Step 1
Write down day one
Note the first day of your last period, for example March 3, 2026.
How to calculate your due date
Most due dates come from one sum: the first day of your last period plus 280 days. Here is where that number comes from, when it shifts, and what an early scan does to it.
The short answer
Take the first day of your last period and add 280 days, which is 40 weeks. That date is your estimated due date. If you know the day you conceived, 266 days from then gives about the same date.
The count assumes you ovulated on day 14, so it starts about 2 weeks before conception. By the time a period is late, you're already around 4 weeks pregnant.[1][3]
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]
Step 2
Add 1 year, take away 3 months
March 3, 2026 becomes December 3, 2026.
Step 3
Add 7 days
December 3 becomes December 10, 2026. In the app you enter the date once and Today shows your week.
Naegele's rule by hand
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.
Questions
How is a due date calculated?
Add 280 days (40 weeks) to the first day of your last period. That's Naegele's rule, and it assumes a regular 28-day cycle with ovulation on day 14.
Can I work out my due date from the day I conceived?
Roughly, yes. The 280-day rule assumes you ovulated on day 14, so 266 days (38 weeks) from conception lands on the same date. After IVF, the date comes from the embryo transfer instead.
Why did my due date change after the scan?
An early ultrasound measures the baby, which gives a more accurate date than your period. If the two differ by more than ACOG's cut-off (5 days before 9 weeks, 7 days from 9 to 14 weeks), your care team goes by the scan.
How many babies are born on their due date?
Very few: an NHS hospital leaflet puts it at 4.4%. Babies can arrive any time from 37 to 42 weeks, so the date is better read as the middle of a window.
Sources
- ACOG: Committee Opinion 700: Methods for estimating the due date Checked October 30, 2026
- ACOG: Committee Opinion 579: Definition of term pregnancy Checked October 30, 2026
- NHS: Due date calculator Checked October 30, 2026
- Royal Berkshire NHS Foundation Trust: How do I work out when my baby is due? (patient leaflet) Checked October 30, 2026
Spotted an error? Write to [email protected]. How we check health information.