Pregnancy Due Date Calculator
Calculate your estimated due date from your last period using Naegele's rule, plus gestational age, trimester and conception date.
About the Pregnancy Due Date Calculator
This pregnancy due date calculator estimates your baby's arrival date from the first day of your last menstrual period (LMP). It uses Naegele's rule — LMP plus 280 days (40 weeks) — and adjusts for your personal cycle length, since ovulation happens later in longer cycles. Enter your LMP and cycle length, and the due date appears instantly along with your current gestational age in weeks and days.
Beyond the due date, the tool shows which trimester you are in today, your estimated conception date (around 14 days after the LMP in a 28-day cycle) and key milestones: the end of the first trimester at 13 weeks and the second at 27 weeks. These dates help you plan scans, screening tests and maternity leave conversations well in advance.
Remember that a due date is a statistical estimate — only about 4–5% of babies arrive exactly on it, and most are born within two weeks either side. Doctors often refine the date after a first-trimester ultrasound. Everything here runs privately in your browser, and nothing you enter is stored or sent anywhere.
How to Use the Pregnancy Due Date Calculator
- 1Select the first day of your last menstrual period (LMP).
- 2Adjust the average cycle length if yours differs from 28 days (21–35 supported).
- 3Read your estimated due date, current gestational age and trimester.
- 4Note the conception estimate and the first- and second-trimester end dates for planning scans.
Frequently Asked Questions
How accurate is a due date calculator?
LMP-based due dates are a good first estimate but not exact — only around 4–5% of babies are born on their due date, and roughly 90% arrive within two weeks either side. A first-trimester ultrasound (dating scan) is more accurate and doctors will usually adjust the due date if the scan differs by more than a few days.
What is Naegele's rule?
Naegele's rule is the classic due-date formula: take the first day of the last menstrual period and add 280 days (40 weeks). It assumes a 28-day cycle with ovulation on day 14. This calculator refines it by adding or subtracting the difference between your actual cycle length and 28 days, which shifts the estimate for shorter or longer cycles.
How many weeks pregnant am I?
Gestational age is counted from the first day of your last period, not from conception, so you are considered 'pregnant' for about two weeks before conception actually occurs. Enter your LMP above and the calculator shows your exact gestational age today in weeks and days, plus which trimester that falls in.
How does cycle length change my due date?
In a 28-day cycle, ovulation happens around day 14. In a 33-day cycle it happens around day 19, so conception — and therefore the due date — shifts about 5 days later. The calculator adds (cycle length − 28) days to the standard 280-day estimate, which is why entering your true average cycle length matters.
Can I use this calculator if my periods are irregular?
It becomes less reliable. Naegele's rule assumes a reasonably consistent cycle, so with very irregular periods the LMP method can be off by a week or more. If you know your conception or ovulation date (for example from IVF or ovulation tracking), your doctor can date the pregnancy from that instead, or rely on an early ultrasound.
When does each trimester start and end?
The most common convention divides pregnancy at the end of week 13 and week 27: the first trimester runs from week 1 to 13, the second from 14 to 27, and the third from 28 until birth. This calculator shows the calendar dates for those transitions based on your LMP so you can plan appointments and tests.