π€° Pregnancy Due Date Calculator
Naegele's rule: last period plus 9 months and 7 days.
For informational purposes only, not medical advice. Consult a healthcare professional.
Your estimated due date is 280 days (40 weeks) after the first day of your last menstrual period, which works out to the LMP plus 9 months and 7 days. A 35-day cycle pushes the date a week later than the textbook 28-day math. Enter your dates above for your EDD, conception estimate, and current week.
How to Use This Calculator
Naegele's rule assumes ovulation on day 14 of a 28-day cycle. The calculator adjusts when your cycle runs longer or shorter, because ovulation shifts with it.
Enter the first day of your last period, not the day it ended and not the day you think you conceived. If you are unsure by a few days, use your best estimate; early ultrasound can refine it later.
Enter your average cycle length. Leave 28 if your cycles are textbook. A 35-day cycle adds 7 days to the due date because ovulation likely happened around day 21 instead of day 14.
Read your due date, the estimated conception date (about two weeks after LMP for a 28-day cycle), your current gestational age in weeks and days, and your trimester.
Naegele's Rule, Explained
Franz Naegele published the rule in the early 1800s, and it survives because the arithmetic is close to the biology: pregnancy lasts about 266 days from conception, and conception happens about 14 days after the LMP in a 28-day cycle, so 266 + 14 = 280 days from LMP. The "9 months and 7 days" version is the same 280 days in calendar form. The cycle adjustment matters more than people expect: with a 35-day cycle, ovulation lands near day 21, so dating from a 28-day assumption would make the pregnancy look a week younger than it is, which can wrongly suggest a post-term pregnancy later.
Worked Example
LMP of July 10, 2026, 28-day cycle: add 280 days. July 10 plus 9 months is April 10, 2027, plus 7 days is April 17, 2027. Conception estimate: July 10 + 14 days = July 24, 2026. With a 35-day cycle instead: EDD shifts 7 days later to April 24, 2027, and conception to July 31. As of October 16, 2026, the 28-day-cycle pregnancy is 98 days along, which is exactly 14 weeks and 0 days, the first day of the second trimester. The calculator performs this same date arithmetic on whatever you enter.
How Accurate Is the Due Date?
| Fact | Detail |
|---|---|
| Deliveries on the EDD | Only about 4 to 5% arrive exactly on the date |
| Deliveries within 2 weeks | Roughly 80% land between 38 and 42 weeks |
| First babies | Average slightly past the EDD, around 40 weeks 5 days in some studies |
| Best dating method | First-trimester ultrasound (crown-rump length), accurate within 5-7 days |
Treat the EDD as the middle of a due month, not an appointment. First-trimester ultrasound beats LMP dating whenever the two disagree by more than a few days, because cycle lengths vary and implantation timing shifts. Late in pregnancy the date drives real decisions: induction is typically discussed at 41 weeks and recommended by 42, and a wrong-by-a-week date can move those decisions. That is why providers re-date with early ultrasound rather than arguing about the arithmetic. For what comes after the date arrives, our pregnancy due date tools pair with ovulation tracking for planning the next one, and our pregnancy weight gain calculator covers the IOM targets by trimester once you know your week.
Trimesters and Milestones by Week
Weeks 1 to 13 are the first trimester, 14 to 27 the second, 28 to delivery the third. The dating quirk that confuses everyone: weeks 1 and 2 of "pregnancy" happen before conception, so at 4 weeks pregnant the embryo is about 2 weeks old. Key milestones fall on the LMP clock: heartbeat visible around 6 weeks, nuchal screening 11 to 14, anatomy scan 18 to 22, viability near 24, glucose screening 24 to 28, group B strep test 36 to 37. If your LMP is uncertain or cycles are irregular, say so at the first prenatal visit; that single early ultrasound settles the dating for the entire pregnancy.
How the Due Date Shapes Late-Pregnancy Decisions
The EDD stops being trivia around week 41. Standard practice in the US: if labor has not started by 41 weeks, providers discuss induction, weighing the small rising risk of stillbirth against the downsides of induction. By 42 weeks and 0 days, delivery is recommended rather than debated, because risks climb meaningfully past that point. This is where accurate dating pays off: a pregnancy misdated a week late could push induction decisions into the wrong week. Non-stress tests and fluid checks (antenatal surveillance) typically start around 40 to 41 weeks for pregnancies going past the date. None of this means the date predicts your baby's birthday; it means the date anchors the safety decisions at the end. Keep the EDD from your earliest ultrasound as the official one and use it for every countdown that matters.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
How is the due date calculated from the last period?
Add 280 days (40 weeks) to the first day of your last menstrual period. The shortcut version: add 9 months and 7 days. For cycles longer or shorter than 28 days, add or subtract the difference, since ovulation shifts with cycle length.
Why does pregnancy count from the last period instead of conception?
Because the LMP is a date people usually know, while conception is rarely observed. The 40-week clock includes the roughly two pre-conception weeks, so at 4 weeks pregnant the embryo is about 2 weeks old. Ultrasound dating uses the same convention.
What is Naegele's rule?
A formula from the early 1800s: EDD = LMP + 9 months + 7 days, assuming a 28-day cycle. It equals 280 days from LMP and remains the standard clinical starting point, refined by early ultrasound when available.
How accurate are due dates?
Only about 4-5% of babies arrive on the EDD itself, but roughly 80% arrive within two weeks of it (38-42 weeks). First-trimester ultrasound dating is the most accurate method, within 5-7 days.
Does cycle length change the due date?
Yes. The 280-day rule assumes ovulation on day 14. With a 35-day cycle, ovulation is nearer day 21, so the due date moves about 7 days later. This calculator applies that adjustment automatically.
What if I don't know my LMP?
Use your best estimate for now, then get a first-trimester ultrasound: crown-rump length measurement dates the pregnancy within 5-7 days and becomes the official dating. Tell your provider the LMP is uncertain so they prioritize the scan.
Can IVF pregnancies use this calculator?
IVF dating is more precise and uses transfer date instead: for a day-5 blastocyst transfer, EDD is the transfer date plus 261 days (or LMP-equivalent = transfer minus 19 days). Standard LMP calculators do not apply cleanly to IVF.
What does it mean to be 40 weeks 3 days?
Gestational age is reported as completed weeks plus extra days. 40 weeks 3 days is 3 days past the due date. Most providers discuss induction options at 41 weeks and recommend delivery by 42 weeks.
Why did my due date change after ultrasound?
Because early ultrasound measures the embryo directly and beats calendar math. If the scan and LMP differ by more than 5-7 days in the first trimester, the scan date replaces the LMP date for the rest of the pregnancy.
Do twins change the due date?
The EDD math is the same, but twins rarely reach 40 weeks: average delivery is around 35-37 weeks depending on type. Providers monitor twin pregnancies more closely and plan delivery earlier regardless of the calculated date.