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Conception Calculator

Estimate the date of conception based on your due date, last period, or ultrasound. Includes fertile window estimation.

Conception Worksheet

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How Conception Date Is Estimated

Conception occurs when a sperm fertilizes an egg, usually within 24 hours of ovulation. In a typical 28-day menstrual cycle, ovulation occurs around day 14. The calculator uses this assumption to estimate conception.

From Due Date

A standard pregnancy is 280 days (40 weeks) from the last menstrual period, or 266 days (38 weeks) from conception. To find the conception date, subtract 266 days from the due date.

From Last Menstrual Period (LMP)

Conception is estimated at LMP + 14 days (day of ovulation in a 28-day cycle). If your cycle is longer or shorter than 28 days, ovulation may occur earlier or later.

From Ultrasound

An early ultrasound (6-9 weeks) measures the embryo to determine gestational age. This is often more accurate than LMP-based dating, especially for women with irregular cycles.

Understanding the Fertile Window

You don't have to have intercourse on the exact day of ovulation to conceive. The fertile window spans about 6 days:

  • 5 days before ovulation: Sperm can survive in the reproductive tract for up to 5 days
  • Day of ovulation: The egg is released and can be fertilized
  • 1 day after ovulation: The egg remains viable for about 12-24 hours

The highest probability of conception is from intercourse 1-2 days before ovulation, when sperm are already present when the egg is released.

Factors That Affect Conception Timing

FactorEffect on Ovulation
Short cycle (21-25 days)Ovulation may occur as early as day 7-11
Long cycle (32-35 days)Ovulation may occur around day 18-21
Irregular cyclesOvulation timing varies month to month
Stress / illnessCan delay or suppress ovulation
BreastfeedingCan suppress ovulation (lactational amenorrhea)

Conception vs Gestational Age

Gestational age (used by doctors) starts from the first day of the last menstrual period — about 2 weeks before conception actually occurs. So at "2 weeks pregnant" by gestational age, conception hasn't happened yet. Fetal age (from conception) is always about 2 weeks less than gestational age.

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Frequently Asked Questions

How do you calculate conception date from due date?

Subtract 266 days (38 weeks) from the due date. The due date is set 280 days after the first day of the last menstrual period, and conception usually happens about 14 days into a 28-day cycle, so counting back 266 days lands on the most likely conception date, give or take a few days.

When did I conceive based on my last period?

In a 28-day cycle, conception most likely happened about 14 days after the first day of your last period. For other cycle lengths, subtract 14 from the cycle length to find the ovulation day: a 31-day cycle puts ovulation around day 17, so conception was probably around 17 days after the period started.

Can you know the exact day you conceived?

No. Sperm can survive up to 5 days in the reproductive tract and the egg lives about 24 hours after ovulation, so intercourse on any of about six days could lead to pregnancy. Fertilization happens when the egg is released, which may be days after intercourse, so only a likely window can be estimated.

Is an ultrasound more accurate for conception date?

Usually yes, especially with irregular periods. An early ultrasound at 6 to 9 weeks measures crown-rump length to give gestational age, and subtracting two weeks from that gives the approximate age of the pregnancy from conception. This is often more reliable than estimates based on cycle dates.