How this calculator works
Subtracting 40 weeks gives an equivalent gestational dating start and subtracting 38 weeks gives an approximate conception date. The five days before conception are shown as an illustrative intercourse window based on the fertile-window convention. Due dates and ovulation timing are uncertain, so the window is not a forensic boundary. This cannot determine parentage, identify a specific encounter or reconstruct dates from a preterm birth. The result is an estimate for information only and is not medical advice.
Formula
Equivalent dating start = EDD - 280 days. Approximate conception = EDD - 266 days. Illustrative intercourse window = conception - 5 days through conception.
Worked example
EDD 2026-10-08 gives an equivalent start of January 1, approximate conception January 15, and an illustrative window January 10 to 15.
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Frequently asked questions
Is the equivalent start my actual last period?
It may differ, especially when the due date was assigned by ultrasound or assisted reproduction.
Can this establish paternity?
No. These dates are approximate and biological timing varies.
Why include an assessment date?
It allows the page to show gestational age reproducibly instead of depending on the device clock.