How it works
This is a forward-looking maternity tool. You supply the date your most recent menstrual bleed began plus your typical cycle cadence, and the calculator projects ahead to your baby's estimated arrival day — the figure obstetricians call the EDD (Estimated Date of Delivery).
Most expectant parents first encounter their EDD at an early prenatal appointment, but this calculator lets you generate that same projection at home the moment you suspect you are pregnant. The output anchors the rest of your maternity planning: when to schedule the 12-week scan, when to start antenatal classes, when to pack a hospital bag, and roughly when to arrange maternity leave at work.
The formula
Due date = LMP + 280 days (+ cycle adjustment)
This is Naegele's rule, the obstetric standard since the 1800s. It counts forward 280 days from the first day of your last menstrual period (LMP). Because the rule assumes a textbook 28-day cycle, the calculator applies a correction when your actual cycle length differs: it adds or subtracts the difference between your cycle and 28 days.
Worked example
Suppose your last menstrual period began on 2024-03-01 and your average cycle runs 30 days.
Standard Naegele base: 2024-03-01 + 280 days = 2024-12-06
Cycle adjustment: 30 − 28 = +2 days
Adjusted EDD: 2024-12-06 + 2 days = 2024-12-08
So your estimated delivery date lands on 8 December 2024. The 30-day cycle pushes ovulation slightly later than the textbook assumption, which shifts the EDD by two days.
| Cycle length | Adjustment | Effect on EDD |
|---|---|---|
| 28 days | 0 | No change |
| 35 days | +7 | One week later |
| 21 days | −7 | One week earlier |
Things to watch
Only around 5% of babies show up on their exact projected day. The EDD is a midpoint of a normal window — spontaneous labour most commonly begins between 37 and 42 weeks of gestation. Treat the date as a planning anchor, not a firm booking.
A few specific pitfalls can throw off the result:
- Misremembering the LMP. Stress, travel, or coming off hormonal contraception can make the start of your last bleed easy to date incorrectly. A pregnancy test, implantation bleeding, or a withdrawal bleed after stopping the pill can all be mistaken for a true period.
- Irregular cycles. If your cycle length swings from 24 to 38 days, a single average loses meaning. In that scenario a first-trimester ultrasound — measuring the fetus crown-to-rump — produces a more dependable EDD.
- Post-implantation spotting. Light bleeding around the time you would expect a period can be misread as menstruation, pushing the LMP — and therefore the EDD — out by weeks.
This calculator produces an estimate, not professional medical advice. Your midwife or obstetrician will refine the date using ultrasound measurements and clinical assessment at your booking visit.