Pregnancy Conception Calculator

Select whether you know your due date or your last period start date.
Select your expected delivery date provided by your doctor.
Select the first day of your most recent menstrual period.
Average number of days between your periods (typically 28 days).
Conception Date Calculated Successfully!
Your Conception Information

How It Works

Calculate your conception date in three simple steps.

Step 1: Choose Input Method
Select whether you want to calculate from your due date or your last menstrual period (LMP) using the radio buttons.
Step 2: Enter Your Date
Provide either your due date or LMP date. If using LMP, also enter your average menstrual cycle length in days.
Step 3: Get Results
Click calculate to see your estimated conception date, fertile window, and gestational age at birth. Copy any result with one click.

Understanding Conception Calculation

Learn the key concepts behind estimating conception dates.

Gestational Age
Gestational age is measured from the first day of the last menstrual period, not from the conception date. It is typically 280 days (40 weeks) at full term.
Conception vs LMP
Conception typically occurs about 14 days after the start of the LMP, assuming a 28-day cycle. This means gestational age is approximately 2 weeks older than the actual fetal age.
Ovulation Timing
Ovulation usually occurs around day 14 of a 28-day cycle. For different cycle lengths, ovulation shifts accordingly — roughly 14 days before the next period begins.
Fertile Window
The fertile window spans approximately 6 days — the 5 days before ovulation plus ovulation day itself. Sperm can survive up to 5 days inside the reproductive tract.
Pregnancy Duration
A full-term pregnancy lasts approximately 266 days (38 weeks) from conception, or 280 days (40 weeks) from the first day of the LMP. Only about 5% of babies are born exactly on their due date.
Cycle Length Variation
The average menstrual cycle is 28 days, but normal cycles range from 21 to 35 days. Longer cycles mean ovulation occurs later, shifting the estimated conception date forward.
Ultrasound Dating
First-trimester ultrasound is the most accurate method for estimating gestational age, especially when LMP is uncertain or cycles are irregular.
Naegele's Rule
Naegele's Rule estimates due date by adding 280 days (or 9 months + 7 days) to the first day of the LMP. It assumes a 28-day cycle with ovulation on day 14.
Due Date Accuracy
Due dates are estimates, not exact deadlines. Only about 4% of babies are born on their calculated due date. Most arrive within 2 weeks before or after the estimated date.
Irregular Cycles
For irregular cycles, LMP-based calculations are less reliable. Using an average cycle length helps, but ultrasound dating in the first trimester provides the most accurate estimate.
Sperm Survival
Sperm can live inside the female reproductive tract for up to 5 days under ideal conditions. This means conception can occur from intercourse that happened several days before ovulation.
Egg Viability
After ovulation, the egg remains viable for only 12 to 24 hours. If fertilization does not occur within this window, the egg dissolves and is absorbed by the body.
Multiple Factors
Actual conception date depends on multiple factors including cycle regularity, stress levels, diet, travel, illness, and hormonal fluctuations that can shift ovulation timing.
Medical Disclaimer
This calculator provides estimates for informational purposes only. It does not replace professional medical advice. Always consult your OB-GYN for accurate pregnancy dating.
Limitations
No conception calculator can pinpoint the exact date of conception. The results represent the most statistically likely range based on standard medical assumptions about cycle timing and ovulation.
Trimester Breakdown
Pregnancy is divided into three trimesters: first (weeks 1-12), second (weeks 13-26), and third (weeks 27-40). Each trimester has distinct developmental milestones for the fetus.

Key Features

Everything you need to estimate your conception date.

Dual Input Modes
Calculate from either your due date or last menstrual period. Switch between modes instantly using the radio button selection.
Fertile Window Estimate
See the estimated 6-day fertile window showing when conception was most likely to occur based on your cycle data.
Custom Cycle Length
Enter your actual cycle length (20-45 days) instead of relying on the standard 28-day assumption for more personalized results.
Click-to-Copy Results
Click any result swatch or use the action buttons to copy the conception date, fertile window, or all results to your clipboard instantly.
Clean Result Display
Results are shown in a clear grid format with date values and a detailed breakdown panel — no confusing formula descriptions cluttering the output.
Gestational Age Info
See the estimated fetal age at birth in both days and weeks for a complete picture of your pregnancy timeline.
Visual Radio Selection
Styled radio buttons with highlighted selection state make it easy to see which input mode is currently active at a glance.
100% Client-Side Privacy
All date calculations happen locally in your browser. No pregnancy data, dates, or personal information are sent to any server.

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Frequently Asked Questions

Find answers to common questions about conception date calculation.

How is the conception date calculated?
From a due date, conception is estimated at 266 days before. From LMP, it is estimated at 14 days after the period start. Both methods are based on the standard gestational age model assuming ovulation on day 14 of the cycle.
What is the fertile window?
The fertile window is the period during each menstrual cycle when conception is possible. It typically spans 6 days — the 5 days leading up to ovulation plus the day of ovulation itself.
Is the conception date always accurate?
No. The conception date is an estimate based on average cycle assumptions. Actual conception can vary by several days due to irregular ovulation, stress, or other factors. ACOG recommends first-trimester ultrasound for the most accurate dating.
What if my cycle length is irregular?
Use your average cycle length for a rough estimate. However, for irregular cycles, LMP-based calculations become less reliable. Your healthcare provider may recommend an ultrasound scan for more precise dating.
Can this calculator determine the exact conception date?
No. No calculator can determine the exact conception date. This tool provides the most statistically likely date range. Only prenatal DNA testing or precise medical records could narrow it down further.
Should I consult my doctor about the results?
Yes. This calculator is for informational purposes only. Always discuss your pregnancy timeline with your OB-GYN. They can confirm dates through physical examination, ultrasound measurements, and clinical assessment.
What is the difference between gestational age and fetal age?
Gestational age counts from the first day of LMP (typically 40 weeks at birth). Fetal age (conceptional age) counts from conception (typically 38 weeks at birth). Gestational age is about 2 weeks more than fetal age.
Is my pregnancy data stored anywhere?
No. All calculations happen entirely in your browser using client-side JavaScript. No dates, cycle information, or personal data are sent to any server or stored in any way.