Pregnancy test at 3 weeks

Invalid or future dates fail with invalid_input and are not billed.

Input contract, the future-date rule, and the response shape

Optional output keys are omitted rather than set to null. POST a JSON object with two required calendar fields. lmp is the first bleeding day of the last menstrual period as a strict four-digit year, hyphen, two-digit month, hyphen, two-digit day string; aliases last_menstrual_period, lmp_date, and period_start are accepted so forms can keep their usual names. as_of is the reference day used as today or as the day you want gestational age reported; aliases reference, on_date, today, and date map to the same parse. Each field is validated as a real Gregorian day in years one through nine thousand nine hundred ninety-nine, including leap-day rules, so values such as thirty-one April or twenty-nine February in a common year fail with invalid_input naming the field. If lmp falls after as_of the last-period date is in the future relative to the reference day and the task fails with invalid_input; that failure is not billed. On success the body echoes the normalised lmp and as_of strings; returns week3_date (last period plus twenty-one whole civil days) and week3_end (plus twenty-seven days); reports gestational_days, weeks, and days at as_of; sets at_week3 when as_of sits inside that twenty-one to twenty-seven day window; returns days_to_week3 as twenty-one minus gestational_days (negative after week three starts); and attaches hcg_min, hcg_max, hcg_mid, hcg_unit, test_sensitivity, likely_result, likelihood_percent, and a formula string. Example: lmp 2026-01-01 with as_of 2026-01-22 is exactly three weeks 0 days, at_week3 is true, and the 5 to 50 mIU/mL band is returned with a thirty percent likelihood against a 25 mIU/mL strip.

Where a 3-week pregnancy-test estimate earns its keep

People search for a pregnancy test at three weeks because that is often the first moment they even consider testing, still before a missed period on a twenty-eight-day cycle. Wellness apps, editorial explainers, and intake forms need a transparent number they can show next to that search: the expected serum range, a home-strip likelihood against a named sensitivity, and the calendar window that three weeks actually occupies. Embedding this calculator gives ISO dates and integer percentages a user interface can render without inventing a doubling model of its own. If a test is negative at three weeks and a period still does not arrive, the usual teaching is to repeat later in the week or at four weeks, when the published band sits much higher. Classroom demos of gestational dating can change the last-period day and immediately see week3_date move, including across February in a leap year, which is exactly the kind of civil-calendar edge a host clock mishandles near time-zone offsets. Automation that archives last-period columns can call the same pure function on the prepaid path at $0.002 per successful request; invalid input does not consume balance. Pin the examples, keep last-period validation strict, and you get a reproducible three-week hCG and home-test estimate without standing up an obstetric SDK of your own for a two-date problem.