EDD Calculator

Calculate your estimated date of delivery using Last Menstrual Period (LMP), conception date, or ultrasound dating methods.

Accurately determine your estimated date of delivery and track gestational age using clinically validated calculation methods for optimal prenatal care planning.

Examples

Click on any example to load it into the calculator.

Standard LMP Calculation

Standard LMP Calculation

Typical pregnancy dating using last menstrual period with 28-day cycle.

LMP Method: lmp

Last Period: 2024-01-01

Cycle Length: 28 days

Known Conception Date

Known Conception Date

Pregnancy with known conception date for most accurate EDD calculation.

Conception Method: conception

Conception Date: 2024-01-15

Cycle Length: 28 days

Ultrasound Dating

Ultrasound Dating

Pregnancy dating based on first trimester ultrasound measurements.

Ultrasound Method: ultrasound

Ultrasound Date: 2024-02-15

Gestational Age: 8 weeks 3 days

Cycle Length: 28 days

Irregular Cycle LMP

Irregular Cycle LMP

Pregnancy dating for women with longer menstrual cycles.

LMP Method: lmp

Last Period: 2024-01-01

Cycle Length: 35 days

Other Titles
Understanding EDD Calculator: A Comprehensive Guide
Master the science of pregnancy dating and gestational age calculation. Learn how to accurately determine estimated dates of delivery using multiple methods for optimal prenatal care.

What is the EDD Calculator?

  • Core Concepts and Pregnancy Dating
  • The Science Behind EDD Calculation
  • Why Accurate Dating Matters
The EDD (Estimated Date of Delivery) Calculator is a sophisticated pregnancy dating tool that helps expectant mothers and healthcare providers accurately determine pregnancy due dates and track gestational age. It uses multiple clinically validated methods including Last Menstrual Period (LMP), conception date, and ultrasound dating to provide the most accurate pregnancy timeline. This calculator is essential for prenatal care planning, ultrasound scheduling, and monitoring fetal development.
The Foundation: Pregnancy Dating Methods
Pregnancy dating can be calculated using three primary methods, each with different levels of accuracy. The Last Menstrual Period (LMP) method is the most commonly used and adds 280 days (40 weeks) to the first day of the last menstrual period. The conception date method adds 266 days (38 weeks) to the known conception date, providing the most accurate estimated date of delivery. Ultrasound dating uses fetal measurements to determine gestational age and calculate the delivery date.
Gestational Age and Pregnancy Timeline
Gestational age is measured from the first day of the last menstrual period, not from conception. A full-term pregnancy is 40 weeks (280 days) from LMP, but only 38 weeks (266 days) from conception. The calculator tracks current gestational age, weeks remaining until delivery, and important pregnancy milestones like trimester boundaries. Understanding gestational age is crucial for monitoring fetal development and scheduling prenatal care.
Accuracy and Clinical Validation
The accuracy of estimated date of delivery calculation depends on the method used and individual factors. LMP dating assumes a 28-day cycle with ovulation on day 14, but many women have different cycle lengths. Conception date dating is most accurate but requires knowing the exact conception date. Ultrasound dating in the first trimester is highly accurate and can correct LMP dates by up to 7 days.

Key Pregnancy Dating Concepts:

  • Pregnancy is dated from the first day of the last menstrual period (LMP)
  • A full-term pregnancy is 40 weeks (280 days) from LMP
  • Conception typically occurs 2 weeks after LMP in a 28-day cycle
  • Ultrasound dating is most accurate in the first trimester

Step-by-Step Guide to Using the EDD Calculator

  • Method Selection and Data Collection
  • Input Methodology
  • Result Interpretation and Application
Maximizing the accuracy of your estimated date of delivery calculation requires understanding the different methods available, collecting accurate data, and interpreting results appropriately. Follow this systematic approach to ensure your pregnancy dating supports optimal prenatal care and monitoring.
1. Choose Your Calculation Method
Select the most appropriate method based on your situation. Use LMP if you have regular periods and know your last period date. Use conception date if you know when conception occurred (most accurate). Use ultrasound dating if you have had a first trimester ultrasound with gestational age measurements. The calculator will guide you through the required inputs for each method.
2. Collect Accurate Data
For LMP dating, record the first day of your last menstrual period. For conception dating, note the date when conception likely occurred. For ultrasound dating, record the ultrasound date and the gestational age determined by the scan. Include your average cycle length if it differs from 28 days, as this affects LMP calculations.
3. Input Your Data Carefully
Enter all required information for your chosen method. Double-check dates and numbers for accuracy. The calculator will validate your inputs and provide error messages if corrections are needed. Ensure gestational age inputs are within reasonable ranges (4-42 weeks for ultrasound dating).
4. Interpret and Apply Results
Review your calculated estimated date of delivery and current gestational age. Note the days remaining until your delivery date and important pregnancy milestones. Share these results with your healthcare provider for prenatal care planning. Remember that only 5% of babies are born on their exact due date—most arrive within 2 weeks before or after.

