Pregnancy Due Date
Calculator
Calculate your estimated due date, current week, trimester, and pregnancy milestones, from your last period, conception date, ultrasound, or IVF transfer.
Pregnancy Due Date Calculator: When Is My Baby Due?
That moment when you see those two lines, one of the very first questions is always: when is my baby due? Your estimated due date (EDD) becomes the anchor of your pregnancy journey, marking milestones and counting down to one of life’s most profound moments. This free pregnancy due date calculator gives you an instant estimate from your last menstrual period, conception date, known due date, or IVF transfer, with a week-by-week timeline, trimester tracking, and key milestones.
Quick answer: Add 280 days (40 weeks) to the first day of your last menstrual period (LMP). If your LMP was 1 January, your estimated due date is 8 October. Use the calculator above for any start date, with automatic adjustment for cycle length.
How Is a Due Date Calculated?
The standard method is Naegele’s Rule: a 280-day (40-week) pregnancy measured from the first day of the last menstrual period. It accounts for the fact that ovulation typically occurs around day 14 of a 28-day cycle, meaning conception happens about two weeks after the LMP. This provides a consistent, measurable reference point for all obstetric dating.
Due Date Formulas
LMP method: Due Date = LMP + 280 days
Conception method: Due Date = Conception + 266 days
Cycle adjustment: If your cycle is longer than 28 days, add the difference. Example: 32-day cycle → add 4 days.
IVF Day 5 blastocyst: Due Date = Transfer Date + 261 days
How the Pregnancy Due Date Calculator Formula Works
This calculator measures one thing: an estimated delivery date, worked out with calendar arithmetic rather than any measurement of your baby. Depending on which mode you choose, it starts from a different known date and applies a fixed offset built around the standard 280-day (40-week) length of a full-term pregnancy.
| Mode | Formula | What it needs |
|---|---|---|
| Last period (LMP) | Due date = LMP + 280 days + cycle adjustment | First day of last period, average cycle length |
| Conception | Due date = conception date + 266 days | Estimated conception or ovulation date |
| Due date known | LMP = due date − 280 days; conception = LMP + 14 days | Due date confirmed by ultrasound or provider |
| IVF transfer | Due date = transfer date + (266 − embryo age in days) | Transfer date, embryo age (day 3, 5, or 6) |
Cycle adjustment only applies to the LMP mode: for every day your average cycle runs longer or shorter than 28 days, the due date shifts by that same number of days, since ovulation happens correspondingly later or earlier. Embryo age in the IVF mode accounts for the fact that a transferred embryo already has a known age, so the countdown to 266 days needs fewer days from the transfer date than it would from a natural conception.
Step-by-step calculation walkthrough (LMP method)
Step 1: Identify the inputs. Last menstrual period started January 1. Average cycle length is 32 days (4 days longer than the 28-day baseline).
Step 2: Apply the formula. Due date = LMP + 280 days + (cycle length − 28). Here that’s January 1 + 280 days + 4 days.
Step 3: Perform the calculation. January 1 plus 280 days lands on October 8. Adding the 4-day cycle adjustment moves the estimate to October 12. Conception is estimated at LMP + 14 days + the same 4-day adjustment, which is January 19.
Step 4: Interpret the result. October 12 is the calculator’s estimated due date for this cycle. It marks the midpoint of a normal delivery window, not a fixed deadline, since ACOG’s clinical guidance on dating pregnancy treats LMP-based dates as an estimate to be confirmed or adjusted by first-trimester ultrasound.
📐 Why the modes give slightly different answers for the same pregnancy: LMP dating assumes textbook ovulation timing. Conception dating uses a known fertilisation date if you have one. Ultrasound dating measures the embryo or fetus directly and is generally the most precise in early pregnancy. IVF dating is the most exact of all, since the embryo’s age at transfer is known to the day.
Assumptions and limitations: the LMP formula assumes ovulation occurred close to day 14 of the cycle (adjusted for cycle length) and that the entered LMP date is accurate. It does not account for irregular ovulation, recent hormonal contraceptive use, breastfeeding-related cycle changes, or multiples (twins and higher-order pregnancies often deliver earlier than singleton pregnancies). None of the four modes predicts the actual delivery date. They estimate the most likely date within a normal range, which spans several weeks either side.
How Accurate Is a Due Date?
