You just received a positive pregnancy test and want to immediately estimate your due date, know your exact current gestational week and day, and understand when your first trimester milestones conclude.
How to use Due Date Calculator step by step
1. Enter first day of last period (LMP)
Select the calendar date when your last menstrual bleeding began.
2. Adjust average menstrual cycle length
Standard obstetric default is 28 days; adjust between 21 and 35 days for exactness.
3. View due date and gestational age
See your estimated delivery date (EDD), completed weeks plus days, and active trimester.
The Step Count: Traditional Friction vs. KorexTools
Calculating due date on paper via Naegele’s Rule (4 steps)
- Write down the first day of your last normal period (LMP).
- Add 7 calendar days to that date.
- Subtract 3 months from that calendar month.
- Add 1 calendar year if the date transitions into the next year.
With KorexTools Pregnancy Due Date Calculator (3 steps in 2 seconds)
- Select the first date of your last menstrual period.
- Adjust cycle length if your regular cycle differs from 28 days.
- Inspect your due date, current gestational week, and days remaining.
Naegele’s rule: the clinical standard for obstetric due date calculation
Clinical obstetrics calculates the Estimated Date of Delivery (EDD) using Naegele’s Rule: start with the first day of the Last Menstrual Period (LMP), add 7 days, and subtract 3 months (equating to 280 gestational days or 40 completed weeks).
Because conception generally occurs roughly 14 days before the next period, women with cycle lengths deviating from the 28-day norm receive an automatic mathematical adjustment reflecting their true ovulation window.
Gestational milestones across the three trimesters
- •First trimester: Week 1 through Week 13 (organogenesis and early development).
- •Second trimester: Week 14 through Week 27 (structural growth and quickening).
- •Third trimester: Week 28 through Week 40 (fetal lung maturation and final weight gain).
Gestational milestones based on Naegele obstetric computation
| Milestone | Gestational Week | Clinical Focus |
|---|---|---|
| First trimester completion | Week 13 | Nuchal translucency scan / organ formation |
| Midpoint anatomy scan | Week 20 | Detailed structural ultrasound evaluation |
| Third trimester transition | Week 28 | Accelerated growth and fetal viability |
| Full term delivery window | Week 37 – 40 | Natural physiological labor expectation |
Common Mistakes and How to Avoid Them
- ⚠️Entering the last day of bleeding instead of the very first day the cycle commenced.
- ⚠️Assuming delivery past the calculated date is abnormal: full-term spans week 37 through week 41.
Frequently Asked Questions
What percentage of infants arrive on their exact calculated due date?
Only about 4% to 5% are born on the exact day. A full-term birth window spans from week 37 to week 42.
Why is gestational age counted from the last period instead of conception?
Because the exact hour of conception is rarely identifiable, whereas the onset of menstrual bleeding provides an objective date.
How does a non-28 day cycle affect my delivery date?
If your cycle averages 32 days, ovulation occurred roughly 4 days later than average, shifting your estimated due date by 4 days.
Does this calculator replace an obstetric ultrasound?
No. An early first-trimester crown-rump length (CRL) ultrasound remains the diagnostic gold standard for obstetric dating.
What I would do if I were in your shoes
“Treat your due date as an arrival month window rather than a rigid deadline. Clinical term delivery safely spans multiple weeks.” — DataKorex Engineering Team.