Data Collection Guidelines:

  • Record the first day of bleeding as day 1 of your menstrual period
  • Track your cycle length for at least 3-6 months for accuracy
  • Note the exact date of conception if known
  • Record ultrasound measurements from first trimester scans

Real-World Applications of EDD Calculator

  • Prenatal Care Planning
  • Medical Decision Making
  • Personal Pregnancy Management
The EDD Calculator serves as a fundamental tool in modern prenatal care, supporting both healthcare providers and expectant parents in making informed decisions throughout pregnancy. Its applications extend beyond simple date calculation to comprehensive pregnancy management and medical planning.
Prenatal Care Scheduling
Healthcare providers use estimated dates of delivery to schedule prenatal appointments, ultrasound scans, and important pregnancy milestones. The calculator helps determine when to perform first trimester screening, anatomy scans, and other diagnostic tests. Accurate dating ensures optimal timing for all prenatal care interventions.
Medical Decision Making
Accurate pregnancy dating is crucial for medical decisions such as induction timing, cesarean section scheduling, and management of high-risk pregnancies. The calculator helps identify pregnancies that may need closer monitoring or early delivery. It also assists in determining fetal viability and appropriate interventions.
Personal Pregnancy Management
Expectant parents use the EDD Calculator to plan maternity leave, prepare for childbirth, and track pregnancy progress. It helps in understanding which trimester they're in and what developmental milestones to expect. The calculator also assists in planning for the baby's arrival and organizing support systems.

Clinical Applications:

  • Scheduling prenatal appointments and ultrasound scans
  • Determining optimal timing for genetic screening
  • Planning induction or cesarean section dates
  • Monitoring high-risk pregnancy progression

Common Misconceptions and Correct Methods

  • Dating Accuracy Myths
  • Method Selection Errors
  • Result Interpretation Mistakes
Understanding common misconceptions about pregnancy dating helps ensure accurate calculations and appropriate expectations. Many people have incorrect assumptions about how pregnancy dating works, which can lead to confusion and inappropriate planning.
Myth: Pregnancy Starts at Conception
A common misconception is that pregnancy begins at conception. In medical terms, pregnancy is dated from the first day of the last menstrual period (LMP), which is approximately 2 weeks before conception typically occurs. This dating system is used because LMP is more reliably known than conception date.
Myth: All Women Have 28-Day Cycles
Many people assume all women have 28-day menstrual cycles. In reality, cycle lengths vary significantly between women and even within the same woman. The EDD Calculator accounts for this by allowing cycle length adjustment, which can change the estimated date of delivery by several days.
Myth: Due Date is Exact
Another common misconception is that the calculated due date is exact. In reality, only about 5% of babies are born on their exact due date. Most babies are born within 2 weeks before or after the estimated date. The calculator provides a target date, not a guaranteed delivery date.

Correct Understanding:

  • Pregnancy is dated from LMP, not conception
  • Cycle length affects due date calculation
  • Due dates are estimates, not exact predictions
  • Ultrasound dating can correct LMP dates

Mathematical Derivation and Examples

  • Calculation Formulas
  • Method Comparisons
  • Accuracy Analysis
The mathematical foundation of EDD calculation is based on established obstetric principles and clinical research. Understanding the underlying formulas helps appreciate the accuracy and limitations of different dating methods.
LMP Method Formula
The LMP method uses the formula: EDD = LMP + 280 days + (cycle length - 28) days. The base 280 days represents 40 weeks of pregnancy. The cycle adjustment accounts for women with non-standard cycles. For example, a woman with a 35-day cycle would have her EDD adjusted by +7 days.
Conception Method Formula
The conception method uses: EDD = Conception date + 266 days. This represents 38 weeks from conception, which is equivalent to 40 weeks from LMP. This method is most accurate when the conception date is known with certainty.
Ultrasound Method Formula
The ultrasound method uses: EDD = Ultrasound date + (280 - gestational age at ultrasound). This method corrects for any discrepancies between LMP dating and actual fetal development. First trimester ultrasounds are most accurate for dating.
Accuracy Comparison
Conception dating is most accurate (±3 days) but requires knowing the exact conception date. Ultrasound dating in the first trimester is very accurate (±5 days) and can correct LMP dates. LMP dating accuracy depends on cycle regularity and can vary by ±7 days or more.

Calculation Examples:

  • LMP: Jan 1 + 280 days + (28-28) = Oct 7
  • Conception: Jan 15 + 266 days = Oct 7
  • Ultrasound: Feb 15 + (280-59) = Oct 7
  • Irregular cycle: Jan 1 + 280 + (35-28) = Oct 14