Due Date Calculator

Calculate your pregnancy due date using Last Menstrual Period (LMP), conception date, or ultrasound dating methods.

Accurately determine your pregnancy due date 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.

Last Period: 2024-01-01

Conception Date: N/A

Ultrasound Date: N/A

Cycle Length: 28 days

Known Conception Date

Known Conception Date

Pregnancy with known conception date for most accurate due date calculation.

Last Period: N/A

Conception Date: 2024-01-15

Ultrasound Date: N/A

Cycle Length: 28 days

Ultrasound Dating

Ultrasound Dating

Pregnancy dating based on first trimester ultrasound measurements.

Last Period: N/A

Conception Date: N/A

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.

Last Period: 2024-01-01

Conception Date: N/A

Ultrasound Date: N/A

Cycle Length: 35 days

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

What is the Due Date Calculator?

  • Core Concepts and Pregnancy Dating
  • The Science Behind Due Date Calculation
  • Why Accurate Dating Matters
The Due Date 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 due date. Ultrasound dating uses fetal measurements to determine gestational age and calculate the due 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 due date, 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 due date 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 Due Date Calculator

  • Method Selection and Data Collection
  • Input Methodology
  • Result Interpretation and Application
Maximizing the accuracy of your due date 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 due date and current gestational age. Note the days remaining until your due 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 any irregularities in your menstrual cycle
  • Keep ultrasound reports with gestational age measurements

Real-World Applications of Due Date Calculator

  • Prenatal Care Planning
  • Pregnancy Monitoring and Milestones
  • Healthcare Provider Coordination
The Due Date Calculator serves multiple essential purposes in pregnancy care and monitoring. It's a valuable tool for expectant mothers, healthcare providers, and pregnancy support teams. Understanding how to use this calculator effectively can significantly improve prenatal care coordination and pregnancy outcomes.
Prenatal Care Planning and Scheduling
Accurate due date calculation is crucial for scheduling prenatal appointments, ultrasounds, and important pregnancy tests. Healthcare providers use the due date to determine when to perform various screenings and assessments. The calculator helps expectant mothers plan their pregnancy timeline and prepare for important milestones like genetic testing, anatomy scans, and glucose screening.
Pregnancy Monitoring and Development Tracking
The calculator tracks current gestational age and pregnancy progress, helping monitor fetal development against expected milestones. It identifies trimester boundaries and important pregnancy phases. This information is essential for understanding what to expect at each stage of pregnancy and recognizing normal development patterns.
Healthcare Provider Communication
Sharing calculated due dates and gestational age with healthcare providers ensures consistent pregnancy dating across all medical records. This coordination is vital for accurate prenatal care, especially if multiple providers are involved. The calculator results help facilitate effective communication between expectant mothers and their healthcare team.

Practical Applications:

  • Schedule prenatal appointments based on accurate gestational age
  • Plan important pregnancy tests and screenings at optimal times
  • Monitor fetal development against expected milestones
  • Coordinate care between multiple healthcare providers

Common Misconceptions and Correct Methods

  • Due Date Accuracy Myths
  • Calculation Method Misunderstandings
  • Pregnancy Timeline Confusion
Many misconceptions exist about pregnancy dating and due date calculation. Understanding these common errors and the correct approaches is essential for accurate pregnancy monitoring and realistic expectations about delivery timing.
Due Date Accuracy and Realistic Expectations
A common misconception is that the due date is the exact day the baby will be born. In reality, only 5% of babies are born on their exact due date. Most babies arrive within 2 weeks before or after the due date. The due date is better understood as a target date around which delivery is expected to occur. This understanding helps manage expectations and reduces anxiety about 'overdue' pregnancies.
LMP vs. Conception Date Confusion
Many people confuse the last menstrual period date with the conception date. Pregnancy is dated from LMP, which is typically 2 weeks before conception occurs. This dating method accounts for the fact that ovulation and conception timing can vary, while LMP is more reliably known. The calculator uses LMP as the standard reference point for pregnancy dating.
Ultrasound Dating Limitations
While ultrasound dating is highly accurate in the first trimester, its accuracy decreases as pregnancy progresses. First trimester ultrasounds can predict due dates within 3-5 days, while third trimester ultrasounds may have an error margin of 2-3 weeks. The calculator prioritizes first trimester ultrasound dating for maximum accuracy.

Common Misconceptions:

  • Only 5% of babies are born on their exact due date
  • Pregnancy is dated from LMP, not conception date
  • Ultrasound accuracy decreases as pregnancy progresses
  • Cycle length affects LMP due date calculations

Mathematical Derivation and Examples

  • Due Date Calculation Formulas
  • Gestational Age Mathematics
  • Clinical Validation Studies
The mathematical foundations of due date calculation are based on extensive clinical research and population studies. Understanding these formulas and their scientific basis helps appreciate the accuracy and limitations of different dating methods.
LMP Due Date Formula
The standard LMP formula adds 280 days (40 weeks) to the first day of the last menstrual period: Due Date = LMP + 280 days. This calculation assumes a 28-day cycle with ovulation on day 14. For women with different cycle lengths, the formula adjusts: Due Date = LMP + 280 days + (Cycle Length - 28) days. This adjustment accounts for variations in follicular phase length while maintaining the standard 14-day luteal phase.
Conception Date Formula
When conception date is known, the due date calculation is: Due Date = Conception Date + 266 days (38 weeks). This method provides the most accurate due date because it eliminates the variability of menstrual cycle length and ovulation timing. The 266-day period represents the actual gestational period from conception to birth.
Ultrasound Dating Mathematics
Ultrasound dating uses fetal measurements to determine gestational age. The due date is calculated as: Due Date = Ultrasound Date + (280 days - Gestational Age at Ultrasound). This method is most accurate when performed in the first trimester, as fetal development is most predictable during early pregnancy. The calculator converts weeks and days to total days for precise calculation.

Calculation Examples:

  • LMP: January 1, 2024 + 280 days = October 7, 2024 due date
  • Conception: January 15, 2024 + 266 days = October 7, 2024 due date
  • Ultrasound: February 15, 2024 at 8 weeks = October 7, 2024 due date
  • Irregular cycle: 35-day cycle adds 7 days to standard LMP calculation