Pregnancy Due Date Calculator
How to use this pregnancy due date calculator
Using this free pregnancy due date calculator takes three short steps. First, choose your method: the first day of your last menstrual period, or your conception date, if you know it. Second, pick that date from the calendar box. Third, read your estimated due date, which appears straight away, with no button to press. If you used the last period method and that date is in the past, the calculator also shows how many weeks and days pregnant you are, your trimester, and your estimated conception date.
Worked example
Suppose the first day of your last menstrual period was 1 January 2026. Naegele’s rule adds 280 days to that date, which is 40 weeks, and lands on 8 October 2026. That is the estimated due date the calculator shows for a last period of 2026-01-01: 2026-10-08. The estimated conception date is 14 days after the last period, which is 15 January 2026. If instead you enter a conception date of 15 January 2026, the calculator adds 266 days, which is 38 weeks, and again arrives at 8 October 2026. The two methods agree, as they should, because conception typically happens about two weeks after the last period begins.
How Naegele’s rule works
Naegele’s rule is the standard way of estimating a due date from the last menstrual period. It assumes a regular 28-day cycle, with ovulation about 14 days after the period starts, and a pregnancy lasting 40 weeks from the first day of the last period. Forty weeks is 280 days. That is why the calculator adds 280 days for the last period method, but only 266 days, or 38 weeks, for the conception method: the conception method starts counting two weeks later, so it needs two fewer weeks to reach the same due date.
The rule is a useful estimate, not a promise. Cycles vary in length, ovulation does not always happen on day 14, and babies do not check the calendar before they arrive. An ultrasound scan, especially an early one, usually gives a more accurate due date than a date calculation alone, and your healthcare provider will use the scan, your history, and their examination together to date the pregnancy.
Trimesters explained
Pregnancy is divided into three trimesters. The first trimester runs from week 1 to week 13, and it is when the baby’s major organs form and when morning sickness is most common. The second trimester runs from week 14 to week 27, and many people find this the most comfortable stage, with more energy and a growing bump. The third trimester runs from week 28 onwards, until birth, when the baby grows rapidly and gains weight, and the parent often feels more tired and uncomfortable. The calculator shows your current trimester alongside your weeks and days, so you can see at a glance where you are in the journey.
Please treat this pregnancy due date calculator as an estimate only, not medical advice. Confirm your due date and your care with a healthcare provider, who can check your dates with a scan and advise you personally.
Frequently asked questions
How accurate is a due date estimate?
A due date is an estimate, not a deadline. Only a small minority of babies arrive exactly on the due date; most arrive in the days or weeks around it. Think of the due date as the middle of a window, and plan for a range rather than a single day.
What if my cycles are irregular?
If your cycles are irregular, a date based on your last period is less reliable, because ovulation may not have happened 14 days after your period started. In that case, the conception method is better if you know the date, and an early ultrasound scan is the most reliable way to date the pregnancy.
What if I know the conception date but not my last period?
Choose the conception date method and enter that date. The calculator adds 266 days, which is 38 weeks, to give your estimated due date. You do not need your last period date for this method.
What if I don’t know either date?
If you know neither date, this calculator cannot give a due date, because it needs a starting date to count from. An ultrasound scan can date the pregnancy from measurements of the baby, so book an appointment with a healthcare provider as soon as you can.
When should I see a doctor?
See a healthcare provider as soon as you know you are pregnant, and seek care urgently if you have bleeding, severe pain, fever, reduced fetal movement later in pregnancy, or any worrying symptom. This calculator is an estimate only and is not a substitute for medical care.