How your due date is worked out
A full-term pregnancy is counted as 40 weeks, or 280 days, from the first day of your last menstrual period (LMP). That sounds odd, because you are not pregnant for the first two weeks of that time. But almost everyone knows when their last period started, and very few people know the exact day they conceived. So doctors and midwives count from the period, and conception is taken to happen about two weeks later.
This rule is named after Franz Naegele, a German obstetrician of the early 1800s. The calculator uses the day-count version, which is exact in every month and in leap years.
The adjustment matters if your cycles are regular but not 28 days long. Ovulation usually happens about 14 days before the next period, so a 35-day cycle moves ovulation, and the due date, about 7 days later.
Other ways to date a pregnancy
| You know | Due date | Why |
|---|---|---|
| First day of last period | LMP + 280 days (± cycle adjustment) | Naegele’s rule |
| Conception date | Conception + 266 days | Conception counts as 2 weeks 0 days |
| IVF, day-3 embryo transfer | Transfer + 263 days | 266 days minus the embryo’s 3 days |
| IVF, day-5 blastocyst transfer | Transfer + 261 days | 266 days minus 5 days |
| IVF, day-6 blastocyst transfer | Transfer + 260 days | 266 days minus 6 days |
| Scan result, e.g. 12 weeks 2 days | Scan date + (280 − 86) days | Counts forward from the measured age |
The American College of Obstetricians and Gynecologists (ACOG), together with ultrasound and maternal-fetal medicine societies, recommends dating IVF pregnancies from the embryo’s age and the transfer date, because both are known precisely. For other pregnancies, ACOG says an ultrasound in the first trimester is the most accurate way to confirm or set the due date. If the scan and the period-based date differ by more than a set number of days, your care team will usually go with the scan.
Worked examples
From the last period. Your last period started on 1 January 2026 and your cycles are 28 days. Add 280 days: the due date is Thursday 8 October 2026. On 28 March 2026 you would be 86 days along, which is 12 weeks 2 days.
Longer cycles. Same start date, but your cycles are 35 days. Add 280 + 7 = 287 days: the due date moves to 15 October 2026.
IVF. A day-5 blastocyst was transferred on 20 January 2026. Add 261 days: the due date is again 8 October 2026. On the transfer day the pregnancy is already counted as 2 weeks 5 days.
Weeks, trimesters and “term”
Pregnancy is described in completed weeks and days: “12 weeks 2 days” (written 12w 2d) means 12 full weeks plus 2 days. This calculator uses the common trimester split:
- First trimester: up to 13 weeks 6 days.
- Second trimester: 14 weeks 0 days to 27 weeks 6 days.
- Third trimester: 28 weeks 0 days until birth.
Some services, including the NHS, describe the first trimester as weeks 1 to 12, so you may see slightly different splits.
ACOG and the Society for Maternal-Fetal Medicine define the end of pregnancy more precisely:
| Label | Gestational age |
|---|---|
| Early term | 37w 0d to 38w 6d |
| Full term | 39w 0d to 40w 6d |
| Late term | 41w 0d to 41w 6d |
| Post-term | 42w 0d or later |
The NHS notes that a pregnancy usually lasts from 37 to 42 weeks. Your due date is the middle of that range, not a deadline, and only a small share of babies are born on the exact day.
Scans and key dates in the UK
The NHS offers a dating scan at around 10 to 14 weeks and a mid-pregnancy (anomaly) scan usually between 18 and 21 weeks. The calculator shows the dates those windows fall on for you, along with the start of each trimester and the term dates above. Other countries use similar schedules, but follow the plan your own maternity service gives you.
Tips
- Enter the day your period started, not the day it ended.
- If your cycles are irregular, or you were recently on hormonal contraception or breastfeeding, the period-based date is less reliable. An early scan will give a better estimate.
- Once you have a scan date, choose “I know my due date” to follow the week-by-week calendar from it.
- Nothing you enter leaves your browser.
If you are trying to conceive, the ovulation calculator estimates your fertile days. To count days between any two dates, use the date calculator. Staying well hydrated matters more in pregnancy; the water intake calculator shows the reference amounts.
This calculator gives estimates for information only. It does not replace care from your midwife or doctor, and you should contact them about any symptoms or concerns.