Pregnancy dating
Use your last menstrual period—or a conception date you genuinely know—to estimate a due date and trimester milestones. Your care team’s confirmed date takes priority.
Your dates stay on this device. Nothing you enter here is saved or sent to Flowy.
The calculation
Clinical pregnancy dating traditionally starts before conception: 280 days are added to the first day of the last menstrual period. If conception is truly known, the equivalent estimate adds 266 days.
The milestone dates are calendar orientations based on that same estimate. Keep the due date confirmed by your maternity team as the source of truth for care.
Good to know
Last-period dating assumes a regular 28-day cycle and accurate recall. Ovulation timing, irregular cycles, recent hormonal contraception, breastfeeding, or an uncertain last period can make that estimate less reliable.
ACOG recommends early ultrasound to establish or confirm gestational age. Assisted reproduction pregnancies should use clinic-provided dating based on the embryo and transfer date.
Common questions
The standard LMP method adds 280 days, or 40 weeks, to the first day of the last menstrual period. It assumes a regular 28-day cycle with ovulation around day 14.
The calculator adds 266 days, or 38 weeks, to a known conception date. Conception is often difficult to date precisely, so use this method only when the date is well established.
ACOG states that first-trimester ultrasound is the most accurate method to establish or confirm gestational age. Your clinician decides which date should guide care.
A due date is an estimate, not an appointment. Many births happen before or after it; your maternity team uses the date as one part of planning and monitoring care.
