Due Date Calculator
This due date calculator estimates when a baby is due from the first day of the last menstrual period, the conception date, or an IVF embryo transfer date. It follows the dating conventions in the American College of Obstetricians and Gynecologists (ACOG) guidance.
Result
Due date: Thursday, October 8, 2026
| Calculated from | First day of last period |
|---|
An estimate. ACOG notes that a first-trimester ultrasound is the most accurate way to date a pregnancy.
How Due Dates Are Estimated
Most pregnancies don’t have a precisely known conception date, so dating starts from something easier to pin down: the first day of the last menstrual period (LMP). By the convention ACOG describes, the estimated due date is 280 days, or 40 weeks, after the first day of the LMP.
Pregnancy weeks are counted from that same day. That’s why someone is described as “2 weeks pregnant” around the time of conception.
The Formulas
| Starting date | Due date |
|---|---|
| First day of last period | + 280 days |
| Conception date | + 266 days |
| IVF transfer, day-5 embryo | + 261 days |
| IVF transfer, day-3 embryo | + 263 days |
The conception-date figure is the 280-day convention minus the 14 days from the start of a period to ovulation that the convention assumes in a 28-day cycle. For IVF, ACOG advises dating from the transfer date and the age of the embryo, which gives the 261- and 263-day figures.
Weeks and days pregnant
Gestational age = today − (due date − 280 days), expressed in completed weeks and days.
Worked Example
The first day of the last period was January 1, 2026:
- Add 280 days: January 1 + 280 days = Thursday, October 8, 2026
- On March 15, 2026, gestational age is 73 days = 10 weeks, 3 days
The same pregnancy dated by other methods gives the same answer:
- Conception on January 15 + 266 days = October 8
- Day-5 embryo transferred January 20 + 261 days = October 8
- Day-3 embryo transferred January 18 + 263 days = October 8
The calculator handles leap years and month lengths automatically.
Things to Know
It’s an estimate
ACOG notes that ultrasound measurement in the first trimester is the most accurate way to establish or confirm gestational age. If an early ultrasound gives a different date, the due date in the medical record may be changed to match it.
Irregular cycles
The last-period method assumes a regular 28-day cycle. It doesn’t account for longer, shorter or irregular cycles, or for uncertainty about when the last period started. Those are common reasons an ultrasound is used to confirm dating.
Your clinician’s date comes first
Once a due date is set in your medical record, that’s the date your care team uses. This calculator is for general information and can’t replace dating by a doctor or midwife.
Related tools
During pregnancy, the adequate intake of total water rises to 3.0 liters a day; see the water intake calculator for details.
Frequently Asked Questions
How is a due date calculated?
By convention, the estimated due date is 280 days (40 weeks) after the first day of the last menstrual period. If the last period started on January 1, 2026, the estimated due date is October 8, 2026.
How do I calculate my due date from conception?
Add 266 days to the conception date. That's the 280-day convention minus the roughly two weeks between the start of a period and ovulation that the convention assumes in a 28-day cycle.
How is the due date calculated after IVF?
ACOG advises using the age of the embryo and the transfer date. The due date is 261 days after transfer of a day-5 embryo, or 263 days after transfer of a day-3 embryo.
How accurate is a due date?
It's an estimate. ACOG notes that a first-trimester ultrasound is the most accurate way to establish or confirm gestational age, and the due date may be revised if an ultrasound differs from the date based on the last period.
What if my cycles are longer or shorter than 28 days?
The 280-day convention assumes a regular 28-day cycle. ACOG notes that the last-period method doesn't account for irregular cycle length, which is one reason an early ultrasound is used to confirm dating.
How many weeks pregnant am I?
Pregnancy is counted from the first day of the last menstrual period, not from conception. After you enter a date, the calculator shows how many weeks and days pregnant you are today.
Related Calculators
Sources
- American College of Obstetricians and Gynecologists. Committee Opinion No. 700, Methods for Estimating the Due Date. Obstetrics & Gynecology 2017 (reaffirmed 2022)
- ACOG — by convention the due date is 280 days after the first day of the last menstrual period; IVF due date is 261 days after a day-5 embryo transfer and 263 days after a day-3 embryo transfer.
- Conception-date method — 266 days, derived from the 280-day convention minus 14 days from the last period to ovulation in a 28-day cycle.
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