Estimate your due date from the first day of your last period, using the standard 280-day count. You also get an estimated conception date, trimester boundaries, and — if you are currently pregnant — how far along you may be today.
Pregnancy Calculator
How the Pregnancy Calculator Works
This calculator estimates how far along a pregnancy is and the expected due date from the first day of the last menstrual period (LMP) only. The standard method counts pregnancy from the LMP, not from conception, because the LMP is usually known while the exact conception day often is not. Pregnancy is taken to last 280 days — 40 weeks — from the LMP, so the due date is the LMP plus 280 days, and the current week is counted from that same start.
From an LMP, the calculation is: due date = LMP + 280 days; gestational age = today’s date − LMP, expressed in weeks and days (for example, 13 weeks and 6 days is written 13w6d). If you know a possible conception date instead, use our Due Date Calculator, which can count forward 266 days from it — this calculator takes the first day of your last period (LMP) only. The trimester is also shown: weeks 1–13 are the first trimester, 14–27 the second, and 28–40 the third.
The same dates drive the other results: the conception window is estimated as the five days before ovulation plus ovulation day itself, and key milestones — first heartbeat scan around 6–7 weeks, anatomy scan around 18–20 weeks, viability around 24 weeks — are placed on the same timeline. All of these are estimates built on regular 28-day cycles; irregular cycles shift ovulation and therefore every date that follows.
How to Use This Calculator
- Enter the first day of your last period (LMP).
- Enter that date.
- Press Calculate to see the due date, current gestational age in weeks and days, trimester, and the estimated conception date.
- Note the milestone dates shown alongside — they are guides for planning appointments, not medical advice.
- If your cycle is irregular, treat the results as approximate and confirm dates with an early ultrasound, which is more accurate.
Example
If the first day of the last period was 1 January 2026, the due date is 8 October 2026 (1 January + 280 days), and on 8 April 2026 the pregnancy is 13 weeks and 6 days (13w6d) — still in the first trimester, one day short of the second.
If you know a possible conception date instead, use our Due Date Calculator, which can count forward 266 days from it — this calculator takes the first day of your last period (LMP) only.
Understanding Your Result
The due date is an estimate, not an appointment. Only about 5% of babies arrive exactly on their due date; most arrive within two weeks either side, and first pregnancies often run a few days past. The 280-day rule assumes a regular 28-day cycle with ovulation on day 14 — if your cycles are longer or shorter, ovulation moves and the true due date moves with it, which is why an early dating scan (usually at 8–12 weeks) often adjusts the date by a few days. Treat the calculator’s date as the planning anchor, not a deadline.
The week-and-day format matters because pregnancy care is organised around it: screening tests, scans and check-ups are scheduled by gestational week, and a difference of a few days can move an appointment. The trimester boundaries are also approximate — some clinicians count the second trimester from week 14 and the third from week 28, as this calculator does, while others use slightly different cut-offs. If a result sits near a boundary, as in the examples above, confirm with your clinician which side of the line your care team uses.
Pregnancy Calculator FAQs
Why does pregnancy start from the last period, not conception?
Because the first day of the last period is usually known, while the exact day of conception often is not. Counting from the LMP adds roughly two weeks before conception, which is why a pregnancy is described as 40 weeks even though the baby develops for about 38 weeks.
How accurate is the due date?
It is an estimate based on a regular 28-day cycle. An early ultrasound dating scan is more accurate and often adjusts the date by a few days; only about 5% of babies are born exactly on their due date.
What if my cycles are irregular?
The calculation assumes ovulation on day 14 of a 28-day cycle. With irregular cycles, ovulation may fall earlier or later, shifting the due date. Use the result as a guide and confirm with a scan.
What does 13w6d mean?
It means 13 weeks and 6 days of pregnancy — 97 days since the first day of the last period. The next day becomes 14w0d, the start of the second trimester.
Can I use this calculator after IVF?
Yes, with care: for IVF, the transfer date plus the embryo’s age gives a more precise start than the LMP method. Many clinics calculate the due date from the transfer date, so use the clinic’s date if you have one.
Related calculators: Due Date Calculator, Pregnancy Conception Calculator, Date Calculator.