Direct Answer
Due date = LMP + 280 days (40 weeks) per Naegele’s rule. LMP January 1 → due date October 8. First-trimester ultrasound can adjust dating by 5-7 days when LMP is uncertain.
Use the Pregnancy Due Date Calculator with LMP or conception date.
Last verified on: June 28, 2026
Editorial note: Health calculators provide general estimates — not medical diagnosis or personalized nutrition or treatment advice. Consult a physician or registered dietitian for individual guidance.
Research method: Daily Calcs reviewed CDC, NIH, and peer-reviewed clinical guidelines and validated calculator formulas on June 28, 2026.
Dating Methods Compared
| Method | Formula | Best when |
|---|---|---|
| LMP (Naegele) | LMP + 280 days | Regular 28-day cycles |
| Ovulation | Conception + 266 days | Tracked ovulation |
| Ultrasound | CRL in 1st trimester | Irregular cycles / unknown LMP |
Naegele’s Rule Step by Step
Naegele’s rule: LMP (last menstrual period) + 1 year − 3 months + 7 days.
Example: LMP January 1, 2026
- Add 1 year → January 1, 2027
- Subtract 3 months → October 1, 2026
- Add 7 days → October 8, 2026 estimated due date
Gestational age counts from LMP — at conception you are already considered 2 weeks pregnant by obstetric dating.
Worked Example: 28-Day Cycle vs 32-Day Cycle
| Cycle length | LMP | Est. ovulation | Est. due date (Naegele) |
|---|---|---|---|
| 28 days | Jan 1 | ~Jan 15 | Oct 8 |
| 32 days | Jan 1 | ~Jan 19 | Oct 8 (LMP method unchanged) |
Naegele assumes ovulation on day 14. If you ovulate late (day 18), true due date may be 4 to 7 days later — first-trimester ultrasound adjusts this.
Week-by-Week Milestones From LMP Jan 1
| Gestational week | Approx. calendar date | Milestone |
|---|---|---|
| 8 | Feb 26 | First prenatal visit typical |
| 12 | Mar 26 | End first trimester |
| 20 | May 21 | Anatomy scan window |
| 40 | Oct 8 | Estimated due date |
What to Do Next
- Enter LMP in the Pregnancy Due Date Calculator.
- Schedule first prenatal visit — often weeks 8 to 10.
- Bring ultrasound dating if LMP is uncertain or cycles are irregular.
- Treat due date as a midpoint — term is 37 to 42 weeks.
- Confirm final EDD with your obstetric provider.
Due Date Planning Checklist
- Calculate EDD from LMP as soon as pregnancy is confirmed
- Schedule first prenatal appointment (weeks 8–10)
- Book first-trimester ultrasound (weeks 8–13)
- Confirm EDD with provider at first visit
Common Mistakes With Due Date Planning
Treating the due date as a deadline causes unnecessary induction pressure — only ~5% of babies arrive on that exact day. Planning work leave for a single date without a 37 to 42 week window creates stress.
Using conception date without adjusting for cycle length when LMP is known can double-error dating.
Assumptions and Limitations
Naegele’s rule assumes a 28-day cycle with ovulation on day 14. Irregular cycles, hormonal contraception recently stopped, and breastfeeding amenorrhea reduce LMP accuracy.
First-trimester ultrasound within 14 weeks is the clinical gold standard when dates conflict. IVF and embryo transfer dating follow embryologist-specific rules not covered by standard LMP entry alone.
What This Means for Your Personal Numbers
Generic examples help you understand the logic — your outcome depends on inputs only you know. Run the related calculator with your age, weight, income, loan amount, or location before treating any table row as a target. Adjust one variable at a time so you can see which lever moves the result most.
If the calculator output surprises you, verify assumptions first: activity level, tax district, insurance quote, cycle length, or credit tier. Small input changes often move results more than rounding differences between published benchmarks and your situation.
When numbers still look wrong after honest inputs, use 2 to 4 weeks of tracking — weight trend, home readings, spending log, or rate quotes — to calibrate. Calculators start the conversation; your data confirms it.
Book prenatal care early — dating ultrasound in the first trimester is most accurate and may adjust your planner EDD by several days.
Calculator Methodology
The Pregnancy Due Date Calculator applies Naegele’s rule to LMP or adds 266 days to conception/IVF transfer date with embryo age adjustment.
Assumptions: Standard 28-day cycle when using LMP alone.
Limitations: Clinical ultrasound dating overrides calculator — educational estimate only.
How to stress-test your result
Run a best case and worst case input side by side. For health calculators, shift activity one level up or down. For finance calculators, add 0.25% to rate or 10% to tax and insurance. If the outcome breaks your budget or health target at the worst case, build margin before committing.
Document the date you ran the numbers and which source you used for tax, insurance, or clinical thresholds. Re-run when your inputs change materially — new job, new prescription, rate lock, or 10+ lb weight change.
Related Reading
- Ovulation Calculator Guide — conception timing
- Sleep Calculator — pregnancy sleep planning
- Pregnancy Due Date Calculator — your EDD
Official and Supporting Sources
Share your calculator inputs and outputs with a professional when decisions have medical, legal, or lending consequences. Bring printed logs, Loan Estimates, or weekly weight averages — not single data points — to those conversations.
Next Step
Enter your LMP in the Pregnancy Due Date Calculator and confirm at your first prenatal visit.
Frequently Asked Questions
How is pregnancy due date calculated?
The standard method adds 280 days (40 weeks) to the first day of the last menstrual period (LMP) using Naegele's rule: LMP + 1 year − 3 months + 7 days. LMP dating assumes a 28-day cycle with ovulation on day 14. Irregular cycles make LMP less accurate — first-trimester ultrasound (crown-rump length) is preferred when available and can shift due date by days to weeks.
How accurate is the due date?
Only about 5% of babies arrive on the exact due date. Term is 37 to 42 weeks — a five-week window is normal. Ultrasound before 14 weeks estimates gestational age within 5 to 7 days. LMP-only dating can err by 1 to 2 weeks with irregular cycles or late ovulation. Treat the due date as a midpoint, not a deadline.
LMP vs ultrasound due date: Which wins?
ACOG prioritizes first-trimester ultrasound when LMP is uncertain or differs from ultrasound by more than 5 to 7 days. If you know ovulation from tracking, ovulation-based dating (LMP equivalent minus 14 days plus 266 days) can beat LMP alone. Your prenatal provider reconciles dates — calculator output starts the conversation, not replaces clinical dating.
What if I do not remember my last period?
Use earliest ultrasound gestational age if available. Without LMP or ultrasound, due date estimation is unreliable — schedule prenatal care for dating scan. Physical exam (uterine size) is rough after 12 weeks. The Pregnancy Due Date Calculator needs LMP or conception date for meaningful output.
How many weeks pregnant am I today?
Gestational age counts from LMP, not conception — you are '2 weeks pregnant' at conception biologically. At positive test (~4 weeks LMP), embryo age is ~2 weeks. Calculators show gestational weeks from LMP. Add current date minus LMP start, divided by 7, for weeks today — or use the calculator's automatic gestational age field.
Conception date vs LMP for due date?
Due date from conception adds 266 days (38 weeks) to conception/ovulation. LMP method adds 280 days because conception is assumed ~14 days after LMP start. If you conceived via IVF, embryologist transfer date plus 266 minus days as embryo age gives due date — IVF clinics provide this directly.
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