistency. With irregular cycles, ultrasound dating is the more reliable method, and this estimate should be treated as rough.