How is the conception date calculated?

If you provide a due date, the calculator subtracts 266 days (38 weeks, or the average fetal age) to find the likely conception date. If you provide your last menstrual period (LMP), it assumes ovulation occurred around 14 days before your cycle ends and adds this to your LMP date. For example, in a 28-day cycle, it adds 14 days to the LMP; in a 32-day cycle, it adds 18 days.

How accurate is this conception calculator?

This tool provides a highly reliable estimate based on clinical data and averages. However, since sperm can live for up to 5 days in the reproductive tract and ovulation can vary slightly from month to month, the 'fertile window' is the most accurate range for conception. For the most precise dating, your healthcare provider will use first-trimester ultrasound measurements.

Can this calculator be used for irregular periods?

Yes, if you use the 'Last Period' method, you can input your average cycle length, which makes the estimate more accurate for those with cycles shorter or longer than the typical 28 days. However, if your periods are highly irregular, an early ultrasound will provide a more reliable date.

Can this calculator tell me who the father is?

No. This calculator provides an estimated date range for conception. Because sperm can survive for several days before fertilization occurs, it cannot be used to pinpoint the exact day of intercourse that resulted in pregnancy or for paternity determination.

Pregnancy Conception Calculator

Q: What is the difference between gestational age and fetal age?

Gestational age is the standard measurement used by doctors, calculated from the first day of your last menstrual period (LMP). It's about two weeks longer than the fetal age. Fetal age is the actual age of the developing baby, starting from the date of conception. Our calculator shows your current gestational age.

Discover your pregnancy timeline. Estimate your conception date, fertile window, and more with our free, easy-to-use calculator.

Calculate from:

Your Estimated Due Date

Understanding Your Pregnancy Journey from Day One

Welcome! If you're wondering about the very beginning of your pregnancy, you're in the right place. Our Pregnancy Conception Calculator helps you pinpoint the date you likely conceived. This key date is the true starting point for your baby's development and helps you understand your pregnancy timeline, from the first trimester to your due date.

Whether you have an estimated due date from your doctor or just know the date of your last menstrual period (LMP), our tool provides an accurate estimate in seconds.

How to Use the Conception Calculator: A Simple Guide

Finding your estimated conception date is easy. Just choose one of the two methods below:

Method 1: Calculate from Due Date. This is the best option if you've already had an ultrasound and your doctor gave you an estimated due date. Select the "Due Date" tab, enter the date, and let the calculator work backward to find the likely conception date.
Method 2: Calculate from Last Period. Perfect if you haven't seen a doctor yet. Choose the "Last Period" tab, enter the first day of your last menstrual period, and adjust the "Average Cycle Length" for a more personalized and accurate result.

After calculating, you'll not only see the conception date but also your "fertile window"—the range of days when intercourse could have led to pregnancy.

Conception, Ovulation, and Your Fertile Window Explained

To understand your results, it's helpful to know how conception works. It's a precise biological event that can only happen during a short period each month.

Ovulation: This is the main event. It’s when one of your ovaries releases a mature egg, which then travels into the fallopian tube. This typically happens about 14 days before your next period is due to start.
Fertilization: For a pregnancy to occur, a sperm must fertilize this egg. A released egg is viable for only about 12-24 hours.
The Fertile Window: While the egg's lifespan is short, sperm can survive inside the female reproductive tract for up to five days. This means your fertile window—the time you can actually get pregnant—is the five days leading up to ovulation plus the day of ovulation itself. Our calculator shows this six-day period as your "Possible Intercourse Dates."

Knowing this window is crucial for understanding how your conception date is estimated. While the calculation is a strong estimate, it is not a guarantee of the exact date of intercourse.

Why Your Conception Date Matters

Pinpointing your conception date does more than satisfy curiosity. It helps you:

Track Developmental Milestones: Knowing the fetal age helps you follow your baby's growth week by week.
Plan for Prenatal Care: It helps in scheduling important tests and check-ups at the right time.
Understand Your Due Date: It clarifies why your due date is set at 40 weeks from your LMP, not from conception.

Frequently Asked Questions (FAQ)

What's the difference between gestational age and fetal age?

This is a common point of confusion. Gestational age is the standard used by all healthcare providers. It is calculated from the first day of your last menstrual period (LMP) and totals 40 weeks. Fetal age is the actual age of your baby from the moment of conception, which is about 38 weeks. Your doctor will always refer to your pregnancy by its gestational age.

Why is my doctor's due date different from a calculator's?

Calculators based on LMP are highly accurate, especially for women with regular 28-day cycles. However, the most accurate method for dating a pregnancy is a first-trimester ultrasound. An ultrasound measures the baby's size (crown-rump length) to determine its gestational age. If your cycle is irregular or you're unsure of your LMP, your doctor will rely on this ultrasound measurement, which may result in a slightly different due date.

Can this calculator tell me exactly who the father is?

No. This calculator provides an estimated date range for when conception most likely occurred. Because sperm can survive for several days before fertilizing an egg, it cannot be used to pinpoint the exact day of intercourse that led to pregnancy or for paternity purposes.