Conception Date Calculator
Count backward from a recorded due date, a last period and cycle length, or dated pregnancy weeks.
The pregnancy dating guide explains why the estimate moves when the due date changes.
Counting backward to an around-date
This page answers a backward question: given a pregnancy date you already have, around when does the standard calendar place fertilization? It does not estimate a due date. That forward sum is the Due Date Calculator. It does not count today’s weeks. That is the Gestational Age Calculator. It does not build a fertile window. That is the Ovulation Calculator.
The Merck Manual describes estimated delivery as 266 days after a known conception date. Running that rule backward, this calculator subtracts 266 days from a recorded estimated due date. Two hundred sixty-six days is 280 minus 14: the 280-day pregnancy calendar minus the two weeks between a conventional period start and ovulation.
From a last period, the calculator first applies the same cycle-length adjustment the due-date page uses, (cycle length − 28) days, and then adds 14 days to that pregnancy-calendar start. On a 28-day cycle the around-date is 14 days after the period. A longer entered cycle moves it later by the same number of days; a shorter cycle moves it earlier. From dated pregnancy weeks, it steps back to the pregnancy-calendar start and again adds 14 days. ACOG Committee Opinion 700 is the reason to treat the result as movable: if ultrasound dating changes the documented due date, the backward estimate moves with it.
The pregnancy dating guide explains this idea without the form. The methodology records the day counts, including the statement that the tool does not publish one universal uncertainty window around the estimate.
Worked example
Recorded due date: October 7, 2024. Around-date = October 7, 2024 − 266 days = January 15, 2024. That is the same point you reach from a 28-day last period of January 1, 2024, because January 1 plus 14 days is January 15, and January 1 plus 280 days is October 7.
Change only the cycle length to 32 days and the pregnancy-calendar start shifts by 32 − 28 = 4 days, to January 5, 2024. Adding 14 days places the around-date on January 19, 2024. The related forward due date would be October 11, 2024. The extra four days are the cycle adjustment, not a measured ovulation.
A record of 8 weeks 4 days on March 1, 2024 is 60 days of gestational age. March 1 minus 60 days is January 1, 2024, and January 1 plus 14 days is again January 15, 2024. All three paths agree only because they were built from the same 28-day story. A revised clinical due date would move all of them together.
How to read the estimate
Read the date as an around-date on the pregnancy calendar. It is the conventional fertilization point implied by the due date or period you entered. ACOG’s fertility-awareness FAQ describes sperm survival on the order of days and a much shorter lifespan for the egg, which is why sex and fertilization need not fall on the same calendar day. ASRM’s committee opinion on optimizing natural fertility describes a six-day fertile window ending on the day of ovulation. Those facts explain the uncertainty. They do not widen this calculator’s single date into a personal probability range.
Use the result to orient a calendar: which cycle, which general part of the month, and how the date would move if the due date were revised. Do not use it as a finding about who contributed to the pregnancy, which act of intercourse led to fertilization, or the day implantation occurred. Parentage and exact fertilization timing are outside a day-count.
If what you actually have is a transfer record, stop here and use the IVF calculator. Embryo age plus transfer date is a different backward path, described in How IVF due dates are calculated.
What this date cannot settle
The estimate follows the due date. When the prenatal record has a current due date, subtract from that date, not from an older period calculation you prefer. Committee Opinion 700 says later changes to a documented due date should be uncommon and should themselves be documented. This page will not choose between two due dates for you.
Uncertain period dates, very irregular cycles, and recent hormonal contraception make the 14-day step a weaker story. A positive pregnancy test does not add the missing precision; test timing is explained on the Pregnancy Test Calculator. Ultrasound measurements are not entered here. If a scan changed the due date, enter the new due date and let the around-date follow it.
Questions about bleeding, pain, or how you feel go to your prenatal clinician or local urgent-care service. This calculator does not interpret symptoms, and it does not confirm that a pregnancy is inside the uterus or progressing.
Frequently asked questions
- Is the conception date exact?
It is an around-date from a 266-day calendar rule. ACOG’s description of sperm survival and egg lifespan is the reason an intercourse date and a fertilization date can differ. The result does not identify either one with certainty, and it says nothing about parentage.
- Why is conception about two weeks after my last period?
On the 28-day model, ovulation is placed 14 days after the period starts, and the due date is 280 days after that period. Subtracting 266 from the due date lands on day 14. A different usual cycle length shifts that day by the difference from 28.
- Can I enter the day we had sex?
This form does not have a sex-date field, because sex and fertilization are not the same event. If you know an ovulation date from a separate record, the due-date calculator can count forward 266 days from that date. The fertile-window estimate, when you only have a period and cycle length, is the ovulation calculator.
- What if my due date changed after an ultrasound?
Recalculate from the due date now written in the record. The around-date moves by the same number of days as the due date. Keeping the old conception estimate beside a new due date mixes two calendars.
Sources
These sources support the 266-day convention and the limits of a calendar fertilization date. They do not review or endorse Pregnancy Metrics.
- Merck Manual — Evaluation of the Obstetric Patient. Estimated delivery 266 days after a known conception date.
- ACOG Committee Opinion 700 — Methods for Estimating the Due Date. The 280-day convention, the day-14 assumption, and documenting a single due date that later ultrasound may revise.
- ACOG — Fertility Awareness-Based Methods of Family Planning. Sperm survival and the short viability of the egg, which separate intercourse from fertilization.
- ASRM — Optimizing Natural Fertility. The six-day fertile window ending on ovulation, and the limits of calendar prediction.