Naegele's rule, step by step
Naegele's rule is the hand version of adding 280 days, and most due dates have come from it for about 200 years:[4]
- Write down the first day of your last period, for example March 3, 2026.
- Add 1 year and take away 3 months: December 3, 2026.
- Add 7 days: December 10, 2026.
Because months have different lengths, the shortcut can land a day or two away from an exact count. Adding 280 days to March 3, 2026 gives December 8, 2026, and that exact count is what apps and calculators use.
The due date calculator does the same sum and shows how many weeks along you are today.
What if my cycle isn't 28 days?
The 280-day rule assumes a regular 28-day cycle with ovulation on day 14.[1][4] That's why due date calculators, the NHS one included, ask for your cycle length and tell you to adjust it if yours is shorter or longer.[3] Ours moves the date by the difference from 28 days:
| Your usual cycle | Change our calculator makes |
|---|---|
| 24 days | 4 days earlier |
| 28 days | no change |
| 32 days | 4 days later |
| 35 days | 7 days later |
If you don't know the date of your last period, or aren't sure of it, the NHS suggests asking your midwife or doctor. The scan at around 12 weeks gives a more accurate estimate anyway.[3]
From a conception date or IVF
- Known conception date: add 266 days (the 280-day rule minus the 14 days it assumes before ovulation).
- IVF, day-5 embryo transfer: add 261 days to the transfer date.
- IVF, day-3 embryo transfer: add 263 days to the transfer date.
The IVF figures are ACOG's: after assisted reproduction, the due date comes from the age of the embryo and the transfer date.[1]
Why a scan can move the date
An early ultrasound measures the baby, and that dates the pregnancy more accurately than a remembered period.[3][4] ACOG's guidance is to change the due date when an early scan and the period date disagree by more than a set number of days: 5 days before 9 weeks, 7 days from 9 weeks to the end of week 13.[1]
Once a date is set from a first-trimester scan, ACOG says a later scan shouldn't change it, with rare exceptions.[1]
Early, full and late term
| Term (ACOG) | Weeks |
|---|---|
| Early term | 37 weeks 0 days to 38 weeks 6 days |
| Full term | 39 weeks 0 days to 40 weeks 6 days |
| Late term | 41 weeks 0 days to 41 weeks 6 days |
| Post term | 42 weeks and later |
These are ACOG's labels.[2] Your due date sits in the middle of full term, and few babies arrive on the day itself: an NHS hospital leaflet puts it at 4.4%, with most born by 41 weeks.[4]
Keeping the weeks in the app
When you tell Her Good Days you're pregnant, the Today screen shows your week and how many weeks are left, next to appointments and symptoms you log. If your care team changes the date after a scan, you change it in the app and the weeks update. More on pregnancy in the app.