The Pregnancy Due Date Calculator is designed to estimate your due date and track your current progress using your choice of starting information. It runs instantly in your web browser to show your estimated due date, current weeks and days along, trimester progress, and a timeline of key pregnancy milestones and term ranges. Your dates stay on this page — the calculation runs in your browser, and nothing is uploaded, stored or shared.
The Science of Gestational Age
In clinical practice, pregnancy dating is calculated from the first day of your last menstrual period (LMP) rather than the date of conception. This convention exists because the exact day of ovulation and subsequent conception is historically difficult to pin down, whereas the start of a menstrual period is a distinct, memorable event.
Under Naegele's rule, the estimated due date (EDD) is calculated by adding 280 days (40 weeks) to the first day of the last menstrual period. This rule assumes a standard 28-day menstrual cycle with ovulation occurring on day 14. Historically, this was taught as adding one year, subtracting three months, and adding seven days to the LMP. If calculating from a known, precise conception date, the EDD is set at conception + 266 days.
Because gestational age begins counting approximately two weeks before conception actually occurs, a person who receives a positive pregnancy test at "4 weeks pregnant" actually conceived roughly 2 weeks prior.
By common clinical convention, the pregnancy is divided into three trimesters:
- First trimester: Ends at 13 weeks, 6 days (13w6d).
- Second trimester: Runs from 14 weeks, 0 days to 27 weeks, 6 days (27w6d).
- Third trimester: Begins at 28 weeks, 0 days and runs until delivery.
The Role of Cycle Length in Pregnancy Dating
The standard 28-day cycle assumption does not reflect every individual's biology. Ovulation timing shifts in direct relation to cycle length, occurring roughly 14 days before the next expected period. Therefore, if your cycle is longer or shorter than 28 days, the timing of conception shifts, and the due date must be adjusted accordingly.
The calculator adjusts for this by shifting the EDD by the difference of (Cycle Length − 28 days).
- Longer cycles: A 32-day cycle means ovulation occurred later, shifting the due date 4 days later.
- Shorter cycles: A 24-day cycle means ovulation occurred earlier, pulling the due date 4 days earlier.
This cycle-length adjustment is a recognized standard, also built into the NHS due-date calculator. The tool accepts an average cycle length input ranging from 20 to 45 days. If the input falls outside this window, it displays the error: "Cycle length runs from 20 to 45 days.".
Understanding IVF Due Dates
For pregnancies achieved through In Vitro Fertilization (IVF), the exact timing of fertilization and embryo development is known. This makes IVF dating the most precise calendar-based method available. According to the American College of Obstetricians and Gynecologists (ACOG) Committee Opinion No. 700, assisted reproductive technology (ART)-derived dating should always be used to assign the EDD.
The calculator determines the due date by adding 266 days to the transfer date and subtracting the embryo's age in days:
- Day 3 embryo: Add 263 days to the transfer date.
- Day 5 embryo: Add 261 days to the transfer date.
- Day 6 embryo: Add 260 days to the transfer date.
Frozen embryo transfers use these exact same rules based on the age of the embryo at the time of freezing; the freeze-thaw process itself does not alter the timeline.
Ultrasound Dating vs. Calendar Methods
While calendar methods provide a helpful starting point, a first-trimester ultrasound measuring crown–rump length (up to 13 weeks, 6 days) is clinically established as the most accurate dating method, accurate to within 5 to 7 days.
According to ACOG Committee Opinion No. 700, the LMP-derived due date should yield to the ultrasound-derived due date if they differ by more than the following thresholds:
| Gestational Age by LMP | Discrepancy Threshold for Redating |
|---|---|
| Up to 8 weeks, 6 days (8w6d) | More than 5 days |
| 9 weeks, 0 days to 13 weeks, 6 days (9w0d–13w6d) | More than 7 days |
| 14 weeks, 0 days to 15 weeks, 6 days (14w0d–15w6d) | More than 7 days |
| 16 weeks, 0 days to 21 weeks, 6 days (16w0d–21w6d) | More than 10 days |
| 22 weeks, 0 days to 27 weeks, 6 days (22w0d–27w6d) | More than 14 days |
| 28 weeks, 0 days and beyond (28w0d+) | More than 21 days |
Once a due date is established by an early ultrasound scan, it is not revised during subsequent later scans. Because deciding whether to redate a pregnancy is a clinical judgment, this calculator preserves the scan-based date and does not change it on its own.
The Reality of the Due Date Window
A due date is a helpful reference point, but it is not a strict deadline. Only about 1 in 20 babies (approximately 4% to 5%) are born on their exact estimated due date. Most babies arrive within a window spanning two weeks on either side of the date, and a normal pregnancy can span anywhere from 37 to 42 weeks. First pregnancies also statistically tend to run a few days past the estimated due date.
To help you understand this window, the calculator categorizes gestational age into clinical term bands defined by ACOG and the Society for Maternal-Fetal Medicine (SMFM) Committee Opinion No. 579:
- Preterm: Before week 37 (prior to 37w0d).
- Early term: Weeks 37–38 (37w0d to 38w6d).
- Full term: Weeks 39–40 (39w0d to 40w6d).
- Late term: Week 41 (41w0d to 41w6d).
- Postterm: Week 42 on (42w0d and beyond).
Pregnancy Milestones and Trimesters
As your pregnancy progresses, the calculator maps out key dates to help you plan:
- Estimated conception: The approximate date the egg was fertilized. Outside of IVF, this date is always an estimate because sperm can survive in the reproductive tract for up to 5 days, meaning intercourse several days before ovulation can result in conception.
- First trimester ends: Transition point to the second trimester at 13w6d.
- Anatomy scan window: The mid-pregnancy ultrasound, typically offered between 18 and 22 weeks, to check the baby's physical development.
- Third trimester begins: The final phase of pregnancy starting at 28w0d.
- Early term from, Full term from, and Postterm from: Key dates marking your transition through the clinical term bands.
If you enter a date that is further back than a normal pregnancy lasts, the tool will display the message: "That date is further back than a pregnancy lasts, so there’s no current week to show — the due date and key dates are below.".
Frequently Asked Questions
How accurate is a calculated due date?
It is the middle of a window, not a deadline. Pregnancy counts as term anywhere from 37 to 42 weeks, and first pregnancies often run a few days past the date. An early ultrasound pins the date down more precisely than any calendar method — so use the estimate to plan around, not to schedule by.
Why does cycle length move the due date?
The classic rule assumes a 28-day cycle with ovulation on day 14. A longer cycle means you ovulate later, so conception — and with it the due date — shifts later by the difference; a shorter cycle pulls it earlier. If you do not track your cycle, leave it at 28 days.
My scan gave a different due date — which one counts?
Usually the scan’s. A first-trimester ultrasound (up to 13 weeks 6 days) dates a pregnancy to within about 5–7 days, which guidelines from ACOG rate as more reliable than even a well-remembered last period; when the two differ by more than set thresholds, the scan date takes over. That call belongs to your midwife or doctor — which is why this calculator never changes a scan-based date on its own.
How is the due date worked out after IVF?
From the transfer date and the embryo’s age: add 266 days minus the embryo age — 263 days for a day-3 embryo, 261 for day-5, 260 for day-6. Frozen transfers count the same, using the age the embryo was frozen at. Because the exact timing is known, IVF dating is the most precise of all, and a later scan would only rarely shift it.
What can this calculator not tell me?
It cannot confirm a pregnancy, check the baby’s health, or decide anything about your care — it is a calendar calculation for learning and planning. Questions about your own dates, a scan that disagrees, or when to be seen belong with your midwife or doctor.