Quick answer: your estimated due date is 280 days (40 weeks) after the first day of your last menstrual period. By hand, add one year, subtract three months and add seven days. A period starting January 10, 2026 gives a due date of October 17, 2026.
Use the calculator below for other cases: a different cycle length, IVF, a known conception date, or an ultrasound. It also shows your current week, trimester and a week-by-week calendar.
This calculator gives an estimate for information only. It is not medical advice. Your doctor or midwife confirms your due date.
How to use this pregnancy calculator
This pregnancy calculator works out your due date and how many weeks pregnant you are. Everything runs in your browser, so your dates are not sent anywhere.
- Choose your dating method. Pick last menstrual period (LMP), conception date, IVF transfer, a known due date, or an ultrasound.
- Enter the date. Type or pick the date. For LMP, also enter your usual cycle length if it is not 28 days. For an ultrasound, add the weeks and days from the report.
- Read your results. See your due date, current week and day, trimester, days to go, estimated conception date and key dates.
- Open the week-by-week calendar. Expand the calendar to see the dates for every week of pregnancy, with your current week highlighted.
- Save or share. Copy the result, copy a link that keeps your dates, or add your due date to your calendar.
Which method should I choose?
LMP
Best when your periods are regular and you know the first day of your last one.
Conception date
Use it when you know the date, for example from ovulation tracking. Due date = conception + 266 days.
IVF transfer
Use the transfer date and the embryo's age: day 3, day 5 or day 6.
Known due date
Enter a date from your doctor to see the weeks, trimester and calendar.
Ultrasound
Use it when the LMP is unknown or unsure. Enter the scan date and the gestational age on the report.
How is a due date calculated?
Pregnancy is dated from the first day of the last menstrual period and lasts about 280 days, or 40 weeks. Naegele's rule assumes a 28-day cycle with ovulation around day 14. If your cycle is longer or shorter, ovulation is later or earlier, so the due date moves by the difference.
Due dates from the first day of your last period
| LMP | Due date (280 days) |
|---|
| January 1, 2026 | October 8, 2026 |
| February 15, 2026 | November 22, 2026 |
| March 20, 2026 | December 25, 2026 |
| April 10, 2026 | January 15, 2027 |
| June 1, 2026 | March 8, 2027 |
| August 15, 2026 | May 22, 2027 |
Adjusting for cycle length (LMP on January 10, 2026)
| Cycle | Adjustment | Due date |
|---|
| 24 days | -4 days | October 13, 2026 |
| 26 days | -2 days | October 15, 2026 |
| 28 days | +0 days | October 17, 2026 |
| 30 days | +2 days | October 19, 2026 |
| 32 days | +4 days | October 21, 2026 |
| 35 days | +7 days | October 24, 2026 |
IVF due dates (transfer on March 1, 2026)
| Embryo | Add to transfer date | Due date |
|---|
| Day-3 embryo | +263 days | November 19, 2026 |
| Day-5 blastocyst | +261 days | November 17, 2026 |
| Day-6 blastocyst | +260 days | November 16, 2026 |
What are the three trimesters?
| Trimester | Weeks |
|---|
| First | Week 0 to 13 weeks 6 days |
| Second | 14 weeks 0 days to 27 weeks 6 days |
| Third | 28 weeks 0 days until birth |
How many months pregnant am I?
Weeks do not divide evenly into months, so the grouping below is a common approximation.
| Month | Weeks |
|---|
| 1 | Weeks 1 to 4 |
| 2 | Weeks 5 to 8 |
| 3 | Weeks 9 to 13 |
| 4 | Weeks 14 to 17 |
| 5 | Weeks 18 to 22 |
| 6 | Weeks 23 to 27 |
| 7 | Weeks 28 to 31 |
| 8 | Weeks 32 to 35 |
| 9 | Weeks 36 to 40 |
What counts as full term?
