Explore an estimated fertilization date and due date from the information you have. This educational calculator cannot determine an exact conception day or confirm pregnancy.
01 / CHOOSE YOUR DATING METHOD
Choose the information you have. Only the relevant fields appear, and results update automatically. Your personal dates stay in your browser.
02 / EDUCATIONAL DATING ESTIMATE
Choose a method and enter its date to see an estimated fertilization date, due date and method-appropriate milestones.
Educational estimate, not a diagnosis. Calculated dates do not confirm pregnancy, prove intercourse timing or identify a biological parent. Seek clinician guidance for pregnancy dating and medical decisions.
03 / METHODS & LIMITATIONS
Clinical gestational age usually starts about two weeks before the fertilization dating anchor. Implantation is a separate later event and is not dated here. For irregular cycles, early ultrasound and clinician-established due dates may be more reliable than LMP.
LMP due date = period date + 280 + (cycle length − 28) calendar days; ovulation/fertilization anchor = period date + cycle length − 14. From a due date, subtract 266 days for fertilization or 280 for an LMP-equivalent. Ultrasound LMP-equivalent = scan date − reported gestational days; due date = scan + 280 − reported gestational days. IVF due date = transfer + 266 − embryo age; fertilization anchor = transfer − embryo age. A known-date anchor gives due date = known date + 266 days.
Only actual Gregorian calendar days are added or subtracted, using UTC date-only arithmetic. No approximate months, local time-of-day calculations, universal ±days uncertainty ranges or percentage accuracy claims are used. Dates and derived milestones must remain within 1900–2100. Ultrasound input uses a software range of 4–42 weeks plus 0–6 days, and requires clinician-reported gestational age.
ACOG: clinical guidance on estimating the due dateEnter your values locally; no inputs are uploaded.
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Review the explanation, copy supported outputs or reset the workspace.
Choose LMP, due date, ultrasound, IVF transfer or a known ovulation/fertilization date. Enter only the relevant dates and details; results update automatically. Review the method assumptions and clinician guidance before using a dating estimate.
Five clearly distinguished dating methods; actual calendar-day UTC arithmetic; a method-appropriate milestone timeline; separate, qualified natural-cycle intercourse windows; no IVF intercourse window; copy/reset and browser-local privacy.
Personal dates remain in browser memory. They are not uploaded, placed in the URL, stored in a profile or sent to a processing API. Copying is performed only when you choose Copy Result.
No. It estimates a fertilization dating anchor from calendar conventions, not an observed biological event. It cannot confirm pregnancy, prove intercourse timing, identify a biological parent or establish paternity. Use clinician guidance for pregnancy dating and medical decisions.
Ovulation releases an egg; fertilization is the joining of egg and sperm; intercourse can occur before fertilization; implantation is a separate later event. The optional known-date method treats the entered date as a dating anchor and does not verify any of these events. Implantation is not dated here.
Estimated due date is LMP plus 280 days plus cycle length minus 28. The ovulation/fertilization anchor is LMP plus cycle length minus 14 days. This assumes a roughly 14-day interval from ovulation to the next period. The supported cycle range is 21–45 whole days. Irregular cycles reduce reliability; the adjusted LMP-equivalent is a model reference, not another actual period.
From a due date, the calculator subtracts 266 days for the fertilization anchor and 280 for an LMP-equivalent. Ultrasound uses scan date minus clinician-reported gestational days for that reference, then adds 280 days for a due date. Gestational age is not age since fertilization. Early ultrasound and clinician-established due dates may be more reliable than LMP for irregular cycles.
For a documented day-3 embryo, due date is transfer plus 263 days and the fertilization anchor is transfer minus three days. For day 5, the corresponding offsets are plus 261 and minus five days. Transfer is not fertilization. A usual natural-cycle fertile intercourse window does not apply, and is omitted; follow your fertility clinic’s established dates.
For non-IVF methods only, it is an approximate natural-cycle model from five days before the estimated ovulation anchor through that day. It is not a proven intercourse or conception interval or a universal uncertainty range. Inputs remain in browser memory; there is no account, upload or third-party processing API. UTC date-only arithmetic avoids daylight-saving shifts.