Pregnancy Due Date Calculator
Estimate your due date and track each week.
The date your last menstrual period began.
Typically 28 days. A longer cycle pushes the due date later.
Estimated due date
- Gestational age
- 10 wk 0 d
- Completed weeks
- 10
- Trimester
- First trimester
Estimate only. The due date is LMP + 280 days, adjusted for your cycle length; gestational age is measured to today. Confirm with your healthcare provider.
How the due date calculator works
The calculator applies the common last-menstrual-period convention: an estimated due date is 280 days from the first day of the last menstrual period (LMP). This is a calendar estimate, not a determination of conception or fetal age. It adds exact calendar days across month boundaries and leap years.
The core formula is EDD = LMP + 280 days. If your average cycle length is not 28 days, the tool applies a cycle adjustment: it adds cycleLength - 28 days. A longer cycle means you ovulate later, so conception and the due date move later by the same amount. The full expression is EDD = LMP + 280 + (cycleLength - 28) days.
Separately, for any reference date you provide, the tool measures gestational age as whole calendar days from the LMP, then reports completed weeks as floor(days / 7) and the matching trimester.
Worked example
Inputs:
- LMP: 5 January 2026
- Average cycle length: 31 days
- Reference date (today): 6 April 2026
Steps:
| Step | Calculation | Result |
|---|---|---|
| Cycle adjustment | 31 − 28 | +3 days |
| Total days to add | 280 + 3 | 283 days |
| Due date | 5 Jan 2026 + 283 days | 15 Oct 2026 |
| Gestational age | 5 Jan to 6 Apr | 91 days |
| Completed weeks | floor(91 / 7) | 13 weeks |
| Trimester | week 13 is in 0 to 13 | First |
With a default 28-day cycle, the same LMP would give a due date of 12 October 2026 (280 days), three days earlier. The cycle adjustment is an additional calculator assumption; clinical dating may not use it in the same way.
How to use it
- Enter the first day of your last period, not the day it ended.
- Set your average cycle length if you track it. Leave it at 28 if you are unsure; that is the standard assumption.
- Use the reference date field to check your gestational week and trimester on any day, past or future.
- Treat the result as a planning anchor for appointments and milestones, then confirm it with your clinician’s dating scan.
Limitations
This estimate assumes regular ovulation on a predictable schedule and relies on an accurately remembered LMP. It cannot account for irregular cycles, early or late ovulation, recent hormonal contraception, postpartum cycles, bleeding mistaken for a period, or assisted reproduction. ACOG recommends using assisted-reproduction dates for pregnancies conceived that way and identifies first-trimester ultrasound as the most accurate method to establish or confirm gestational age. Do not use this calculator to time clinical tests, medication, induction, or other care; use the due date documented by your clinician.
Sources and scope
- Jurisdiction
- General obstetric dating; US clinical source
- Data current to
- July 30, 2026
- Last updated
- Methods for Estimating the Due Date — American College of Obstetricians and Gynecologists
Frequently asked questions
Why is the due date counted from my last period and not from conception?
Most people know the start of their last period but rarely know the exact day of conception, which can vary by several days. Dating from the last menstrual period (LMP) gives a consistent, reproducible starting point. The 280-day count already builds in the roughly two weeks between the LMP and ovulation, so a pregnancy is conventionally measured as about 40 weeks even though fertilization happens around week 2.
How accurate is an estimated due date?
It is a planning estimate, not a prediction of the day labor will begin. Last-menstrual-period dating can be wrong when cycle timing or recall is uncertain. ACOG states that first-trimester ultrasound, through 13 weeks and 6 days, is the most accurate method to establish or confirm gestational age.
What if my cycle is not 28 days long?
Enter your real average cycle length. The calculator shifts the due date by the difference from 28 days, because a longer cycle means you ovulate later and conceive later, pushing the due date back. A 31-day cycle moves the date 3 days later; a 26-day cycle moves it 2 days earlier.
What does my gestational week mean for trimesters?
Gestational weeks are completed full weeks since your LMP. The first trimester covers weeks 0 to 13, the second covers weeks 14 to 27, and the third runs from week 28 onward. The calculator reports your completed weeks and the matching trimester for any reference date you give it.
Can I use this if I conceived through IVF?
This tool is built for natural cycle dating from an LMP, so it is not the right method for IVF. With IVF the transfer date and embryo age are known precisely, and clinics date the pregnancy from those instead. Use the date your clinic provides rather than this estimate.