Only about 4 to 5% of babies are born on their exact estimated due date. Normal delivery spans 37 to 42 weeks. About 57% of births occur within one week either side of the due date, and 87% within two weeks. The due date is best understood as the midpoint of a normal delivery window, not a precise deadline. ACOG’s guidance on dating pregnancy recommends early ultrasound, ideally in the first trimester, as the most accurate single method available.
Pregnancy Timeline by Trimester
| Trimester | Weeks | Key developments |
|---|---|---|
| 1st Trimester | Weeks 1–12 | Organ formation, heartbeat detectable. Morning sickness common. |
| 2nd Trimester | Weeks 13–27 | Baby grows rapidly. Movements felt. Anatomy scan at ~20 weeks. |
| 3rd Trimester | Weeks 28–40 | Rapid brain development. Baby settles head-down. Labour preparation begins. |
| Early term | Weeks 37–38 | Baby is developed but still benefits from the final weeks in the womb. |
| Full term | Weeks 39–40 | Best-documented outcomes occur in this window. Delivery expected any time. |
| Late term | Week 41 | Provider monitoring typically increases; induction may be discussed. |
| Postterm | Week 42+ | Slightly elevated risks; induction is commonly recommended by this point. |
Conception Date vs LMP: Key Difference
Your LMP date is the first day of your last period. This is how doctors calculate pregnancy weeks. Your conception date is when fertilisation occurred, about 14 days after LMP. If you’re “8 weeks pregnant,” your baby was conceived approximately 6 weeks ago. This 2-week discrepancy is entirely normal: pregnancy is simply dated from LMP by convention.
IVF Due Date Calculation
IVF pregnancies are dated precisely from the transfer date and embryo age: Day 3 embryo = Transfer + 263 days. Day 5 blastocyst = Transfer + 261 days. Day 6 blastocyst = Transfer + 260 days. IVF due dates are often more precise than LMP-based dates because the fertilisation moment is known exactly.
3 Real-Life Examples
These walk through the calculator’s four modes with realistic numbers. Enter the same values yourself to see matching output.
| Situation | Inputs | Result | What it means |
|---|---|---|---|
| Regular 28-day cycle, tracking from LMP | LMP: February 10. Cycle: 28 days. | Due date: November 17. Conception: February 24. | No cycle adjustment needed since 28 days is the baseline. She’ll bring this date to her first prenatal visit for ultrasound confirmation, per standard care. |
| Longer 35-day cycle, planning ahead | LMP: May 1. Cycle: 35 days. | Due date: February 12 (280 + 7 days). Conception: May 22. | The 7-day cycle adjustment pushes both the conception estimate and the due date a week later than a plain 28-day calculation would suggest, since her ovulation happens later in a longer cycle. |
| IVF day 5 blastocyst transfer | Transfer date: September 3. Embryo age: 5 days. | Due date = September 3 + (266 − 5) = September 3 + 261 days = May 22. | Because the embryo’s exact age is known, this date carries less uncertainty than an LMP-based estimate and typically isn’t revised by early ultrasound the way natural-conception dates sometimes are. |
These are illustrative calculations, not medical guidance for any specific pregnancy. Your healthcare provider will confirm and, if needed, adjust your due date based on clinical exam and ultrasound findings.
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Important Notes
- These are estimates, not predictions. No calendar formula can tell you the exact day labour will start. It gives you the most statistically likely date within a normal range.
- Rounding. All dates round to the nearest whole day. Weeks and days shown (“Week 24, Day 3”) are calculated from midnight on the entered date.
- Terminology has changed. ACOG and the Society for Maternal-Fetal Medicine now define early term (37 to 38 weeks 6 days), full term (39 to 40 weeks 6 days), late term (41 weeks), and postterm (42 weeks and beyond), replacing the older blanket “term” label.
- Multiples date differently. Twin and higher-order pregnancies are commonly delivered before 40 weeks; this calculator’s dates apply to a singleton pregnancy timeline.
- Ultrasound can revise the date. First-trimester ultrasound measurements often take precedence over LMP-based dating when the two disagree by more than a few days.
- Not a substitute for prenatal care. Only a healthcare provider can confirm gestational age, monitor pregnancy health, and adjust a due date based on clinical findings.
- Data privacy. All calculations run in your browser. Your dates aren’t sent to a server, and the PDF is generated locally on your device.
Frequently Asked Questions
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