How this NHS due date calculator works
This pregnancy due date calculator estimates your expected date of delivery (EDD) using standard NHS-style methods. Most due dates are first calculated from the first day of your last menstrual period and then confirmed (or adjusted) at your dating scan.
In a typical 28-day cycle, pregnancy is counted as 40 weeks (280 days) from the first day of your last period. If your cycle is usually shorter or longer, your estimate can shift by a few days.
Calculation methods included
1) Last menstrual period (LMP)
This is the most common method. The calculator adds 280 days to your LMP date, then adjusts for cycle length. For example, a 30-day cycle can move your due date roughly 2 days later than a 28-day cycle.
2) Known conception date
If you know when conception occurred, the calculator adds 266 days (38 weeks). This can be useful if ovulation timing is clear.
3) IVF transfer date
IVF pregnancies are usually dated from transfer day and embryo age. For a 5-day transfer, the due date is generally transfer date + 261 days. For a 3-day transfer, transfer date + 263 days.
What your result means
- Estimated due date: your best calendar estimate of 40 weeks.
- Estimated gestational age: how many weeks and days pregnant you are today.
- Trimester: first, second, or third trimester based on gestational age.
- Key milestone dates: estimated week-12 and week-20 points.
Why due dates can change
A due date is an estimate, not a guarantee. Many babies are born before or after the due date. Your dating scan (usually around 8 to 14 weeks) is often more accurate than period-based dating, especially if your cycles are irregular.
It is normal for your care team to adjust your due date after scan measurements. This helps track growth and antenatal care timing more accurately.
When to contact your maternity team
- If you are unsure about your dates and need guidance for booking appointments.
- If your periods are highly irregular and you want more accurate dating support.
- If you have pain, bleeding, or concerning symptoms at any stage of pregnancy.
- If your baby’s movements change later in pregnancy—seek urgent advice.
Frequently asked questions
Is this exactly the same as my hospital due date?
Not always. This calculator follows standard clinical formulas, but your ultrasound dating result may be different and is usually preferred.
Do all pregnancies last exactly 40 weeks?
No. Full-term birth often occurs between 37 and 42 weeks. The due date is a central estimate used for planning and monitoring.
Can I use this calculator after IVF?
Yes. Use the IVF method and select embryo age at transfer. It gives a strong estimate commonly used in fertility and maternity settings.
Important note
This tool is for information and planning only, and it does not replace medical advice. For personal care decisions, speak to your midwife, GP, or maternity unit.