How the due date is worked out
Pregnancy is counted from the first day of your last period, not from conception. The estimated due date is 280 days (40 weeks) after that day, assuming a 28-day cycle. If your cycle is longer or shorter, the calculator shifts the date by the difference. This is the same method described by the American College of Obstetricians and Gynecologists and used by the NHS due date calculator.
- From conception: 266 days after the conception date.
- From IVF: 266 days after egg retrieval, which is 261 days after a day-5 transfer or 263 days after a day-3 transfer.
How accurate is it?
A due date is an estimate. Only a small share of babies arrive on the exact day, and a birth any time from 37 to 42 weeks is common. An ultrasound in the first trimester is the most accurate way to date a pregnancy, and your doctor or midwife may change your due date after that scan. If the scan date and the period date disagree, the scan date is usually the one used.
Trimesters at a glance
| Trimester | Weeks |
|---|---|
| First | Up to 13 weeks and 6 days |
| Second | 14 weeks to 27 weeks and 6 days |
| Third | 28 weeks to birth |
Once you have your dates, see our guide to gestational diabetes: tests, diet and later risk, which is usually screened for between 24 and 28 weeks.
Get medical help straight away if
- You have vaginal bleeding, severe tummy pain, or pain in one side or at the tip of the shoulder in early pregnancy.
- Your baby's movements slow down or stop later in pregnancy.
- You have a severe headache, vision changes, or sudden swelling of the face and hands.
- Your waters break or you have regular contractions before 37 weeks.
Frequently asked questions
What if I don't know the date of my last period?
An early ultrasound can date the pregnancy by measuring the baby. Tell your doctor or midwife that your dates are uncertain.
How many weeks is a full-term pregnancy?
Around 40 weeks from the last period. Babies born from 37 weeks are generally not considered premature.
Can my due date change?
Yes. It's often adjusted after the first-trimester scan, and that revised date is the one your care team will use.
These tools give general estimates for adults and are not a diagnosis or a substitute for advice from a doctor. If you have symptoms or a health condition, please consult a qualified healthcare professional.