Family calculators: cycle and pregnancy
Two calendars start from the same point: the first day of the last period. One places ovulation on day 14 of a 28-day cycle; the other counts weeks of amenorrhea and the trimester. They do not replace a urine test, an ultrasound or an appointment.
Obstetric convention counts 280 days from the period, not from conception. A longer cycle shifts ovulation and therefore the due date. If an ultrasound already set a date, that is what follow-up uses. Each page shows the formula, a worked example and what the figure does not measure.
Most used
Subcategories
Full list
Questions
Why start from the period, not from conception?
Because the conception date is rarely known to the day. The obstetric calendar starts on the first day of the period and places a theoretical ovulation on day 14. That is a convention, not a measurement.
Do these tools replace pregnancy care?
No. They apply a 28-day and 280-day calendar. A test, an ultrasound or a clinician may date things differently. If in doubt, medical follow-up comes first.