Your estimated due date (EDD) is usually 40 weeks (280 days) after the first day of your last period, assuming a 28-day cycle. If your cycle is longer or shorter, the date moves by the difference. From conception, add 266 days (38 weeks). Your dating scan at about 10 to 14 weeks will confirm a more accurate date.
Due Date Calculator
NHS 40-week methodHow due dates are worked out
Pregnancy is counted from the first day of your last period, not from conception – so at the moment of conception you are already about “2 weeks pregnant”. A full-term pregnancy lasts from 37 to 42 weeks. Only about 1 in 20 babies is born on the due date itself.
Key NHS pregnancy dates
| Weeks | What happens |
|---|---|
| By 10 weeks | Booking appointment with a midwife |
| 10–14 weeks | Dating scan – confirms your due date; combined screening test offered |
| 16 weeks | Antenatal appointment |
| 18–21 weeks | Anomaly (20-week) scan |
| 28 weeks | Third trimester begins; blood tests and anti-D if needed |
| 37 weeks | Full term |
| 40 weeks | Estimated due date |
| 41–42 weeks | Membrane sweep and induction may be discussed |
Trimesters
First
Weeks 1 to 13. Baby’s organs form; tiredness and sickness are common.
Second
Weeks 14 to 27. You may feel the baby move from around 16–24 weeks.
Third
Weeks 28 to birth. Baby gains most of their weight.
Your next steps
- Contact your GP or self-refer to your local midwifery service as soon as you know you are pregnant.
- Take 400 micrograms of folic acid daily until 12 weeks, and 10 micrograms of vitamin D daily.
- Avoid alcohol and smoking.
Adult BMI is not used during pregnancy. Expected birth weights are explained in average baby weight at birth.
Frequently asked questions
How is my due date calculated?
By adding 280 days (40 weeks) to the first day of your last period, adjusted if your cycle is longer or shorter than 28 days.
How accurate is a due date calculator?
It gives an estimate. Your dating scan at 10 to 14 weeks is more accurate, and most babies arrive between 37 and 42 weeks.
Can my due date change?
Yes. If the dating scan measurement differs from your period dates, your midwife will usually use the scan date.
How many weeks pregnant am I?
Enter your dates in the calculator – it shows weeks and days today.
Related tools and guides
About this page
How this tool works
From your last period: due date = first day of last period + 280 days + (cycle length − 28). From conception: + 266 days. IVF: day-3 transfer + 263 days, day-5 transfer + 261 days. Weeks of pregnancy are counted from the equivalent first day of the last period.
Sources
- NHS – Pregnancy due date calculator
- NHS – Trying for a baby
- NICE NG201 – Antenatal care (2021).
Medical disclaimer: this tool gives an estimate for adults and is not a diagnosis or medical advice. bmicalculatornhs.net is independent and not affiliated with the NHS. Speak to your GP, pharmacist or another qualified health professional about your own health. Full disclaimer.