A due date gives you a timeline to plan around — when to book your first scan, start antenatal check-ups, arrange maternity leave and prepare the home for the baby’s arrival. The standard medical method, called Naegele’s rule, adds 280 days (40 weeks) to the first day of your last menstrual period. It assumes a regular 28-day cycle with ovulation around day 14, which makes it a reliable starting estimate for most women with regular cycles.
Enter the first day of your last period and this calculator applies Naegele’s rule to estimate your due date, along with your current week of pregnancy. Keep perspective on the result: only about 5% of babies arrive on the exact due date, and most are born within two weeks on either side. Babies arriving between 37 and 42 weeks are considered full term, so the ‘date’ is really the middle of a healthy window rather than an appointment.
Your doctor will refine this estimate with an early dating ultrasound, usually around 8–12 weeks, which measures the baby directly and is more dependable than cycle maths — especially if your periods are irregular. Treat this calculator’s result as a helpful first guide: write it down, share it at your first antenatal visit, and let your healthcare provider set the official schedule from there.
How it works
Naegele’s rule is simple arithmetic with a long medical pedigree: estimated due date = first day of last menstrual period + 280 days. Equivalently, add one year, subtract three months and add seven days to that date. Pregnancy is therefore counted as 40 weeks from the last period, even though conception typically happens about two weeks later — which is why ‘gestational age’ is already two weeks at the moment of conception. This convention gives every doctor and midwife a common language for dating pregnancies and timing scans, blood tests and milestones.
The rule’s weakness is its assumption of a textbook 28-day cycle. If your cycles run 35 days, ovulation likely happened a week later than assumed, pushing the true due date about a week beyond the calculated one; with irregular cycles the estimate gets shakier still. That is why an early ultrasound, which dates the pregnancy from the embryo’s measured size, overrides the calculation whenever the two disagree. Use this tool for planning and excitement — and your provider’s dating for medical decisions.
How to use this tool
- Enter the first day of your last menstrual period.
- If your cycle is regularly longer or shorter than 28 days, enter your average cycle length.
- Click Calculate to see your estimated due date.
- Note the current week and day of pregnancy shown with the result.
- Write the date down and confirm it with your doctor, ideally via an early ultrasound.
- Use the confirmed date — not this estimate — for scheduling scans and leave.
Practical tips
- If your cycles are irregular, treat this date as rough and push for an early dating ultrasound for accuracy.
- Start folic acid (as advised by your doctor) as early as possible — ideally before conception — since it matters most in the first weeks.
- Download nothing and sign up nowhere to use this tool; your dates stay in your browser.
- Count milestones in weeks (e.g. 'week 20'), the way clinics do, to avoid confusion at appointments.
Frequently asked questions
Why is the due date 280 days after the last period?
Pregnancy is conventionally measured as 40 weeks from the first day of the last menstrual period, even though conception typically happens about two weeks later. This convention, known as Naegele's rule, gives doctors and midwives a standard way to date pregnancies and schedule care, which is why scans and milestones are all counted from the last period rather than conception.
What if my periods are irregular?
Naegele's rule assumes a regular 28-day cycle, so irregular cycles make the estimate less reliable — sometimes off by weeks. If your cycles vary widely, treat the calculator's date as a rough guide only. An early dating ultrasound around 8–12 weeks measures the baby directly and gives a far more dependable due date.
How accurate are due date estimates?
They are estimates, not appointments. Only about 5% of babies arrive on the exact due date; most are born within two weeks before or after it. Babies arriving between 37 and 42 weeks are generally considered full term. Your healthcare provider tracks your progress and adjusts the plan as the pregnancy develops.
What is gestational age?
Gestational age is how far along the pregnancy is, counted in weeks and days from the first day of the last period — not from conception. So at the moment of conception, gestational age is already about two weeks. The calculator shows your current gestational age alongside the due date to help you follow your pregnancy week by week, the same way clinics track it.
My ultrasound gave a different date — which is right?
Trust the early ultrasound. A dating scan in the first trimester measures the embryo directly and is accurate to within a few days, while Naegele's rule depends on cycle assumptions that often don't hold. When the two differ by more than a few days, clinicians use the scan date for all scheduling.
What should I do first after getting my due date?
Book your first antenatal appointment if you haven't already — ideally before 10 weeks. Your provider will confirm the dating, start routine screening, discuss nutrition including folic acid and iron, and plan your scan schedule. Bring the first day of your last period to the visit; it's the single most useful date for your care team.
Note: This is an estimate only and is not medical advice. Confirm your due date with your doctor or midwife — preferably with an early ultrasound — and follow their guidance for your pregnancy care plan.