LMP Calculator

Professional Last Menstrual Period calculator used by healthcare providers. Calculate due date, gestational age, and pregnancy timeline.

Last Menstrual Period Calculator

Enter your LMP date to calculate due date, gestational age, and pregnancy timeline using medical standards

First Day of Last Menstrual Period (LMP)

Select the first day of your last menstrual period. This is the standard medical reference point for pregnancy dating.

Professional Features

Medical-grade LMP calculation using Naegele's Rule

Accurate gestational age in weeks and days format

Estimated due date (EDD) calculation

Automatic trimester identification

Clinical notes and medical information

Understanding LMP

LMP Definition: First day of last menstrual period

Medical Standard: Universally used by healthcare providers

Naegele's Rule: LMP + 280 days = Due date

Gestational Age: Includes 2 weeks before conception

Professional Usage Guidelines

Optimal Conditions: Regular 28-30 day cycles with known LMP

First Prenatal Visit: Standard calculation for initial dating

Medical Documentation: Required format for healthcare records

Insurance Forms: Standard gestational age calculation method

Weekly Tracking: Monitor pregnancy progression milestones

Accuracy Considerations

Most Accurate: Regular cycles, certain LMP date

Less Accurate: Irregular cycles, PCOS, breastfeeding

Ultrasound Override: Early dating scan takes precedence

Margin of Error: ±3-7 days for regular cycles

How to Use the LMP Calculator

Step-by-Step Instructions

Identify Last Menstrual Period

Find the first day of your last normal menstrual period - this is your LMP date.

Enter the LMP Date

Input the date using the date picker or type in MM/DD/YYYY format accurately.

Calculate Gestational Age

Click calculate to determine current pregnancy weeks, trimester, and due date estimation.

Review Pregnancy Timeline

Understand current gestational age, trimester status, and estimated due date.

Track Progress

Use results to monitor pregnancy progression and plan prenatal care appointments.

Save Documentation

Generate PDF reports for healthcare provider visits and personal pregnancy records.

Pro Tips for Accurate Dating

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Record keeping: Track your menstrual cycles to have accurate LMP dates for future pregnancies.

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Normal cycles assumed: LMP dating works best with regular 28-day cycles - irregular cycles may affect accuracy.

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Ultrasound confirmation: Early pregnancy ultrasounds provide more precise dating than LMP calculations.

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Trimester tracking: Use gestational age to understand which pregnancy trimester you're in.

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Medical verification: Always confirm pregnancy calculations with healthcare professionals for accurate care.

Frequently Asked Questions

What is LMP in pregnancy?

LMP stands for Last Menstrual Period. It refers to the first day of your last menstrual period before conception. This date is the standard starting point for calculating pregnancy weeks and due date in medical practice.

How does LMP calculator work?

The LMP calculator uses Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period to calculate the due date. It also calculates current pregnancy week and gestational age.

Why do doctors use LMP for pregnancy dating?

Doctors use LMP because it's easier to remember than conception date, provides a standardized calculation method, and has been proven accurate for most women with regular menstrual cycles.

What if I don't remember my LMP date?

If you don't remember your LMP date, your doctor may use ultrasound measurements for pregnancy dating, especially early ultrasounds (6-12 weeks) which are most accurate for determining gestational age.

Is LMP calculator accurate for irregular periods?

LMP calculator is most accurate for women with regular 28-30 day cycles. For irregular periods, ultrasound dating or conception date methods may provide more accurate results.

What is gestational age vs fetal age?

Gestational age is calculated from LMP and includes the 2 weeks before conception. Fetal age (conceptual age) is calculated from actual conception date and is about 2 weeks less than gestational age.

When should I recalculate using LMP?

Recalculate if your healthcare provider adjusts your due date based on ultrasound measurements, if you realize your LMP date was incorrect, or if there's a significant discrepancy between LMP and ultrasound dating.

How accurate is LMP dating compared to ultrasound?

Early ultrasound (6-12 weeks) is more accurate than LMP dating, with precision within 3-5 days vs 5-7 days for LMP. Healthcare providers often adjust due dates based on early ultrasound measurements.

Can I use this calculator after IVF or fertility treatments?

For IVF pregnancies, conception date is known precisely, making the conception date calculator more accurate. However, many providers still use adjusted LMP dates for standardized documentation.

What factors can affect LMP calculation accuracy?

Irregular cycles, PCOS, recent birth control use, breastfeeding, stress, significant weight changes, and certain medications can all affect cycle regularity and thus LMP calculation accuracy.

Important Medical Disclaimer

This LMP calculator provides estimates only and should not replace professional medical advice.LMP-based calculations are most accurate for women with regular menstrual cycles. Pregnancy dating can vary based on individual factors. Always consult your healthcare provider for accurate pregnancy dating, monitoring, and medical advice throughout your pregnancy journey.