Due Date Calculator
Calculate your estimated due date from your last menstrual period or conception date. Get your complete pregnancy timeline.
What Is a Due Date Calculator?
A due date calculator is a pregnancy planning tool that estimates your Estimated Due Date (EDD) — the day your baby is expected to be born. It uses scientifically proven medical formulas, primarily Naegele's Rule, which predicts birth occurs approximately 280 days (40 weeks) after the first day of your last menstrual period (LMP).
This free due date calculator provides instant estimates without requiring signup, helping expectant parents plan prenatal care, schedule doctor visits, prepare for milestones, and arrange maternity leave. It works for any country — the underlying biology of pregnancy is universal.
For week-by-week tracking, see our Pregnancy Calculator. To work backward from your due date to find conception, use our Conception Calculator.
How Due Date Is Calculated: Naegele's Rule
The standard medical formula used worldwide is Naegele's Rule, developed by German obstetrician Franz Naegele in 1830:
Due Date = LMP + 280 days
Or expressed differently:
Due Date = LMP − 3 months + 7 days + 1 year
This calculation assumes a standard 28-day menstrual cycle with ovulation occurring on day 14. The formula adds 14 days for ovulation plus 266 days of actual gestation (from conception to birth).
Calculation Example
If your last menstrual period started on February 15, 2026:
- February 15 − 3 months = November 15, 2025
- November 15 + 7 days = November 22, 2025
- November 22 + 1 year = November 22, 2026
Your estimated due date would be November 22, 2026.
Methods of Due Date Calculation
Several methods exist beyond Naegele's Rule, used depending on available information:
| Method | Input Required | Accuracy | When to Use |
|---|---|---|---|
| LMP (Naegele's Rule) | First day of last period | ±1-2 weeks | Regular 28-day cycles |
| Conception Date | Date of conception | ±1 week | If you know exact date |
| Ultrasound Measurement | Fetal measurements | ±3-5 days (first trimester) | Most accurate method |
| IVF Transfer Date | Embryo transfer date | ±2-3 days | IVF pregnancies |
Adjusting for Irregular Cycles
If your menstrual cycle is shorter or longer than 28 days, adjust accordingly:
- Shorter cycle (e.g., 24 days): Add 280 + (24-28) = subtract 4 days
- Longer cycle (e.g., 32 days): Add 280 + (32-28) = add 4 days
- Very irregular cycles: Rely on early ultrasound for accurate dating
Pregnancy Timeline from Due Date
Once you know your due date, here are key milestones to track backwards and forwards:
| Time Before Due Date | Gestational Week | Important Event |
|---|---|---|
| 280 days | Week 0 | LMP (start of pregnancy) |
| 266 days | Week 2 | Conception/Fertilization |
| 238 days | Week 6 | Heartbeat detected |
| 217 days | Week 9 | End of embryonic period |
| 189 days | Week 13 | End of first trimester |
| 126 days | Week 22 | Viability threshold |
| 91 days | Week 27 | End of second trimester |
| 21 days | Week 37 | Full term begins |
| 0 days | Week 40 | Estimated due date |
What If My Due Date Changes?
Doctors may adjust your due date during early ultrasounds. This is normal and happens to many women. Reasons include:
- Irregular menstrual cycle: Standard formula assumes 28-day cycles
- Inaccurate LMP date: Many women don't precisely remember their last period
- Conception timing variations: Ovulation may not occur on day 14
- Implantation differences: Embryo may implant earlier or later
- Ultrasound measurements: First-trimester scans provide more accurate dating
If your ultrasound dating differs from your LMP-based estimate by more than 7 days in the first trimester, doctors typically use the ultrasound date as the new due date. This change is normal and doesn't indicate any problem.
Likelihood of Delivering on Due Date
The truth: only about 4% of babies arrive exactly on their estimated due date! Most births fall within a "term" window:
| Term Category | Weeks | % of Births |
|---|---|---|
| Preterm | Before 37 weeks | 10-12% |
| Early Term | 37-38 weeks | 26% |
| Full Term | 39-40 weeks | 57% |
| Late Term | 41 weeks | 6% |
| Post Term | After 42 weeks | <1% |
Doctors typically don't allow pregnancies to continue beyond 42 weeks due to increased risks. Induction is often offered around 41 weeks.
How to Use This Due Date Calculator
- Enter your LMP date — First day of your last menstrual period
- Specify cycle length (if known) — Default is 28 days
- View your results:
- Estimated Due Date (EDD)
- Current pregnancy week
- Days until delivery
- Current trimester
- Bookmark and track — Return weekly to monitor progress
Confirm your due date with a doctor during your first prenatal appointment. Early ultrasounds (before 13 weeks) provide the most accurate dating.
Planning Around Your Due Date
Use your due date to plan important pregnancy preparations:
- First prenatal visit: Schedule at 8-10 weeks of pregnancy
- Maternity leave: Start planning 3-6 months before due date
- Hospital tour: Around weeks 30-34
- Pediatrician selection: By week 32-34
- Baby essentials shopping: Complete by week 36
- Hospital bag preparation: Ready by week 36
- Birth plan finalization: Discuss with doctor by week 36
- Childbirth classes: Complete by week 38
Multiple Pregnancy Adjustments
Due dates differ slightly for multiple pregnancies (twins, triplets):
- Twins: Average delivery at 36-37 weeks (3-4 weeks early)
- Triplets: Average delivery at 32-33 weeks (7-8 weeks early)
- Quadruplets: Average delivery at 29-30 weeks
The calculator provides EDD based on singleton pregnancy assumptions. Adjustment for multiples is best done by your obstetrician based on ultrasound progression.
FAQs
How accurate is the due date?
Only about 4% of babies arrive on the exact due date. Most arrive within 2 weeks of the estimated date.
Can I go into labor before my due date?
Yes. Anything from 37 weeks is considered full-term. Preterm birth is before 37 weeks.
What if I have irregular cycles?
Use your best estimate of cycle length, or use a conception date if known. An early ultrasound gives the most accurate dating.