How is a due date calculated?
The calculator uses Naegele’s rule: a pregnancy lasts on average 280 days (40 weeks) from the first day of the last menstrual period.
Estimated due date = first day of last period + 280 days + (cycle length − 28 days)
The last part is the cycle-length correction. Naegele’s rule assumes ovulation about 14 days after the period starts, as in a 28-day cycle. With a longer cycle ovulation happens later, so the due date moves later. With a shorter cycle it moves earlier. If you leave the cycle length empty, 28 days is used and no correction is applied.
Weeks pregnant today are counted from the same date, with the same correction, and shown as weeks plus days. The trimester follows from the weeks.
How to use the due date calculator
- Enter the First day of last period: the day bleeding started, not the day it ended.
- If your cycle is usually not 28 days, enter your Usual cycle length (optional), from 21 to 45 days. Otherwise leave it empty.
- Read your Estimated due date, how far along you are (Pregnant today) and your Trimester.
- If the date you entered is in the future, or puts the pregnancy at 42 weeks or more, the calculator tells you so. It’s worth checking the date.
Worked examples
Example 1 – regular 28-day cycle. Last period started Thursday, 1 January 2026. Due date: 1 January 2026 + 280 days = Thursday, 8 October 2026. On 12 March 2026 you would be 10 weeks 0 days pregnant, in the 1st trimester.
Example 2 – longer 32-day cycle. Last period started Sunday, 1 February 2026. Correction: 32 − 28 = +4 days. Due date: 1 February 2026 + 280 + 4 = Thursday, 12 November 2026. On 28 October 2026 you would be 37 weeks 6 days pregnant, in the 3rd trimester. Without the correction, the calculator would show 38 weeks 3 days and a due date four days earlier.
Example 3 – shorter 25-day cycle. Last period started Wednesday, 1 July 2026. Correction: 25 − 28 = −3 days. Due date: 1 July 2026 + 280 − 3 = Sunday, 4 April 2027. On 28 October 2026 you would be 17 weeks 3 days pregnant, in the 2nd trimester.
Trimesters and key weeks
| Weeks pregnant | Stage |
|---|---|
| 0+0 to 13+6 | 1st trimester |
| 14+0 to 27+6 | 2nd trimester |
| 28+0 onwards | 3rd trimester |
| before 37+0 | Preterm |
| 37+0 to 38+6 | Early term |
| 39+0 to 40+6 | Full term |
| 40+0 | Estimated due date |
| 41+0 to 41+6 | Late term |
| 42+0 and beyond | Post-term |
“13+6” means 13 weeks and 6 days. Not every source draws the trimester lines in the same place. Some end the first trimester at 12 weeks and the second at 26 or 27. The differences are small and do not change your care.
Limits
- Due dates are approximate. A birth anywhere from 37+0 to 41+6 counts as term, so think of a due window rather than a single day.
- Early ultrasound is more accurate. Measuring the baby in the first trimester (crown–rump length) dates a pregnancy to within about 5–7 days, better than the last period. Your official due date may be updated after that scan.
- Irregular or unknown cycles. Recent hormonal contraception, breastfeeding or a recent pregnancy can make the last period a poor guide.
- Assisted conception. IVF dates come from the embryo transfer, not from the last period.
- Not a medical assessment. If you think you are pregnant, we recommend contacting your GP, midwife or doctor to arrange antenatal care.
Your weight before pregnancy also matters for antenatal care, and the BMI calculator shows where it sits. After birth, midwives check the baby’s condition with the Apgar score.
Common misconceptions
- “The baby will come on the due date.” Only a small share do. Arriving a week or two either side is completely normal.
- “Pregnancy lasts nine months.” 40 weeks from the last period is closer to nine months and one week.
- “Weeks pregnant start at conception.” They start from the last period, about two weeks before conception in a typical cycle.
- “Adding 9 months and 7 days gives the same date.” The month shortcut (last period + 7 days − 3 months + 1 year) can differ from 280 days by a day or more, because months have different lengths. The calculator counts the days exactly.
Frequently asked questions
How is a due date calculated?
By adding 280 days (40 weeks) to the first day of your last period. This is Naegele’s rule, the standard method. If your cycle is usually longer or shorter than 28 days, the calculator moves the date later or earlier by the difference. A 32-day cycle, for example, adds four days.
What does EDD mean?
EDD stands for estimated date of delivery, the day you reach 40 weeks of pregnancy. You may also see EDC or simply due date; in Italy it is called DPP. It is called estimated for a reason: birth can quite normally happen a few days or weeks before or after it.
How accurate is a due date?
It is an estimate, and few babies arrive on the exact day. A date from the last period is less accurate than one from an early ultrasound scan. This is especially true if your cycles are irregular or you are unsure of the date.
How many weeks pregnant am I?
Count from the first day of your last period, not from conception. Enter that date and the calculator shows how far along you are today in weeks and days, for example 10 weeks 0 days. Midwives and doctors often write this as 10+0 in your notes.
Why am I already 2 weeks pregnant at conception?
Because the count starts at your last period, and ovulation usually happens about two weeks later in a 28-day cycle. This convention is used worldwide. It means your weeks match the ones your midwife or doctor uses, and the ones printed on scan reports.
What if my periods are irregular or I don’t know the date?
Then the calculation is less reliable. Recent hormonal contraception, breastfeeding or a recent pregnancy can all blur the date. An early dating scan, ideally in the first trimester, gives a more accurate due date. That is the date your care team will normally use.
Can the due date change after an ultrasound?
Yes. If the baby’s measurements on an early scan point to a date more than about a week away from the date by period, the scan date is usually adopted. Once set by an early scan, the due date is not normally changed by later scans, even if the baby measures big or small.
What counts as full term?
A baby born from 37 weeks 0 days onwards is considered term; before that, the birth is preterm. Doctors further describe 37 to 38 weeks as early term, 39 to 40 weeks as full term, 41 weeks as late term, and 42 weeks or more as post-term.
What if I conceived by IVF?
Your clinic will date the pregnancy from the embryo transfer instead. It uses the transfer date and the age of the embryo, which is more precise than the last period. We recommend using the date the clinic gives you, because this calculator is based on the last period only.
Sources
- ACOG Committee Opinion No. 700 – Methods for Estimating the Due Date (2017)
- NHS – Pregnancy due date calculator
- ISSalute – Gravidanza: controlli, disturbi e suggerimenti
- ACOG Committee Opinion No. 579 – Definition of term pregnancy. Obstet Gynecol 2013;122(5):1139–1140
- WHO – Recommendations on antenatal care for a positive pregnancy experience (2016)