Pregnancy Conception Calculator
The Pregnancy Conception Calculator estimates the date of conception based on the expected due date of the pregnancy, last period date, or ultrasound date. The calculator also estimates a possible range of days during which sexual intercourse might have led to conception based on sperm being viable for 3-5 days within a woman's body.
Last updated: January 2026
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When Did I Conceive?
When exactly did I get pregnant? Many women ask this question when trying to figure out the exact day their pregnancy began. Often, gestational age, or the age of the baby, is calculated from the first day of the mother's last menstrual period. Biologically, the baby was not conceived until ovulation and the fertilization of the egg, which usually happens at least 10 days after the first day of the mother's last menstrual period.
Because there are many factors surrounding conception, it is difficult to pinpoint an exact date of conception. This calculator helps to estimate the date range of real conception and a possible range of days during which sexual intercourse might have led to conception. There are a few methods that can be used to estimate the conception dates.
Last Menstrual Period
Conception usually occurs around 11-21 days after the first day of the last period of a woman who has a regular period. The estimation of conception date is based on this, but is rarely ever exact since it is difficult to know exactly when ovulation occurs. Estimating conception date can also be more difficult for women who have irregular periods, or for those who don't remember the first day of their last period. In cases like these, one of the more accurate ways to estimate gestational age is through the use of an ultrasound.
Due Date
An estimated due date, usually based on a sonogram, is typically obtained from a healthcare provider during a prenatal visit. Based on this due date, a range of possible conception dates can be calculated. Conception typically occurs approximately 266 days before the due date, since the standard pregnancy length is 280 days from the last menstrual period, and conception happens about 14 days after the LMP.
Ultrasound
An ultrasound, on a basic level, uses sound waves to create images of internal body structures. An ultrasound used in the context of examining the progress of the fetus in pregnant women is referred to as an "obstetric ultrasound." This is done in real-time and is a fairly standard procedure that is part of prenatal care in many countries, as it can provide information about the health of the mother and embryo/fetus, as well as the overall progress and timing of the pregnancy. Ultrasounds can be used to measure a fetus as early as five or six weeks following the mother's last menstrual period. It is a more accurate measure in early pregnancy than it is in later stages. Ultrasound can be used in conjunction with estimates based on the last menstrual period to determine a more accurate gestational age.
Fertilization, Conception, and Pregnancy
Fertilization and conception are terms that are often used interchangeably. This calculator uses the medical and scientific definition of the terms. Fertilization occurs when the sperm and the egg combine in the fallopian tube, producing a fertilized egg, and beginning the process in which an embryo develops.
Conception refers to the process of becoming pregnant, which scientifically involves both fertilization as well as implantation into the wall of the uterus. On the state level (in the U.S.) there is some confusion between the use of the terms conception and fertilization due to differences in the definitions used. In some cases, a woman is considered pregnant after fertilization, but this is not in line with well-established scientific and medical definitions. The long-standing view of those in the medical profession, as well as federal policy (in the U.S.) considers a woman pregnant when a fertilized egg has implanted into the wall of her uterus. When this has occurred, the woman is said to have conceived. In other words, conception, medically and scientifically, requires both fertilization and implantation, and a woman is not considered pregnant until both have occurred.
How Our Conception Calculator Works
Our pregnancy conception calculator uses established medical formulas to estimate when conception likely occurred based on the information you provide. The calculation accounts for the natural variability in menstrual cycles and ovulation timing.
Calculation Methods
- From Due Date: Subtracts 266 days from the estimated due date, since conception typically occurs about 14 days after the last menstrual period, and pregnancy is 280 days from LMP.
- From Last Period: Adds 11-21 days (average 14) to the first day of your last period, adjusting for cycle length. Longer cycles mean later ovulation and conception.
- From Ultrasound: Uses the gestational age at the time of the ultrasound to calculate backwards to when conception likely occurred, providing the most accurate estimate when done in early pregnancy.
Understanding the Intercourse Date Range
The calculator provides a range of possible intercourse dates because sperm can survive in the female reproductive tract for 3-5 days. This means:
- Sexual intercourse up to 5 days before ovulation can result in pregnancy
- The most fertile window is typically 2-3 days before ovulation through the day of ovulation
- Eggs are viable for about 12-24 hours after ovulation
- The calculated range accounts for maximum sperm survival time
Important Note
These calculations are estimates and should not be used for medical or legal purposes. For accurate pregnancy dating and conception estimates, always consult with your healthcare provider. Ultrasound measurements in early pregnancy provide the most accurate dating.
Frequently Asked Questions
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