Due Date Calculator
Enter the first day of your last menstrual period or your conception date to calculate your estimated due date and current week of pregnancy.
How Your Due Date Is Calculated
This calculator uses Naegele's rule — the standard method used by midwives and obstetricians worldwide. The formula adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). The 40 weeks includes the approximately two weeks between your LMP and ovulation, which is why pregnancy is counted from the LMP rather than from conception.
If your cycle is longer or shorter than 28 days, the calculator adjusts accordingly. For a 30-day cycle, ovulation occurs around day 16 rather than day 14, so 2 days are added to the estimated due date. For a 25-day cycle, 3 days are subtracted.
If you know your conception date (for example, from IVF or tracked ovulation), the calculator adds 266 days (38 weeks from conception, equivalent to 40 weeks from LMP).
Pregnancy Timeline
| Stage | Weeks | Key milestones |
|---|---|---|
| First trimester | 1 – 13 | Embryo forms; heartbeat detectable from ~week 6; most miscarriage risk |
| Second trimester | 14 – 27 | Anatomy scan (~week 20); movement felt; lower miscarriage risk |
| Third trimester | 28 – 40 | Rapid growth; lung maturation; birth preparation |
| Early term | 37 – 38 | Baby is considered early term |
| Full term | 39 – 40 | Optimal timing for birth |
| Late term | 41 | Monitoring typically increased |
| Post term | 42+ | Induction usually recommended |
Worked Examples
Example 1: LMP 1 January 2026, 28-day cycle. Add 280 days: due date = 8 October 2026. At 8 weeks from LMP, the pregnancy is in the first trimester.
Example 2: LMP 1 January 2026, 35-day cycle. Cycle is 7 days longer than 28, so add 7 days: due date = 15 October 2026.
Example 3: Conception date 15 January 2026. Add 266 days: due date = 8 October 2026. Conception on day 14 of a 28-day cycle starting 1 January gives the same result as example 1.
Frequently Asked Questions
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Only about 4% of babies are born on their exact due date. Most births occur within two weeks either side. A due date calculated from LMP is accurate to within about 2 weeks. An early ultrasound (before 14 weeks) is more accurate — it measures the embryo directly and can correct for irregular cycles or uncertain LMP.
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If you are unsure of your LMP, an ultrasound is the best way to estimate gestational age and calculate a due date. A first-trimester ultrasound (before 14 weeks) that measures crown-rump length is the most accurate dating method available.
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Your healthcare provider may revise your due date based on early ultrasound measurements. If the ultrasound date differs from the LMP date by more than 5–7 days in the first trimester (or more than 10–14 days in the second trimester), the ultrasound date is usually used instead.
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Gestational age counts from the first day of the LMP — so at "4 weeks pregnant", the embryo is actually only about 2 weeks old (foetal age). This can be confusing, but gestational age is the standard used by all healthcare providers because LMP is easier to know than conception date.
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Book your first antenatal appointment (booking appointment) as soon as you have a positive pregnancy test — ideally before 10 weeks. Your midwife or doctor will confirm the pregnancy, take a full health history, arrange blood tests, and schedule your first ultrasound scan.