How a Due Date Is Estimated
Pregnancy is counted from the first day of the last menstrual period, not from conception. This is a historical convention that predates any means of detecting ovulation, and it persists because the date of a period is something a patient can usually report while the date of conception generally is not.
The consequence is that at the moment of conception you are already counted as two weeks pregnant. A "40-week" pregnancy is therefore about 38 weeks of actual gestation.
Naegele's Rule
The standard method, published by the German obstetrician Franz Naegele in 1806:
Due date = first day of last period + 1 year − 3 months + 7 days
which is simply 280 days, or 40 weeks
The rule assumes a 28-day cycle with ovulation on day 14. For longer or shorter cycles the calculator shifts the date accordingly — a 35-day cycle pushes the due date about a week later, because ovulation happens later while the luteal phase stays roughly constant at 14 days.
Dating from Other Starting Points
| Known date | Add | Why |
|---|---|---|
| Last menstrual period | 280 days | The standard convention |
| Conception or ovulation | 266 days | Conception is 14 days after the LMP |
| IVF, day-3 embryo transfer | 263 days | The embryo is already 3 days old |
| IVF, day-5 blastocyst transfer | 261 days | The embryo is already 5 days old |
| Ultrasound measurement | 280 days − measured age | Works backwards to a notional LMP |
IVF dates are the most accurate of all, because the exact age of the embryo is known rather than inferred.
Accuracy of Each Method
| Method | Typical accuracy | Notes |
|---|---|---|
| IVF transfer date | ±1 day | Embryo age is known exactly |
| First-trimester ultrasound (8–13 weeks) | ±5–7 days | The clinical gold standard for dating |
| Known ovulation date | ±3 days | From tracking or an LH test |
| Second-trimester ultrasound | ±10–14 days | Growth variation widens the range |
| Last menstrual period | ±2 weeks | Assumes a regular 28-day cycle |
| Third-trimester ultrasound | ±3 weeks | Too imprecise for dating |
Dating accuracy falls as pregnancy advances because babies grow at increasingly different rates. This is why an early scan sets the dates and later scans do not override them.
How Likely Is the Due Date Itself?
Only about 4% of babies are born on their estimated due date. Roughly:
| Delivery timing | Share of births |
|---|---|
| Before 37 weeks (preterm) | ~10% |
| 37–38 weeks (early term) | ~26% |
| 39–40 weeks (full term) | ~38% |
| 41 weeks (late term) | ~20% |
| 42 weeks or later (post-term) | ~6% |
The due date is better understood as the midpoint of a likely window than as a prediction. Around 80% of births fall within two weeks either side of it.
Trimesters and Key Milestones
| Stage | Weeks | What it marks |
|---|---|---|
| First trimester | 0–13 | Organ formation; highest miscarriage risk, falling sharply after a heartbeat is seen |
| Second trimester | 14–27 | Growth and movement; the anatomy scan is usually at 18–22 weeks |
| Viability | 24 | The point from which survival outside the womb becomes possible with intensive care |
| Third trimester | 28–40 | Rapid weight gain and lung maturation |
| Early term | 37–38 | Delivery is safe but outcomes improve slightly with more time |
| Full term | 39–40 | Optimal timing for planned delivery |
| Late term | 41 | Monitoring usually increases |
| Post-term | 42+ | Induction is normally recommended by this point |
Frequently Asked Questions
My cycle is not 28 days. Does that matter?
Yes, and the calculator adjusts for it. The luteal phase after ovulation is fairly constant at about 14 days, so a longer cycle means later ovulation and a later due date.
Why does my scan give a different date?
Because the LMP method assumes textbook timing. If a first-trimester scan differs from the LMP date by more than about 5–7 days, clinical guidelines say the scan date replaces it.
Can the due date change later in pregnancy?
Once established by an early scan, the due date is not normally revised. Later scans assess growth against the established dates rather than redating the pregnancy.
How many weeks pregnant am I?
The result above shows this as completed weeks and days since the LMP. Note that "12 weeks pregnant" clinically means 12 weeks completed, so you are in your thirteenth week.