Due Date Calculator

How the due date is calculated

The standard method, often called Naegele's rule, adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). It assumes a 28-day cycle with ovulation around day 14. For an LMP of 1 March 2026, the due date is 6 December 2026. If your cycles are longer or shorter, ovulation shifts, so the calculator adjusts: with 32-day cycles the due date moves 4 days later, to 10 December 2026.

Because pregnancy is counted from the last period, you are already considered about 2 weeks pregnant when conception happens. That is why '40 weeks pregnant' is about 38 weeks after conception.

Due date from conception, IVF or ultrasound

For IVF pregnancies, the American College of Obstetricians and Gynecologists (ACOG) recommends dating from the embryo age and transfer date, because these are known exactly.

When an early scan changes your due date

Measuring the embryo (crown–rump length) on a first-trimester ultrasound is the most accurate way to date a pregnancy. ACOG's guidance is to change the LMP-based due date if the scan differs by more than 5 days before 9 weeks, or by more than 7 days between 9 weeks and 13 weeks 6 days. Later scans are less precise for dating. If your provider gives you a revised date, use the ultrasound method in the calculator.

How many weeks pregnant am I?

Gestational age is the number of days since the LMP, written as weeks + days. On 15 June 2026, with an LMP of 1 March 2026, it has been 106 days – 15 weeks and 1 day – which is in the second trimester. The trimesters are usually counted as weeks 1–13 + 6, 14–27 + 6 and 28 until birth.

Pregnancy milestones by week

Will my baby arrive on the due date?

Probably not exactly. The due date is the middle of a normal range, and only a small share of babies are born on it. Most arrive within about two weeks either side. ACOG defines the stages of term as:

• Early term: 37 weeks 0 days to 38 weeks 6 days
• Full term: 39 weeks 0 days to 40 weeks 6 days
• Late term: 41 weeks 0 days to 41 weeks 6 days
• Post-term: 42 weeks 0 days and later
• Babies born before 37 weeks are preterm.

First pregnancies, long cycles and some health conditions can influence when labour starts. Your care team will discuss what happens if you go past your due date.

What to do next

• Book your first prenatal appointment as soon as you know you are pregnant.
• Ask about folic acid and any medicines you take.
• Keep a note of your LMP and cycle length to share with your provider.
• Use the key dates from the calculator to plan scans and time off work.

This calculator provides an estimate for general information. It is not medical advice and does not replace dating by your doctor or midwife. Contact your care provider with any concerns during pregnancy.

Working backwards: when did I conceive?

If you know your due date, conception was most likely about 266 days (38 weeks) earlier – roughly two weeks after the start of your last period. A due date of 6 December 2026 points to conception around 15 March 2026. Because ovulation timing varies and sperm can survive for several days, this is an estimate within a window of several days, not an exact date.

Trimesters at a glance

Twins and IVF due dates

Twin and other multiple pregnancies use the same due-date calculation, but they are more likely to be born before the due date, and care teams often plan delivery earlier. Your provider will give you guidance specific to your pregnancy. For IVF with frozen embryos, use the transfer date and the embryo's age (day 3 or day 5) – the calculator handles both.

Related guides

• When to Take a Pregnancy Test for the Most Accurate Result
• How Many Days After Your Period Can You Get Pregnant?
• Animal Gestation Periods: How Long Are Different Animals Pregnant?

Sources

1. ACOG Committee Opinion No. 700 – Methods for estimating the due date — acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date
2. NHS – Your pregnancy and baby guide — nhs.uk/pregnancy
3. Office on Women's Health – Pregnancy tests — womenshealth.gov/a-z-topics/pregnancy-tests