Pregnancy Week Calculator
Count how many weeks and days pregnant you are from the first day of your last period, with the trimester, the estimated due date and the days to go.
Weeks and days pregnant today, counted from your last period.
How the pregnancy week calculator works
Pregnancy is dated from the first day of the last menstrual period (LMP), about two weeks before conception, so a "40-week" pregnancy is 280 days from that date. The calculator counts the days between the LMP and the date you choose, converts them to weeks and days, names the trimester (weeks 1–13, 14–27, 28 onwards) and gives the due date at 280 days.
If your cycle is not 28 days, open Detailed mode: the due date shifts by the difference, since ovulation happens about 14 days before the next period. An early ultrasound can re-date the pregnancy, in which case use that date instead.
Formula: gestational age = date − LMP · due date = LMP + 280 days + (cycle − 28)
Worked examples
| Inputs | Weeks pregnant | Note |
|---|---|---|
| LMP 1 March 2026, checked 30 August 2026 | 26 weeks 0 days | 182 days = 26 weeks |
| LMP 15 June 2026, checked 30 August 2026 | 10 weeks 6 days | first trimester |
| LMP 1 March 2026, 32-day cycle | 26 weeks 0 days | due date moves 4 days later |
FAQFrequently asked questions
Why count from my last period rather than conception?
Because the LMP is a date most people know, while conception is not observed. Convention dates pregnancy from the LMP, so at conception you are already "2 weeks pregnant".
How accurate is the due date?
Only about 4% of babies arrive on it; most come within two weeks either side. A first-trimester ultrasound is the most accurate way to date a pregnancy.
What if my cycles are irregular?
The LMP method assumes ovulation two weeks after the period starts. With irregular cycles the estimate can be off by a week or more; an ultrasound date is better.
When do the trimesters start?
Definitions vary slightly. This calculator uses weeks 1–13 for the first, 14–27 for the second and 28 onwards for the third.
Where these figures come from
- Mifflin MD et al., "A new predictive equation for resting energy expenditure in healthy individuals", Am J Clin Nutr 1990 — the BMR equation used here
- World Health Organization — Waist circumference and waist–hip ratio (2008 expert consultation) — waist-to-hip cut-offs
- Tanaka H, Monahan KD, Seals DR, "Age-predicted maximal heart rate revisited", J Am Coll Cardiol 2001 — 208 − 0.7 × age
- Nathan DM et al., "Translating the A1C assay into estimated average glucose values" (ADAG), Diabetes Care 2008 — eAG = 28.7 × A1c − 46.7
- Mosteller RD, "Simplified calculation of body-surface area", N Engl J Med 1987 — BSA = √(height × weight / 3600)
- Australian Department of Health and Aged Care — national health guidance
Last checked: September 2026. The equations are fixed by their published papers and do not change with tax years or regulations; the thresholds quoted in verdicts name the body that set them.