How a pregnancy due date calculator works
A pregnancy calculator turns one date into an estimated due date and a count of how far along the pregnancy is. The most common method starts from the first day of your last menstrual period, often shortened to LMP. Doctors and midwives count the weeks of pregnancy from that day, not from conception, because the LMP is usually known while the day of conception usually is not. Adding 280 days, which is 40 weeks, gives the estimated due date, also called the EDD.
This page offers two ways to use that. In last-period mode it works out the due date from the date you enter. In due-date mode you enter a due date that your doctor or midwife has already assigned, perhaps after a scan, and the calculator uses it as it is. In both cases you can pick any date to see how many weeks and days the pregnancy covers on that day, such as a scan or an appointment.
The formula in plain terms
Estimated due date = first day of last period + 280 days. This is the same as adding 40 weeks. The conventional version assumes a regular 28-day cycle with ovulation around day 14. For a last period that started on 1 January 2026, the estimated due date is Thu 8 Oct 2026. Gestational age on any later date is the number of days since that first day, shown as completed weeks and days. On Thu 29 Jan 2026, 28 days after the start, the pregnancy is 4 weeks 0 days along.
Worked examples
- From the last period. Last period 1 January 2026, shown on Thu 9 Apr 2026: the dating covers 14 weeks 0 days, which is the second trimester.
- Mid-pregnancy. Shown on Thu 16 Jul 2026: 28 weeks 0 days, in the third trimester, with 84 days to the estimated due date.
- From a clinician's due date. A due date of Thu 8 Oct 2026 implies that the dating started on Thu 1 Jan 2026, and on Thu 29 Jan 2026 it is 4 weeks 0 days. The calculator does not replace the due date you give it.
- After the due date. Two weeks after the estimated due date, shown on Thu 22 Oct 2026, the calculator reports 42 weeks 0 days, and says plainly that the date is past the estimated due date.
- Before the dating starts. If the chosen date is earlier than the last period date, the calculator says the pregnancy dating has not started rather than showing negative weeks. For a last period on 1 March 2026 and a view on 29 January 2026, it reports that the dating starts in 31 days.
Weeks, trimesters and key dates
Pregnancy is divided into three trimesters. In this tool the first runs to just before week 14, the second from week 14 to just before week 28, and the third from week 28 until birth. Sources differ slightly on the exact boundaries, so treat them as convenient markers. The calculator lists the dates when the second and third trimesters begin and when week 40 is reached, all counted from the first day of the last period.
You will see two ways of talking about age. Gestational age counts from the last period and is what clinicians use. Fetal age counts from conception, which is usually about two weeks later. If your doctor says you are 8 weeks pregnant, they mean gestational age.
Why the due date is only an estimate
A due date is the middle of a window, not an appointment. Most babies are born within a few weeks either side of it, and only a minority arrive exactly on the day. The 280-day rule assumes a 28-day cycle with ovulation on day 14. If your cycles are longer or shorter, or ovulation happened earlier or later, the real timeline shifts. Clinicians may also adjust the due date after an early ultrasound, particularly when the scan and the last-period dates disagree by more than a small margin. This page does not do that.
If your clinician gives you a due date, use that one. In due-date mode this calculator takes your clinician's date as the truth and works from it, so it never overwrites it with a calculated figure.
What this calculator does not cover
- It does not redate a pregnancy using an ultrasound measurement.
- It does not calculate from IVF transfer dates or embryo age.
- It does not correct for irregular cycles or an unknown last period.
- It does not predict the birth date, the baby's size or sex, or any health outcome.
- It is not medical advice and does not replace prenatal care.
If you are not sure of your last period date, or have had irregular cycles, speak to a doctor or midwife about dating the pregnancy with a scan. If you want to understand the cycle dates that lead up to pregnancy, the period calculator shows expected start dates and the ovulation calculator estimates a fertile window. You can also browse the fitness and health calculators.
Using the dates in practice
Write down your due date and the date of your last period, and show both to your doctor or midwife at the first appointment so that they can confirm or adjust the dating. Use the "show the pregnancy on this date" box to check how many weeks you will be on the day of a booked scan or appointment, and to see when each trimester begins so you can plan work, travel or leave. Remember that the figures are counted in completed weeks and days, so 12 weeks 6 days is still the twelfth week, not the thirteenth.
What the weeks mean in practice
Pregnancy care is organised by gestational weeks. Early in pregnancy, many people have a first appointment and a dating scan, which is when a clinician may confirm or adjust the due date. Later, routine checks and tests are scheduled by week. The exact schedule depends on your country, your health and your provider, so use the calculator to know which week you are in and let your care team tell you what happens when.
Term is usually described as the period from 37 weeks to just under 42 weeks. A pregnancy that reaches 40 weeks is at the estimated due date, and one that goes beyond it is called post-dates, which your care team will monitor. This calculator shows weeks up to 42 weeks 6 days, which is the limit of what the tool supports, and it asks you to check the dates for anything beyond.
Using the key dates
The trimester dates in the results table are convenient markers for planning leave, travel and appointments. The start of the third trimester, at week 28, is a common point at which people plan time off or reduced travel, though your own plans should follow your health and your clinician's advice. The week 40 date is not a deadline, and it is better thought of as the middle of a window than as a date to organise everything around.
Why the first weeks are counted before conception
The count starts at the last period, so when conception happens, around two weeks later in a typical 28-day cycle, the pregnancy is already counted as two weeks old. That surprises many people. It is simply a convention, chosen because the date of the last period is usually known and the date of conception usually is not. If you conceived on a date you know for certain, a clinician can still work from the last period or from an early scan, and they will tell you which date they use.
Frequently asked questions
How do I calculate my due date from my last period?
Add 280 days, or 40 weeks, to the first day of your last period. For a period starting on 1 January 2026 that is Thu 8 Oct 2026. This assumes a regular 28-day cycle.
How many weeks pregnant am I?
Count the days from the first day of your last period up to the date you are interested in, then divide by seven. The whole number is the weeks and the remainder is the days.
Why do weeks start before conception?
Doctors count from the last period because it is usually known with more certainty than the day of conception. As a result, the first two weeks of the count happen before the egg is even fertilised.
My doctor gave me a different due date. Which is right?
Follow your doctor. They may have used an ultrasound or other information. Use the "due date from my clinician" option to see weeks and dates based on their date.
Can the calculator handle IVF or an ultrasound date?
No. IVF and ultrasound dating use different starting points, and a clinician should work those out for you.
Will my baby be born on the due date?
Probably not on the exact day. The date is an estimate near the middle of a range, and many babies arrive earlier or later.
Sources and limits of this page
The 280-day convention and the idea of a clinician's due date taking priority follow standard obstetric practice as described by organisations such as ACOG, the NHS and the US Office on Women's Health. The original pages could not be fetched when this page was written, so no figure is attributed to them directly. This page is educational information and not medical advice. Always confirm dating with your doctor or midwife.