Pregnancy Due Date Calculator
Estimate your baby's due date and current week from your last period.
What is the Pregnancy Due Date Calculator?
A pregnancy due date calculator estimates when a baby is due and how far along a pregnancy currently is, based on the first day of the last menstrual period (LMP). It works using Naegele's rule, the standard clinical method of adding 280 days (40 weeks) to the LMP date, since a full-term pregnancy is conventionally measured from this date rather than from conception, which usually occurs roughly two weeks later. To calculate it manually, count forward 280 days (or 9 months plus 7 days) from the first day of your last period; current gestational age is simply the number of days elapsed since the LMP, expressed in weeks and days. A midwife may adjust this estimate after a dating scan, which can be more accurate than LMP-based dating alone.
How it works
The most common method estimates a due date as 40 weeks (280 days) from the first day of your last menstrual period (LMP), assuming a standard 28-day cycle with ovulation around day 14. This is sometimes called Naegele's Rule. Because cycle length and ovulation timing vary between individuals, this is a statistical estimate based on averages, not a precise prediction for any specific pregnancy.
UK context
In UK antenatal care, your midwife or doctor typically confirms or adjusts your estimated due date using an early dating ultrasound scan, which measures the size of the developing baby — this is generally considered more accurate than the LMP-based calculation alone, especially if your cycle is irregular or you're unsure of your last period's exact start date.
Tips
- Only a small percentage of babies are actually born on their exact calculated due date — it's best understood as the midpoint of a normal range, not a fixed deadline.
- If your menstrual cycle is irregular or longer/shorter than 28 days, the LMP-based estimate is less reliable, and a dating scan gives a more accurate estimate.
- Your midwife or doctor's estimate from a dating scan should always take priority over a calculator estimate for any actual medical or care planning purposes.
Frequently asked questions
How accurate is a due-date calculator?
It's a statistical estimate based on averages (a 40-week pregnancy from a 28-day cycle), so it gives a useful planning estimate but isn't precise for any individual pregnancy — only a small proportion of babies arrive exactly on the calculated date.
What if my menstrual cycle isn't a standard 28 days?
The standard LMP-based calculation assumes a 28-day cycle; if yours is longer or shorter, the estimate becomes less reliable. A dating ultrasound scan, which measures the baby directly, gives a more accurate estimate in this situation.
Is a dating scan more accurate than the LMP calculation?
Generally yes, particularly in early pregnancy — a dating scan measures the baby's actual size, which is a more direct indicator of gestational age than working backwards from the last period, especially for irregular cycles.
A quick note
This is a general estimate based on standard averages, not a medical assessment. Your midwife or doctor's dating scan is the more accurate reference for your specific pregnancy.