What are the sources for conception calculations?

The 280-day due date and the 14th-day ovulation assumption are ACOG Committee Opinion 700, https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date. The six-day window ending on the ovulation day is Wilcox, Weinberg, and Baird, New England Journal of Medicine, 1995, https://www.nejm.org/doi/full/10.1056/NEJM199512073332301.

Conception Calculator

A conception window from a due date or a last period, counted backward. An estimate, not a diagnosis.

Smart Tips

• This page only counts backward to a conception window. The pregnancy calculator adds 280 or 266 days going forward.

• A due date minus 266 days is the same 14-day offset as last period plus 14 days.

• The window is six days ending on that estimated ovulation day: five days before, through that day.

• The offset assumes the 28-day cycle in ACOG Committee Opinion 700. A different cycle length is not a field here.

• Ultrasound dating is the dating method when it disagrees with a period date. This page does not read a scan.

• The window is not a paternity test and not a diagnosis.

The Formula

From a due date: ovulation = due date − 266 days
From a last period: ovulation = last period + 14 days
Window = ovulation − 5 days through ovulation

280 Days from last period to due date
ACOG Committee Opinion 700

266 Days from ovulation estimate to due date
280 minus 14

14 Days from last period to the estimate
Same offset as the pregnancy page

6 days Window ending on ovulation day
Wilcox, Weinberg, and Baird, 1995

1 January 2024 plus 14 days is 15 January 2024. The window is 10 January through 15 January. The same last period plus 280 days is 7 October 2024, and that due date counts back to the same window.

Key Insights

Reverse only

The pregnancy page accepts a conception date and adds 266 days. This page starts from a due date or a last period and returns a window.

A range, not one day

The six-day window ends on the estimated ovulation day. A single date would overclaim what the count can show.

Same 14-day offset

Last period plus 14 days, and due date minus 266 days, are the same relationship the pregnancy page already uses.

Not a scan

ACOG notes that the 280-day count assumes a 28-day cycle. Ultrasound dating is what changes a due date when the dates disagree. This page does not do that.

Medical Disclaimer

Estimate only: This page is an estimate, not a diagnosis, a lab test, or a treatment plan.

Not prenatal dating: This window is not a diagnosis, not a paternity test, and not a substitute for ultrasound dating.

Frequently Asked Questions

The estimated ovulation day is the due date minus 266 days. The window starts five days before that day and ends on that day. 266 is 280 minus 14, the same offset the pregnancy calculator uses when it goes forward.

The estimated ovulation day is the first day of the last period plus 14 days. The window is the six days ending on that date. The implied due date, shown beside it, is the last period plus 280 days. That forward date is not the hero result.

Wilcox, Weinberg, and Baird reported that conception occurred only when intercourse took place during a six-day period that ended on the estimated day of ovulation. A single day would hide that range. The egg side of that finding is a short survival time after ovulation.

No. The pregnancy calculator goes forward: last period plus 280 days, conception plus 266 days, or an ultrasound age. This page only counts back to a conception window.

ACOG Committee Opinion 700 says the 280-day count assumes a regular 28-day cycle with ovulation on the 14th day after the beginning of the cycle. This page keeps that assumption. It does not add a cycle-length field, and it does not override an ultrasound date.

The 280-day due date and the 14th-day ovulation assumption are ACOG Committee Opinion 700. The six-day window ending on the ovulation day is Wilcox, Weinberg, and Baird, New England Journal of Medicine, 1995, Timing of Sexual Intercourse in Relation to Ovulation.

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Conception window

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Ovulation estimate

Implied last period

Note:The window is six days ending on last period plus 14 days, or on due date minus 266 days. It assumes the 28-day cycle in ACOG Committee Opinion 700. An estimate, not a diagnosis or a paternity test.