Retrace Pregnancy Timeline
Provide an estimated due date and average cycle length to calculate the likely last menstrual period, ovulation, and conception dates.
Adjusts LMP estimate for cycles shorter or longer than 28 days.
Leave blank to calculate gestational age as of today.
How to Use This Calculator
Enter the estimated due date
Use the due date given by your provider, ultrasound, or IVF clinic.
Adjust for your cycle length
Longer cycles ovulate later; shorter cycles ovulate sooner. Enter your average length for a more tailored estimate.
Review conception and LMP timing
Use the calculated window to recall intercourse dates or align with fertility treatments.
Formula
Estimated LMP = EDD − 280 days + (Cycle length − 28)
Ovulation = LMP + (Cycle length − 14)
Conception window = Ovulation − 2 days to Ovulation + 1 day
Gestational age (days) = 280 − (EDD − Reference date)
Full Description
Reverse due date calculations help patients recall conception timelines, confirm paternity windows, or document pregnancy history. These estimates assume a standard 14-day luteal phase and may vary for irregular cycles, ovulation induction, or assisted reproduction. Ultrasound dating (especially first trimester crown-rump length) and IVF transfer dates provide more precise timing than calendar calculations. Use this tool for guidance and corroborate with clinical data.
Frequently Asked Questions
How accurate is this method?
Accuracy depends on cycle regularity. Ultrasound dating is preferred when available.
What if I conceived with IVF?
Use the IVF due date supplied by your clinic; conception corresponds to retrieval or transfer minus embryo age.
Can this determine paternity?
It narrows the window but cannot confirm paternity. DNA testing provides definitive answers.
Why adjust for cycle length?
People with longer cycles ovulate later than day 14; adjusting aligns the conception estimate with individual biology.