Pregnancy Due Date Calculator
Estimate your baby's due date and current gestational age from your last menstrual period.
The pregnancy due date calculator estimates when your baby is likely to be born using Naegele\u2019s rule: add 280 days (40 weeks) to the first day of your last menstrual period. It also shows how far along you are today and which trimester you are in.
How to use the calculator
- Select the first day of your last period.
- The calculator adds 40 weeks to estimate the due date.
- See your current gestational age in weeks and days.
- Check which trimester you are in right now.
How it is calculated
Gestational age = days since LMP, expressed in weeks and days.
Worked example
Last period on 1 January 2026
Estimated due date = around 8 October 2026
The pregnancy reaches full term between 37 and 42 weeks.
Who should use this calculator
- Expectant parents wanting an approximate date to plan appointments, work leave and family logistics around.
- Anyone tracking how far along the pregnancy is and which trimester they are in right now.
- People comparing the LMP-based estimate with a dating scan so they know which date to plan by.
- Anyone who wants to understand the standard 40-week timeline that clinicians use.
Common mistakes to avoid
- Treating the date as a deadline. Only about 4% of babies arrive exactly on the estimated date. Most are born within two weeks either side, which is why it is called an estimated due date.
- Relying on it when your cycle is irregular. The 280-day rule assumes a 28-day cycle with ovulation around day 14. Longer, shorter or irregular cycles shift the estimate considerably.
- Skipping the dating scan. An ultrasound between about 8 and 13 weeks is more accurate than any calculation and is what clinicians use to set the date.
- Confusing gestational age with time since conception. Pregnancy is dated from the first day of the last period, so at four weeks pregnant, conception happened roughly two weeks ago.
- Using the estimate for medical decisions. Decisions about induction, growth checks and monitoring depend on scan findings and clinical judgement, not on a calculator.
How the 280-day rule works
Naegele's rule — add 280 days, or 40 weeks, to the first day of the last menstrual period — has been used for two centuries because it is simple and, for regular cycles, reasonably accurate. A last period on 1 January gives an estimated due date around 8 October.
The weakness is the assumption built into it: that ovulation happened on day 14. If your cycle runs 35 days, ovulation is closer to day 21, and the true date is likely a week later than the calculation suggests. That is exactly why an early dating scan is the standard. If the scan date and the LMP date disagree by more than about a week, clinicians go with the scan.
Pregnancy terms explained
- LMP
- Last Menstrual Period — specifically the first day of the most recent period, which is where pregnancy dating starts.
- Gestational age
- How far along the pregnancy is, counted from the LMP rather than from conception.
- Naegele's rule
- Adding 280 days to the LMP to estimate the due date.
- Full term
- Between 37 and 42 weeks. Before 37 weeks is premature, and after 42 weeks is post-term.
- Trimester
- A third of the pregnancy: weeks 1–13, 14–27 and 28–40, each with different screening and monitoring milestones.
What to do with the estimate
Treat the date as a planning window rather than a fixed appointment. Use it to schedule the first scan, to book antenatal appointments and to inform your employer about maternity leave, keeping a couple of weeks of flexibility on either side. Record the dating scan result when you have it, because that becomes the date your clinician works from. And if you pass 41 weeks, expect the conversation to turn to monitoring and possible induction — that is normal practice, not a sign that anything has gone wrong.
Frequently asked questions
How accurate is the due date?
Only about 4% of babies arrive exactly on the due date, and most are born within two weeks either side. An early ultrasound gives the most accurate estimate, especially if your cycle is longer or shorter than 28 days.
What is full-term pregnancy?
A pregnancy is considered full term between 37 and 42 weeks. Babies born before 37 weeks are premature, and after 42 weeks are post-term, which is why doctors may induce labour around 41 weeks.
Can I use this if my cycle is irregular?
You can, but the estimate will be less reliable. In that case an ultrasound dating scan, usually done between 8 and 13 weeks, is the better way to set the due date.