Why pregnancy is dated from before conception
Pregnancy is conventionally dated from the first day of your last menstrual period (LMP), not from conception — which is a little counter-intuitive the first time you meet it, but it exists for a practical reason: the LMP is a date most people can state precisely, while the actual date of conception usually is not known. Because ovulation typically happens around two weeks after the period starts, this convention adds roughly two extra "pregnant" weeks before conception has actually occurred. A pregnancy "dated" at 4 weeks by LMP is usually only about 2 weeks post-conception.
Current week = (today − LMP) ÷ 7, rounded downThe trimesters
| Trimester | Weeks (by LMP) | Roughly |
|---|---|---|
| First | 1–13 | Months 1–3 |
| Second | 14–27 | Months 4–6 |
| Third | 28–40+ | Months 7–9 |
A full-term pregnancy is 40 weeks by this convention — 38 weeks from actual conception. Babies born anywhere from 37 to 42 weeks are considered full term; the exact boundaries used clinically vary slightly by source.
Why the estimate can be off by a week or more
- Irregular cycles. The LMP-plus-two-weeks assumption is built around a textbook 28-day cycle with ovulation at day 14. Shorter or longer cycles shift the real conception date without shifting the LMP-based calculation.
- Uncertain LMP. Light spotting can be mistaken for a period, or the date is simply misremembered, which throws off a calculation built entirely on that one date.
- Late ovulation. Stress, illness or normal cycle variation can delay ovulation within a single cycle, which an LMP-only estimate cannot see.
An early ultrasound, typically in the first trimester, measures the fetus directly and is generally considered more accurate than LMP dating alone, particularly for anyone with irregular cycles. If a scan date and an LMP-based date disagree by more than about a week, clinical guidelines in most places favour the scan.
What this calculator does and does not do
It answers one question — which week and trimester a pregnancy is in right now, based on the date you give it — using the same LMP convention every due-date calculator and most early prenatal paperwork uses. For the estimated delivery date itself rather than the current week, see Due Date Calculator. For estimating when conception itself likely occurred, working backwards from a due date, see Conception Date Calculator. None of these replace dating confirmed by a healthcare provider, which is what actual medical decisions should be based on.