How this page is reviewed
| Risk tier | High YMYL |
|---|---|
| Author | Calculover Editorial Team Health education |
| Editorial owner | Calculover Reproductive Health Desk Pregnancy and fertility methodology owner |
| Reviewer | Calculover Editorial Review Medical-source review |
| Last reviewed | 2026-05-11 |
| Last verified | 2026-05-11 |
| Data effective date | 2026-05-11 |
Methodology
Pregnancy Due Date Calculation Resource follows the pregnancy or cycle formula described on the page, such as last-menstrual-period dating, IVF transfer dating, cycle-window estimation, hCG trend context, or pregnancy-weight categories. It keeps date and range outputs educational because clinical dating and pregnancy assessment require obstetric history and, often, ultrasound or lab follow-up.
Assumptions
- Cycle-based estimates assume the entered period dates, cycle length, luteal phase, and pregnancy dates are accurate and reflect the user rather than a population average.
- LMP due-date estimates generally assume a 28-day cycle with ovulation around day 14 unless the calculator provides alternate inputs such as ovulation, conception, ultrasound, or IVF transfer date.
- Pregnancy and fertility outputs assume a singleton pregnancy or typical cycle unless the user has entered data that the calculator specifically supports.
Limitations
- Irregular cycles, breastfeeding, postpartum changes, perimenopause, PCOS, fertility treatment, pregnancy loss, multiple gestation, and uncertain dates can make estimates inaccurate.
- Ovulation and fertile-window estimates are not reliable contraception and do not confirm pregnancy, miscarriage, ectopic pregnancy, fetal growth, or pregnancy viability.
- hCG levels and pregnancy-weight ranges vary widely; symptoms, bleeding, severe pain, high blood pressure, or concerning lab trends need prompt clinical review.
Sources
- Methods for Estimating the Due Date, American College of Obstetricians and Gynecologists
- Pregnancy Fact Sheet, Eunice Kennedy Shriver National Institute of Child Health and Human Development
- Fertility Awareness-Based Methods of Family Planning, American College of Obstetricians and Gynecologists
Professional guidance: Pregnancy Due Date Calculation Resource is for reproductive-health education and date planning only. It is not obstetric, fertility, contraceptive, diagnostic, or emergency medical advice; discuss results and symptoms with an obstetrician-gynecologist, midwife, fertility specialist, or other licensed clinician.
The due date is one of the first things expectant parents want to know, but it is also one of the most misunderstood numbers in medicine. Only about 4% of babies are born on their exact due date. The due date is better understood as the middle of a window rather than a precise prediction.
How Due Dates Are Calculated
Due Date = LMP + 280 days (40 weeks) LMP = first day of your Last Menstrual Period. This assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the estimate shifts accordingly.
Calculation Methods
| Method | Accuracy | Notes |
|---|---|---|
| Last Menstrual Period (LMP) | ±2 weeks | Most common method; assumes regular 28-day cycle |
| First trimester ultrasound | ±5–7 days | Most accurate method; measures crown-rump length |
| Conception date (if known) | ±3–5 days | Add 266 days from conception |
| IVF transfer date | ±1–2 days | Most precise; exact embryo age known |
The Real Delivery Window
Full-term pregnancy ranges from 37 to 42 weeks. About 80% of babies are born between 38 and 42 weeks. First-time mothers tend to deliver slightly later (average 41 weeks and 1 day). The due date is simply the 40-week mark, not a deadline.
Track your timeline and milestones with the Pregnancy Calculator.
Key Takeaways
- Due date = LMP + 280 days (Naegele's Rule), but only 4% deliver on the exact date.
- First trimester ultrasound is the most accurate dating method.
- Normal delivery range is 37–42 weeks.
- Think of it as a due month rather than a due date.
Frequently Asked Questions
Why is the due date calculated from the last period, not conception?
Because the date of the last menstrual period is usually known with certainty, while the exact date of conception is often unknown. The LMP-based calculation includes the approximately 2 weeks before ovulation and conception actually occurred.
Can the due date change during pregnancy?
Yes. If a first-trimester ultrasound shows the baby measuring more than 7 days different from the LMP-based estimate, most providers will adjust the due date to match the ultrasound measurement.
Is it dangerous to go past the due date?
Most pregnancies that continue to 41 weeks are completely normal. After 42 weeks, risks increase slightly. Healthcare providers typically recommend induction between 41-42 weeks if labor has not started naturally.
Looking for more? Browse all free resources including guides, comparisons, and glossary terms.