Pregnancy week from last period

Knowing how many weeks pregnant someone is almost always starts from the first day of the last menstrual period, not from the conception date itself. Midwives, prenatal apps, and triage forms express that age as completed weeks plus remaining days—for example twelve weeks and three days—so every chart, ultrasound label, and patient message can agree on the same number. This pregnancy week from last period calculator accepts an explicit LMP date and an explicit reference date as strict ISO calendar strings, counts whole Gregorian days between them in a UTC-neutral civil calendar, and returns the gestational week, day remainder, a compact display string, trimester band, and a standard two-hundred-eighty-day estimated due date. Nothing is inferred from the server clock: the client supplies both dates, so batch jobs, historical chart reviews, and free browser checks stay reproducible.

How gestational week and day are defined from the LMP

Obstetric gestational age is customarily counted from the first day of the last menstrual period even though fertilization usually occurs about two weeks later. That convention keeps dating consistent across clinics when the exact conception day is unknown. This capability implements the completed-weeks form of that rule: take the whole number of civil calendar days from the LMP date to the reference date, divide by seven, and report the integer quotient as weeks and the remainder as days.

On the LMP day itself the difference is zero, so the result is zero weeks and zero days. Seven days later the result is one week and zero days; ten days later it is one week and three days. A trimester field follows a common educational split: completed weeks zero through thirteen map to the first trimester, fourteen through twenty-seven to the second, and twenty-eight or more to the third.

Those bands are convenience metadata for dashboards; clinical staging can differ by protocol, so treat them as derived labels rather than diagnostic codes. Leap days are validated and included when they fall between the two dates, so February twenty-nine in a leap year never silently becomes March first.

Where knowing the pregnancy week helps

Prenatal intake forms and patient portals need a deterministic oracle so mobile, web, and back-office tools print the same 12w+3d for the same LMP and visit date. Charting pipelines that re-score historical encounters must not depend on when the worker process ran, which is why both dates are explicit inputs and why the implementation never calls Date.now. Research assistants cleaning self-report exports can convert LMP and survey dates into gestational weeks without standing up a full obstetrics stack.

Sibling KIT capabilities estimate due date alone or pregnancy weight-gain bands; this landing targets the distinct search intent of current gestational week and day from LMP. Failed validation does not consume balance.