Estimate a due date and gestational age from a last menstrual period and 20–45 day cycle, with calendar-safe arithmetic and clear ultrasound-dating limits.
A quick decision brief for this specific tool
First day of a valid LMP within the past 42 weeks and a whole usual cycle length from 20 to 45 days
LMP-based estimated due date, completed gestational weeks/days, trimester, days to or past the estimate, cycle-based ovulation window, and general milestone dates
UTC calendar-day serial arithmetic; due offset = 280 + (cycle length − 28), with no local-hour or daylight-saving subtraction
Choose your path
Produce a calendar-stable LMP-based due-date and gestational-age estimate with cycle adjustment and clear clinical dating limitations.
Estimate due date and current gestational age without timezone drift.
Enter the LMP and usual whole cycle length, then review dates and completed weeks/days.
Can reproduce the cycle-adjusted calendar result and treats it as an estimate.
See how the conventional 280-day estimate changes.
Enter a supported 20–45 day cycle and inspect the adjusted due offset and ovulation window.
Understands cycle timing is an assumption rather than confirmation of conception.
Avoid false reassurance from a web calculator.
Review the irregular-cycle, fertility-treatment, uncertain-LMP, and ultrasound caveats.
Uses prenatal care for clinical dating and does not treat milestones as appointments or advice.
This tool is part of these guided projects. Each project provides step-by-step instructions with checklists and all the tools you need in one place.
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The conventional LMP estimate adds 280 days for a 28-day cycle and assumes ovulation around day 14. This tool adjusts the offset by the difference between the entered cycle and 28 days.
Estimated due date = LMP + 280 days + (cycle length − 28)Obstetric gestational age is conventionally counted from the LMP. The displayed ovulation/conception date is only an estimate based on cycle length, not a known conception date.
Completed weeks and days = calendar days since LMP ÷ 7The calculator converts ISO calendar dates to UTC day serials before subtraction, avoiding daylight-saving-time hour shifts. It accepts LMP dates from today back through 42 weeks.
Calendar-day difference = UTC date serial(end) − UTC date serial(start)Updated: August 2026
Calculating estimated due date at the first obstetric appointment.
→ Due date: ~October 22, 2026 (40 weeks)
Comparing the LMP estimate for a 35-day cycle with the conventional 28-day assumption.
→ Estimated due date is 7 calendar days later than the 28-day-cycle result
A user reviews the calculator's limitations and takes the estimate to a clinician rather than treating it as confirmed dating.
Obstetric gestational age is conventionally counted from LMP. The calculator labels its cycle-based ovulation/conception date as an estimate and does not accept it as a confirmed conception date.
The due date is an estimate, not a scheduled delivery date. A clinician may revise dating—especially using first-trimester ultrasound—and provides individualized follow-up guidance.
Enter the first day of the last menstrual period and usual cycle length to estimate gestational age, due date, and a cycle-based ovulation window. Leave uncertain dates blank and use prenatal care and ultrasound for clinical dating.