Pregnancy Due Date: How the LMP Calculator Works
Most pregnancy due date calculators use Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period. But why 40 weeks from a period that happened before conception? And when does an ultrasound date override the LMP? Full explanation here.
Naegele's Rule: The Core Formula
Every standard pregnancy due date calculation β whether from your GP, NHS app, or online calculator β is based on Naegele's Rule, published by German obstetrician Franz Naegele in 1812 and still the international standard:
EDD = LMP + 280 days
Or equivalently: EDD = LMP + 9 months + 7 days (the calendar approximation)
Step-by-step example
- LMP starts: 1 March 2026
- Add 280 days (or 40 weeks)
- EDD = 5 December 2026
That's it. No complex calculation needed β the formula is embedded in every pregnancy wheel, app, and clinical system.
Why 40 Weeks From the Last Period?
This is the question that confuses most people. If conception happens around day 14 of the cycle (ovulation), why is the pregnancy dated from day 1 of the period β two weeks earlier?
The answer is historical: in the 19th century, women knew when their period started but not when conception occurred. LMP was the only consistent reference point. The formula counts from LMP to give a 40-week gestational age, which corresponds to approximately 38 weeks from conception.
| Event | Weeks from LMP | Weeks from conception |
|---|---|---|
| LMP begins | 0 | β2 |
| Ovulation / conception | 2 | 0 |
| Implantation | 3β4 | 1β2 |
| Positive pregnancy test | 4β5 | 2β3 |
| Dating scan (nuchal) | 11β13 | 9β11 |
| Due date | 40 | 38 |
This is why when a doctor says "you're 8 weeks pregnant," they mean 8 weeks from LMP β the embryo itself may only be ~6 weeks old.
Adjusting for Non-Standard Cycles
Naegele's Rule assumes a 28-day cycle with ovulation on day 14. If your cycle is different, the LMP-based EDD will be off.
Adjusted EDD = LMP + 280 days + (cycle length β 28)
| Cycle length | Adjustment | Effect on EDD |
|---|---|---|
| 21 days | β7 days | Due date 7 days earlier |
| 25 days | β3 days | Due date 3 days earlier |
| 28 days | 0 | Standard EDD |
| 30 days | +2 days | Due date 2 days later |
| 35 days | +7 days | Due date 7 days later |
| 40 days | +12 days | Due date 12 days later |
Women with polycystic ovary syndrome (PCOS) or highly irregular cycles should note that their LMP-based date may be significantly off. The dating ultrasound becomes particularly important in these cases.
Trimesters: How Pregnancy Is Divided
Pregnancy is conventionally divided into three trimesters based on gestational age (weeks from LMP):
| Trimester | Weeks | Key developments |
|---|---|---|
| First trimester | Weeks 1β12 | Organ formation, highest miscarriage risk |
| Second trimester | Weeks 13β27 | Rapid growth, fetal movement |
| Third trimester | Weeks 28β40+ | Lung maturation, final growth, birth preparation |
Gestational age milestones (NHS):
- Extremely preterm: Before 28 weeks
- Very preterm: 28β31 weeks
- Moderate to late preterm: 32β36 weeks
- Early term: 37β38 weeks
- Full term: 39β40 weeks (the target window)
- Late term: 41 weeks
- Post-term: 42 weeks and beyond (induction typically offered at 41β42 weeks)
The Dating Scan: When Ultrasound Overrides LMP
The 11β14 week dating scan (offered to all pregnant women on the NHS as part of the combined screening test) measures the fetus's crown-rump length (CRL) to estimate gestational age with much greater precision than LMP.
How the scan date is used
| Difference between scan and LMP | What happens |
|---|---|
| β€5 days different | LMP date retained as EDD |
| >5β7 days different | Scan date becomes the new EDD |
| LMP uncertain/irregular cycle | Scan date always used |
Earlier scans (before 10 weeks) are even more accurate for dating because the rate of fetal growth in the first trimester is relatively consistent across pregnancies. After 14 weeks, variation in individual fetal size makes dating less reliable.
Why the scan might give a different date
- Your cycle was longer or shorter than 28 days
- Ovulation happened earlier or later than day 14
- The LMP date was estimated or recalled incorrectly
- Implantation variation
NHS IVF / Assisted Conception Dates
For pregnancies achieved via IVF, the due date calculation is different:
| Transfer type | How EDD is calculated |
|---|---|
| Fresh transfer (day 5 blastocyst) | Transfer date + 266 days (38 weeks from conception) |
| Frozen transfer (day 5 blastocyst) | Transfer date + 266 days |
| Day 3 transfer | Transfer date + 268 days |
IVF due dates are more precise because the exact date of fertilisation (or transfer) is known. No LMP adjustment is needed. The same gestational age milestones apply.
How Accurate Is the Due Date?
The EDD is an estimate, not a deadline. Statistical distribution of births around the EDD:
| Timing | Approximate percentage |
|---|---|
| On the exact due date | ~4% |
| Within Β±3 days | ~30% |
| Within Β±1 week | ~70% |
| Within Β±2 weeks (37β42 weeks) | ~95% |
| Before 37 weeks (preterm) | ~8% |
| After 42 weeks (post-term) | ~5% |
The NHS typically offers induction at 41β42 weeks to avoid risks associated with post-term pregnancy (placental deterioration, increased stillbirth risk). This is offered as a choice, not mandatory.
UK Antenatal Appointments by Gestational Week
| Appointment | Gestational age | What happens |
|---|---|---|
| Booking appointment | 8β12 weeks | History, blood tests, referral for scan |
| Dating scan | 11β14 weeks | CRL measurement, nuchal fold, Down's screening |
| Midwife appointment | 16 weeks | Results, blood pressure |
| Anomaly scan | 18β21 weeks | Structural abnormalities screening |
| 25 weeks (first-time parents) | 25 weeks | Growth, BP, urine |
| 28 weeks | 28 weeks | Blood tests (glucose, Hb), anti-D if Rhβ |
| 31 weeks (first-time) | 31 weeks | |
| 34 weeks | 34 weeks | |
| 36 weeks | 36 weeks | Position check |
| 38 weeks | 38 weeks | |
| 40 weeks (first-time) | 40 weeks | Membrane sweep offered |
| 41 weeks | 41 weeks | Induction discussion |
Second-time parents have fewer routine appointments in second trimester.
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