According to ACOG, early term is 37w0d to 38w6d, full term is 39w0d to 40w6d, late term is 41w0d to 41w6d and post-term is 42w0d or more.
How accurate is a due date?
A due date is the middle of a range, not a deadline. Only about 5% of babies are born on their due date. An early ultrasound is the most accurate way to date a pregnancy. If you are unsure of your LMP, or your cycles are irregular, your provider may use a scan instead. Twins and other multiples often arrive earlier, so follow your provider's plan.
Frequently asked questions about due dates
How do you calculate a due date?
Add 280 days (40 weeks) to the first day of your last menstrual period. This is Naegele's rule, which you can also do by hand: add one year, subtract three months, and add seven days. For example, a last period starting January 10, 2026 gives a due date of October 17, 2026.
How do I calculate how many weeks pregnant I am from my LMP?
Count the days from the first day of your last menstrual period to today and divide by 7. The whole number is your weeks and the remainder is your days. For example, 94 days is 13 weeks and 3 days. Doctors count pregnancy from the LMP, not from conception, so you are counted as about two weeks pregnant at conception.
How accurate is a pregnancy due date calculator?
A calculator gives an estimate, not a delivery date. Only about 5% of babies arrive on their due date, and most are born between 37 and 42 weeks. The estimate is most reliable with a regular cycle and a known LMP. Your provider may adjust the date after an early ultrasound.
What if my cycle is not 28 days?
Naegele's rule assumes ovulation about 14 days after your period starts. If your cycle is longer or shorter, shift the due date by the difference. With a 32-day cycle, add 4 days; with a 25-day cycle, subtract 3 days. This calculator makes that adjustment when you enter your cycle length.
How is the due date calculated for IVF?
IVF due dates use the embryo transfer date, not the LMP, because the embryo's age is known. Add 261 days to a day-5 blastocyst transfer, 263 days to a day-3 embryo transfer, or 260 days to a day-6 blastocyst transfer. This calculator offers all three options. Your clinic will confirm the final date.
What if I do not know my last menstrual period?
Use an ultrasound. Choose the ultrasound method, enter the scan date and the gestational age the report gives, and the calculator works out the due date. An ultrasound in the first trimester is the most accurate dating method. ACOG guidance is to change an LMP-based date when the scan differs by more than 5 days before 9 weeks, or more than 7 days from 9 to 13 weeks and 6 days.
How do I work out my conception date?
Conception usually happens about 266 days before the due date, or about 14 days after the start of your last period in a 28-day cycle. This calculator shows an estimated conception date. It is approximate, because ovulation timing varies from cycle to cycle.
Why is pregnancy counted from before conception?
The LMP is a date most people know, while conception is rarely certain. Counting from the LMP gives a standard 40-week pregnancy. It also means the first two weeks happen before conception, so the baby's actual age is about two weeks less than the gestational age.
When is a baby considered full term?
ACOG defines early term as 37 weeks 0 days to 38 weeks 6 days, full term as 39 weeks 0 days to 40 weeks 6 days, late term as 41 weeks 0 days to 41 weeks 6 days, and post-term as 42 weeks 0 days or more. The calculator shows the date range for each.
How many months is 40 weeks of pregnancy?
Forty weeks is about 9 months, or 9 months and a week or so, because months are longer than 4 weeks. Doctors use weeks, since they are more precise. The first trimester runs to 13 weeks 6 days, the second from 14 weeks to 27 weeks 6 days, and the third from 28 weeks until birth.
Is my information private?
Yes. The calculation runs in your browser, and the dates you enter are not sent to a server or stored. The Copy link button only puts your dates in a web address on your own device.
Sources and method
Methods follow ACOG Committee Opinion 700, Methods for Estimating the Due Date and ACOG Committee Opinion 579, Definition of Term Pregnancy. This tool is for general information and does not replace care from a doctor or midwife. If you have bleeding, severe pain or concerns about your pregnancy, contact your provider or local emergency service.
Last updated October 2026