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Conception Date Calculator

Estimate your conception date and fetal age

Based on peer-reviewed medical formulas.

Choose Your Method

Enter Due Date
Enter Last Period
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Estimated Date of Conception
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Gestational Age
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Fetal Age
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Due Date
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How Conception Date Is Calculated

The estimated conception date is calculated using two common methods. If you know your due date, subtracting 266 days (38 weeks) gives you the approximate conception date. If you know the first day of your last menstrual period (LMP), adding 14 days accounts for the typical ovulation window.

Both methods rely on averages. In reality, ovulation can occur anywhere from day 10 to day 21 of your cycle, and sperm can survive in the reproductive tract for up to 5 days. This means the actual conception date may differ from the estimated date by several days. Early ultrasound measurements in the first trimester provide the most accurate dating.

Understanding Gestational vs Fetal Age

Healthcare providers typically use gestational age, which is measured from the first day of your last menstrual period. This makes pregnancy appear to last 40 weeks, even though actual development from conception is about 38 weeks (266 days). Gestational age is used because the LMP is usually easier to remember than the exact date of ovulation.

Fetal age (or embryonic age) is counted from the date of conception and more accurately reflects the baby's actual development. For example, at 12 weeks gestational age, the fetus is approximately 10 weeks old from conception. Understanding both measurements helps you better track your pregnancy milestones.

Timeline of Early Pregnancy

Use our Baby Feeding Calculator to plan ahead for when your baby arrives, and try our BMI Calculator to track healthy weight during pregnancy.

Frequently Asked Questions

When did I conceive?
You can estimate conception by subtracting 266 days (38 weeks) from your due date, or by adding 14 days to the first day of your last menstrual period. Remember that these are estimates — actual conception may occur a few days earlier or later.
What is the difference between gestational and fetal age?
Gestational age is counted from the first day of your last menstrual period (LMP) and is typically 2 weeks longer than fetal age. Fetal age is counted from the date of conception. A pregnancy at 10 weeks gestational age is about 8 weeks along in fetal development.
Can conception happen on a different day?
Yes. Sperm can survive up to 5 days in the reproductive tract, and the egg is viable for about 12 to 24 hours after ovulation. This means conception can happen several days after intercourse. The estimated conception date is based on averages and may not match the actual date of intercourse.
How accurate is the conception date?
Conception date estimates based on LMP or due date are accurate to within about 1 to 2 weeks. Early ultrasound measurements (first trimester) are the most accurate way to determine gestational age, with a margin of error of only 3 to 5 days.
Does ovulation always happen on day 14?
No. Day 14 is an average for a 28-day cycle, but ovulation timing varies widely between women and between cycles. Ovulation can occur anywhere from day 10 to day 21, depending on cycle length, stress, hormones, and other factors. This is why the estimated conception date is an approximation.
How do I calculate my due date from conception?
Add 266 days (38 weeks) to the estimated date of conception to get your due date. Alternatively, add 280 days (40 weeks) to the first day of your last menstrual period. Both methods yield the same result since ovulation typically occurs about 14 days after the LMP.

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BMI Calculator — Track weight during pregnancy

Weight Loss Calculator — Postpartum weight goals

Water Intake Calculator — Hydration during pregnancy

Calorie Deficit Calculator — Plan nutrition targets

Protein Calculator — Protein needs during pregnancy

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