How this works
Pregnancy is dated in weeks since the first day of the last menstrual period (LMP), not since conception. This convention dates back to before reliable conception-date tracking was possible and remains the global obstetric standard. A "full term" pregnancy is 40 weeks (280 days) from LMP — though anything between 37 and 42 weeks is considered term. Naegele's rule is the standard formula: due date = LMP + 280 days. It assumes a regular 28-day cycle with ovulation on day 14 — so conception is roughly 2 weeks after LMP, which is why the "weeks pregnant" number is always about 2 weeks larger than the time since conception. If you know your conception date instead (from IVF or precise tracking), the calculator simply adds 38 weeks (266 days). For irregular cycles, real ovulation may be earlier or later than day 14 — the calculator includes a cycle-length adjustment when needed. Trimesters are conventionally split at 13 weeks (1st → 2nd) and 27 weeks (2nd → 3rd). The first trimester is when most organogenesis happens and most miscarriages occur; the second is generally the most comfortable; the third is when the baby gains most of its weight and the mother experiences the most physical demand. The numbers a clinician will quote — viability around 24 weeks, term at 37 — also use the LMP-based dating shown here.
The formula
LMP is the first day of the last menstrual period; conception_date is the day of fertilisation (often estimated as ovulation, ~14 days after LMP for a 28-day cycle). The 280-day constant assumes a 28-day cycle; for shorter or longer cycles, add or subtract (cycle_length − 28) days. Trimester boundaries: 0–13 weeks (1st), 13–27 weeks (2nd), 27–40+ weeks (3rd).
Example calculation
- LMP: 2026-01-15. Today: 2026-05-02.
- Days pregnant: 107 → 15 weeks + 2 days.
- Trimester: 2nd (13–27 weeks).
- Due date: 2026-01-15 + 280 days = 2026-10-22. Days remaining: 173.
Frequently asked questions
How accurate is this calculator?
For dating purposes, this calculator matches what an obstetrician would quote at a routine appointment — same Naegele's rule, same week numbering, same trimester boundaries. The calculator's due date estimate is what a clinician would use until a first-trimester ultrasound; from week 8 onwards, ultrasound dating (crown-rump length measurement) is more accurate and clinicians may revise the due date by a few days. Only ~5% of babies are born exactly on their due date — the normal range is 37 to 42 weeks. Treat the date as a midpoint, not a deadline.
I had IVF — should I enter LMP or conception date?
Enter the conception date (or transfer date) — it's far more accurate. For IVF, conception is conventionally taken as the day of egg retrieval (the egg is fertilised within 24 hours, and embryo transfer happens 3–5 days later). The calculator adds 38 weeks (266 days) to that date. Some clinics quote a "transfer date + 263 days" for day-3 transfers and "transfer date + 261 days" for day-5 (blastocyst) transfers — the difference is small. If you only have an LMP, the calculator falls back to Naegele's rule, but with IVF the conception-based estimate is closer to the clinical due date.
My cycle isn't 28 days — does that matter?
Yes, slightly. Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle is regularly longer or shorter, ovulation shifts — and so does the conception date. For a 32-day cycle, ovulation is around day 18, so the due date is about 4 days later than Naegele's rule would suggest. For a 24-day cycle, ovulation is around day 10, and the due date is about 4 days earlier. Use the cycle-length input to apply this correction. If your cycles are very irregular, ultrasound dating in the first trimester is more reliable than any LMP-based calculation.
When does each trimester start and end?
There's no universal definition, but the most common convention used clinically is: 1st trimester from week 0 to end of week 12 (so 0–13 weeks), 2nd trimester from week 13 to end of week 26 (13–27 weeks), 3rd trimester from week 27 to delivery. Some sources split at 14/28, others at 13/27 — the calculator above uses 13/27. Functionally: 1st = early symptoms, organ formation, highest miscarriage risk; 2nd = "honeymoon" period, energy returns, anatomy scan around week 20; 3rd = baby gains most weight, mother gets larger and more uncomfortable, viability passes around week 24, term reached at week 37.