How is a pregnancy due date calculated?
The most common method is Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. If you know your ovulation or conception date instead, add 266 days (38 weeks), since ovulation typically occurs about 14 days after the period starts.
This is a statistical estimate based on average cycles, not a medical diagnosis — actual delivery dates vary by roughly two weeks in either direction even in a normal pregnancy, and an early ultrasound is generally more accurate than a menstrual-date estimate alone. Always confirm your due date and monitor the pregnancy with a qualified obstetrician or midwife.
How do I calculate my due date?
To estimate your due date using Naegele's rule, add 280 days (40 weeks) to the first day of your last menstrual period. Example: last period started January 1 -> due date is October 8 (280 days later). If you know your ovulation date instead, add 266 days (38 weeks) from that date.
Steps to estimate a due date
- Find the first day of your last menstrual period (LMP), or your known ovulation/conception date.
- If using LMP, add 280 days; if using ovulation date, add 266 days.
- If your average cycle is longer or shorter than 28 days, adjust the LMP-based estimate by the difference (a 35-day cycle pushes the estimate about 7 days later).
- Count the days from your LMP to today and divide by 7 to get your current week of pregnancy.
- Compare the week number to trimester ranges: weeks 1-12 first trimester, 13-26 second trimester, 27+ third trimester.
Formula
Due date = LMP + 280 days (Naegele's rule), or Due date = Ovulation date + 266 days
- LMP = first day of the last menstrual period
- 280 days = 40 weeks, the average length of a full-term pregnancy counted from LMP
- 266 days = 38 weeks, counted from ovulation/conception, about 14 days after LMP in an average cycle
Quick due date answers
If the last period started January 1, what is the estimated due date?
October 8 — 280 days after January 1 (Naegele's rule).
If ovulation happened on a known date, how is the due date calculated?
Add 266 days to the ovulation date; for example, ovulation on January 15 gives the same October 8 due date as a January 1 last period, since ovulation typically falls about 14 days after the period starts.
How many weeks pregnant is 280 days?
280 days ÷ 7 = 40 weeks, the standard full-term length counted from the last menstrual period.
What week of pregnancy does the third trimester start?
Week 27, following this calculator's ranges: weeks 1-12 first trimester, 13-26 second trimester, 27 onward third trimester.
Does calculating from 266 days after conception give the same due date as 280 days from the last period?
Yes, assuming ovulation occurred 14 days after the period started — the two counting methods are designed to agree for an average 28-day cycle.
How does a longer cycle change the due date?
Each day beyond 28 in your average cycle length pushes the estimate later by the same number of days — a 35-day cycle (7 days longer) moves the due date about 7 days later than the standard 280-day estimate.
If the last period started March 1, what is the due date?
December 5 — 280 days later.
If the last period started June 10, what is the due date?
March 17 of the following year — 280 days later, crossing into the next calendar year.
Example due date calculations
| LMP or ovulation date | Method | Estimated due date |
|---|
| Jan 1 (LMP) | +280 days | Oct 8 |
| Jan 15 (ovulation) | +266 days | Oct 8 |
| Mar 1 (LMP) | +280 days | Dec 5 |
| Jun 10 (LMP) | +280 days | Mar 17 (next year) |
Frequently asked questions
How accurate is a due date calculated this way?
It is a statistical average, not a precise prediction — only about 5% of babies are born on their exact estimated due date, and full-term birth is considered normal anywhere from 3 weeks before to 2 weeks after it. An early first-trimester ultrasound is generally considered more accurate than a menstrual-date estimate alone, especially for irregular cycles.
Why does cycle length matter for the calculation?
Naegele's rule assumes a standard 28-day cycle with ovulation around day 14. If your average cycle is longer, ovulation (and conception) likely happened later, so the whole pregnancy timeline shifts later too — this tool adjusts the LMP-based estimate by the difference between your cycle length and 28 days.
What is the difference between calculating from LMP versus ovulation date?
Calculating from LMP adds 280 days because it counts from the start of the cycle, before conception actually occurs. Calculating from ovulation or a known conception date adds 266 days because it starts counting closer to actual fertilization — the two methods should point to a similar due date if your cycle is close to 28 days.
What are the three trimesters?
The first trimester runs from week 1 through week 12, the second trimester from week 13 through week 26, and the third trimester from week 27 until birth around week 40 — these ranges are commonly used but clinics may define the exact week boundaries slightly differently.
This tool provides a statistical estimate only and is not a medical diagnosis or a substitute for prenatal care. It does not account for irregular cycles beyond a simple length adjustment, medically assisted conception dating, or individual health factors. Confirm your due date and monitor your pregnancy with a qualified obstetrician, gynecologist, or midwife, and rely on ultrasound dating where available.
Sources: American College of Obstetricians and Gynecologists (ACOG) — Methods for Estimating the Due Date