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Conception Calculator

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

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 is the conception date calculated?

Conception typically occurs about 14 days after the first day of the last menstrual period (LMP) in a 28-day cycle. From a due date, subtract 266 days (38 weeks) to estimate conception.

How accurate is the conception date?

The estimated date is approximate. Ovulation can vary by several days depending on cycle length. The fertile window (5 days before to 1 day after ovulation) accounts for this variability.

What is the fertile window?

The fertile window is the period during which intercourse could lead to pregnancy. Sperm can survive 3-5 days in the reproductive tract, and the egg is viable for about 24 hours after ovulation.

Can I determine the exact day of conception?

Not precisely. Even with ovulation tracking, the exact moment of fertilization is unknown. The calculator provides the most likely date based on standard cycle assumptions.