Due Date Calculator: How It Works
A due date is a single day standing in for a range. Only about 4% of babies arrive on it, and roughly 90% arrive within two weeks either side. Knowing how the date is calculated — and how it gets revised — makes the number far more useful than treating it as a deadline.
Naegele's rule
The standard calculation starts from the first day of your last menstrual period (LMP):
Due date = LMP + 280 days
Equivalently: take the LMP, add one year, subtract three months, add seven days. An LMP of 10 March gives a due date of 17 December.
The rule assumes a 28-day cycle with ovulation on day 14. Both assumptions are averages rather than facts about any individual, which is the main source of error in LMP-based dating.
Adjusting for cycle length
If your cycles are consistently longer or shorter than 28 days, ovulation shifts and the due date shifts with it:
Adjusted due date = LMP + 280 + (cycle length − 28) days
| Cycle length | Adjustment |
|---|---|
| 24 days | 4 days earlier |
| 28 days | No change |
| 32 days | 4 days later |
| 35 days | 7 days later |
Other dating methods
| Known date | Add |
|---|---|
| Conception or ovulation date | 266 days |
| IVF day-3 embryo transfer | 263 days |
| IVF day-5 blastocyst transfer | 261 days |
IVF dates are the most precise available, because the timing of fertilisation is known exactly rather than inferred.
Why ultrasound usually wins
An ultrasound in the first trimester measures crown-rump length, and early embryonic growth is remarkably consistent between pregnancies. Before about 14 weeks, this dating is accurate to within roughly five to seven days.
| When scanned | Accuracy |
|---|---|
| 8 – 13 weeks | ± 5 – 7 days — most reliable |
| 14 – 20 weeks | ± 7 – 10 days |
| 20 – 28 weeks | ± 2 weeks |
| After 28 weeks | ± 3 weeks or more |
Clinicians generally revise an LMP-based date if an early scan differs by more than about five to seven days, because growth variation between individual babies increases later in pregnancy — a large baby at 30 weeks may simply be large rather than further along.
The terminology
| Term | Weeks |
|---|---|
| Preterm | Before 37 |
| Early term | 37 0/7 – 38 6/7 |
| Full term | 39 0/7 – 40 6/7 |
| Late term | 41 0/7 – 41 6/7 |
| Post term | 42 0/7 and beyond |
Why weeks are counted from a date before conception
Gestational age is measured from the LMP, which precedes conception by roughly two weeks. So at the moment of conception you are already 'two weeks pregnant', and a positive test typically appears around week four. This convention exists because the LMP is a date most people can identify, while conception usually is not. Pregnancy is therefore described as 40 weeks while the embryo has existed for about 38.
This page is general information, not medical advice. Your midwife or doctor's dating, based on your history and scans, takes precedence over any calculator. Contact your maternity care provider with any concerns about your pregnancy.