About the Pregnancy Due Date Calculator
This estimates a pregnancy due date, how far along you currently are, and which trimester you're in — all from the first day of your last menstrual period (LMP). It's the same method clinicians use for an initial estimate, and it's a reassuring way to picture the timeline early on. Enter your LMP date and, if you know it, your cycle length.
How it works — Naegele's rule
It uses Naegele's rule, the standard method: the due date is 280 days (40 weeks) after the first day of your last period, based on a typical 28-day cycle. If your cycle differs, it adjusts by the difference (a 30-day cycle pushes the date two days later). Gestational age is counted in weeks and days from the LMP, and trimesters are split as first (weeks 0–12), second (13–26) and third (27+).
Assumptions and behaviour
- Dating is from the first day of the last period (LMP), not from conception — by convention, pregnancy is counted from LMP, so you're already considered "2 weeks pregnant" at conception.
- The default assumes a 28-day cycle with ovulation around day 14; a cycle-length adjustment shifts the estimate.
- The result is a single estimated date; only about 1 in 20 babies actually arrive on it.
- Trimester boundaries follow the common weeks-based split.
Limitations
- It's an estimate, not a prediction. Most births happen within a couple of weeks either side of the due date; "full term" spans 37–42 weeks.
- LMP dating is less accurate with irregular or long cycles, uncertain period dates, or recent hormonal contraception — an early ultrasound gives a more reliable due date and is what clinicians rely on.
- It doesn't account for IVF timing (which uses the transfer date) or known conception dates.
- This is general information, not medical advice — always confirm dates and get care from your doctor or midwife.
Privacy
The calculation runs entirely in your browser. Your dates are never uploaded or stored.
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