How is a due date calculated?
A pregnancy normally lasts about 40 weeks (280 days) counted from the first day of your last menstrual period (LMP) — not from the day you conceived. That is because most people know when their period started, but not exactly when ovulation and conception happened. Conception usually takes place around two weeks after the period starts, so at the moment of conception you are already counted as “2 weeks pregnant”.
This free due date calculator uses the same rules doctors and midwives use, and also works from a known conception date or an IVF embryo transfer date.
How the due date calculator works
- From your last period (Naegele’s rule): due date = first day of LMP + 280 days. If your cycle is longer or shorter than 28 days, the date moves by the difference — for a 32-day cycle it moves 4 days later.
- From conception: due date = conception date + 266 days (38 weeks).
- IVF day-5 (blastocyst) transfer: due date = transfer date + 261 days.
- IVF day-3 transfer: due date = transfer date + 263 days.
Example: if your last period started on 1 January 2026 and your cycle is 28 days, your estimated due date is 8 October 2026. On 12 March 2026 you would be exactly 10 weeks pregnant.
Pregnancy stages: weeks and trimesters
| Stage | Weeks | What it means |
|---|---|---|
| First trimester | Weeks 1 – 13 | Baby’s organs form; nausea and tiredness are common |
| Second trimester | Weeks 14 – 27 | Energy often returns; anatomy scan; baby starts moving |
| Third trimester | Weeks 28 – 40 | Rapid growth; preparing for birth |
| Preterm birth | Before 37 weeks | Baby may need extra care |
| Full term | 39 – 40 weeks (37–41 normal) | Ideal time for birth |
| Post-term | 42 weeks and over | Higher risks — induction is usually offered |
Benefits of knowing your due date
- Plan your antenatal care: scans, blood tests and the glucose test happen at set weeks.
- Track your baby’s growth and know what is normal at each stage.
- Prepare practically for maternity leave, travel limits and family support.
- Eat for the right stage: calorie needs rise in the second and third trimesters — see the pregnancy calorie calculator and pregnancy weight gain calculator.
Health warnings
- This is an estimate. Your early ultrasound (dating scan) is more accurate and your doctor may change your due date after it.
- Irregular cycles, recent hormonal contraception or breastfeeding can make LMP-based dates less reliable.
- Seek urgent care for heavy bleeding, severe abdominal pain, fluid leaking, reduced baby movements, severe headache or vision changes.
- Start folic acid (400 micrograms a day) as soon as you know you are pregnant — ideally before conception — and see a doctor early to begin antenatal care.
Eating well after a positive test
In the first trimester focus on folate, iron, iodine and vitamin D, eat small frequent meals if you feel sick, and avoid raw or undercooked meat, eggs and unpasteurised cheese. Read our guides to the first trimester, second trimester and third trimester.
Frequently asked questions
How accurate is a due date calculator?
It gives a good estimate, but only about 4–5% of babies are born on their exact due date. Most arrive between 38 and 42 weeks. An ultrasound in the first trimester is the most accurate way to date a pregnancy.
Why is pregnancy counted from my last period?
Most people know the date their last period started but not the exact day of ovulation. Counting from the period adds about two weeks before conception, which is why pregnancy is said to last 40 weeks.
How do I calculate my due date with irregular periods?
If your cycles are regular but longer or shorter than 28 days, enter your average cycle length. If they are very irregular, the calculator is less reliable and your dating scan will give a better due date.
How is an IVF due date calculated?
For a day-5 blastocyst transfer, add 261 days to the transfer date. For a day-3 transfer, add 263 days. IVF due dates are usually very accurate because the embryo’s age is known.
How many weeks pregnant am I?
Count the weeks from the first day of your last period. The calculator above shows your current weeks and days automatically once you enter the date.
Sources
- American College of Obstetricians and Gynecologists. Committee Opinion 700: Methods for Estimating the Due Date. 2017.
- NHS. Your pregnancy and baby guide – due date.
- WHO recommendations on antenatal care for a positive pregnancy experience. 2016.