An estimate only, not medical adviceYour midwife or doctor confirms your due date, often with an ultrasound scan, and it may change. Most babies are born before or after it, not on it. If you have bleeding, severe pain, fluid leaking, or your baby is moving less than usual, contact your midwife, doctor or local emergency service straight away.
The short answer
A due date calculator estimates when your baby is due by adding 280 days (40 weeks) to the first day of your last period, adjusted if your cycle is not 28 days. You can also start from a conception date, an IVF embryo transfer or an ultrasound scan. Only an estimate, confirmed by your midwife or doctor.
Formulas and sources are named on this page. How we calculate.
How this is calculated
A typical pregnancy lasts about 280 days, which is 40 weeks, counted from the first day of your last menstrual period (LMP). Naegele's rule is the usual shortcut: subtract three months from that date and add seven days. The calculator simply adds 280 days, which gives the same answer.
Due date = first day of last period + 280 days (+ cycle length − 28 days)
Due date = conception date + 266 days
Due date = embryo transfer date + 261 days (5-day embryo) or + 263 days (3-day embryo)
Due date = scan date + (280 − gestational age on the scan, in days)
Worked examples
Last period began 1 January 2026, 28-day cycle. 1 January + 280 days = 8 October 2026. Naegele's rule agrees: 1 January − 3 months = 1 October, + 7 days = 8 October.
The same last period, but a 26-day cycle. Ovulation, and so conception, probably happened two days earlier than in a 28-day cycle, so the due date moves back two days: 8 October − 2 days = 6 October 2026.
Conceived on 15 January 2026. 15 January + 266 days = 8 October 2026.
5-day embryo transferred on 20 January 2026. 20 January + 261 days = 8 October 2026.
Scan on 1 March 2026 showing 8 weeks 3 days (59 days). 1 March + (280 − 59) = 1 March + 221 days = 8 October 2026.
Why the cycle length matters
Counting from the last period assumes ovulation about 14 days later, as in a 28-day cycle. If your cycles are longer, ovulation usually comes later, so the calculator adds the extra days (and takes them off for shorter cycles). This is sometimes called Parikh's adjustment. It helps only when your cycles are regular. If your cycles are irregular or you are unsure of your last period, an early ultrasound dates the pregnancy more reliably.
Why scans can change the date
ACOG advises that an early ultrasound measurement is usually the most accurate way to date a pregnancy. If a scan and your last period give different dates, your care team decides which to use, and they will tell you your confirmed due date. Use the one they give you.
Gestational age, not baby age
"Weeks pregnant" counts from the first day of your last period, about two weeks before conception. That is why a baby conceived on the 14th day of your cycle is already 2 weeks pregnant on paper.
Trimesters and term definitions
Pregnancy stages by gestational age (ACOG)| Stage | Gestational age |
|---|
| 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 |
| Preterm | Before 37 weeks 0 days |
| 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 0 days and later |
The NHS describes a normal pregnancy as lasting 37 to 42 weeks from the first day of the last period.
Frequently asked questions
How is my due date calculated?
Standard dating adds 280 days (40 weeks) to the first day of your last period. This is the same as Naegele's rule: subtract three months and add seven days. The calculator adjusts it if your cycle is longer or shorter than 28 days, and can also work from a conception date, IVF transfer or ultrasound scan.
How accurate is a due date?
It is an estimate. Pregnancy normally lasts from 37 to 42 weeks, and only a minority of babies arrive on the exact due date. Early ultrasound scans usually give the most accurate dating, and your midwife or doctor will confirm or change your date.
What if my cycle is not 28 days or is irregular?
For regular cycles, enter your average cycle length and the calculator adjusts the date by the difference from 28 days. If your cycles are irregular, you are not sure of your last period, or you conceived soon after stopping hormonal contraception, dating from the last period is less reliable, and a scan is the better guide.
How do I work out a due date after IVF?
Choose the IVF option and enter the transfer date. The calculator adds 261 days for a 5-day embryo (blastocyst) or 263 days for a 3-day embryo, because the embryo is already that many days old. For a fresh cycle you can also use 266 days from egg retrieval or fertilisation.
What do early term, full term and late term mean?
ACOG defines early term as 37 weeks 0 days to 38 weeks 6 days, full term as 39 weeks 0 days to 40 weeks 6 days, late term as 41 weeks 0 days to 41 weeks 6 days, and post-term as 42 weeks 0 days or later. Babies born before 37 weeks are preterm.
When does each trimester start?
The first trimester runs to 13 weeks 6 days, the second begins at 14 weeks 0 days and the third at 28 weeks 0 days. The result panel shows the exact date each begins for your due date.
Disclaimer and editorial note
For information only, not medical advice. This calculator gives general estimates from the numbers you enter. It cannot diagnose, treat or prevent any condition and does not replace advice from a doctor, midwife, registered dietitian or other qualified professional who knows your history.
This calculator does not diagnose or confirm a pregnancy. If you think you may be pregnant, take a pregnancy test and speak with a midwife or doctor, who can arrange a dating scan and your antenatal care.
Written and maintained by the HealthFitnessHQ editorial team. We name the source of every formula and category used, and we review each calculator when guidance changes. Read how we work in our editorial policy and methodology.