The standard method for estimating a due date is Naegele's rule, developed by German obstetrician Franz Karl Naegele in the 19th century and still used worldwide today. It assumes a 28-day menstrual cycle with ovulation on day 14.
The formula:
Or equivalently: take the first day of your LMP, subtract 3 months, add 1 year, and add 7 days.
Example: LMP = 1 January 2026 → Due date = 8 October 2026.
This is the most confusing aspect of pregnancy dating for most people. Pregnancy weeks are counted from the first day of the last menstrual period, which typically precedes ovulation and conception by about 2 weeks.
Why? Because the LMP is a reliable known date, whereas conception often cannot be pinpointed precisely. When a doctor says you're "6 weeks pregnant," the embryo is typically around 4 weeks old from conception.
This means weeks 1 and 2 of "pregnancy" actually occur before conception — they represent the phase between the previous period and ovulation.
| Trimester | Weeks | Key Developments |
|---|---|---|
| First trimester | Weeks 1–12 | All major organs form; risk of miscarriage highest; morning sickness typical; first scan at ~12 weeks |
| Second trimester | Weeks 13–26 | Movement felt (~18–22 weeks); anomaly scan at ~20 weeks; generally most comfortable trimester |
| Third trimester | Weeks 27–40 | Rapid growth and weight gain; baby positions for birth; full term from week 37 |
| Week | Milestone |
|---|---|
| 4 | Typical timing of first positive pregnancy test |
| 6 | Heartbeat detectable on ultrasound |
| 8 | All major organs are forming; embryo becomes a foetus |
| 12 | End of first trimester; miscarriage risk drops significantly; dating scan |
| 16–20 | Quickening — first fetal movements felt |
| 20 | Anomaly scan (structural survey) |
| 24 | Threshold of viability (with medical support) |
| 28 | Start of third trimester |
| 37 | Full term — delivery at any point after this is considered term |
| 40 | Estimated Due Date (EDD) |
| 42 | Post-term; induction typically offered |
Only about 4–5% of babies are born on their exact EDD. This surprises most people who treat the due date as a specific target. In reality it is the midpoint of a normal distribution:
The EDD is better understood as a planning date and clinical reference point — not a prediction of when labour will start.
If an early ultrasound (usually at 8–12 weeks) gives a gestational age that differs from the LMP-calculated date by more than 5–7 days, most guidelines (including NICE in the UK) recommend adjusting the EDD to the ultrasound date. This is because:
In cycles longer than 28 days (e.g. a 35-day cycle), the EDD calculated from LMP will typically be about 7 days earlier than the actual EDD — the baby will tend to arrive around a week "late" by LMP dating because ovulation happened a week later than assumed.
Naegele's rule assumes a 28-day cycle. If your cycle is different, the EDD should be adjusted:
Our due date calculator accepts custom cycle lengths to adjust for this automatically.
Only about 4–5% of babies arrive on their exact estimated due date. Around 80% are born within the two-week window either side (weeks 38–42). The EDD is a clinical reference point and planning tool, not a precise prediction. An early ultrasound scan (before 14 weeks) is more accurate than LMP dating and will be used by your care team to set the official EDD if there is a discrepancy.
From the first day of the last menstrual period (LMP), not from conception. This is a convention based on the fact that the LMP is a reliable known date, whereas the exact date of conception often cannot be confirmed. At the time of a positive test (around week 4), the embryo is typically about 2 weeks old from conception but counted as 4 weeks pregnant by gestational age.
Gestational age counts from the LMP — the standard medical measure used in all clinical contexts and on your scan reports. Fetal age (embryonic age) counts from conception and is typically 2 weeks less. When your midwife says "you're 10 weeks," that is gestational age; the embryo is approximately 8 weeks old from fertilisation.
Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer, ovulation occurs later, and the EDD should be pushed forward accordingly. A 35-day cycle would move the EDD 7 days later than the standard calculation. Our calculator lets you enter your cycle length to adjust for this. If there is significant uncertainty, your 12-week dating scan will provide the most reliable estimate.