Author & Reviewer Dr. Anuja Dokras, MBBS, DNB (Obs & Gynae)
Oxford-trained specialist in Infertility & Women’s Health.
Founder of LMP Calculator.
“As a doctor, whenever a woman confirms her pregnancy, my very first question is, ‘What was the first day of your last period?’ This simple question is the foundation for mapping out your entire pregnancy journey. In this guide, we will demystify the crucial term ‘LMP’ in simple language.”
What is LMP in Pregnancy? Understanding the Basics
When you first visit your doctor to confirm pregnancy, one of the first questions you’ll hear is: “What was your LMP date?” This seemingly simple question is actually the foundation of your entire pregnancy care plan.
LMP stands for “Last Menstrual Period” – but here’s what many women get wrong: LMP doesn’t refer to when your period ended. Instead, LMP means the first day your last menstrual period started.
LMP Date Example:
- Period started: April 10th
- Period ended: April 14th
- Your LMP date = April 10th (first day it started)
Why is Your LMP Date So Important During Pregnancy?
Your LMP date isn’t just another piece of information – it’s a critical medical tool that helps healthcare providers:
1. Calculate Your Estimated Due Date (EDD)
The most common use of LMP is determining when your baby is expected to arrive. Doctors use your LMP to calculate the estimated date when you’ll complete 40 weeks of pregnancy.
2. Determine Gestational Age Accurately
LMP helps doctors know exactly how many weeks and days pregnant you are at any given time. This week-by-week tracking is essential because:
- Fetal development follows specific milestones
- Medical care is tailored to each pregnancy stage
- Potential complications can be identified early
3. Schedule Critical Pregnancy Tests
Important prenatal tests must be performed within specific timeframes:
- NT scan (Nuchal Translucency): 11-13 weeks
- Anatomy scan: 18-22 weeks
- Glucose tolerance test: 24-28 weeks
Without an accurate LMP date, you might miss these crucial testing windows.
4. Monitor Baby’s Growth and Development
By knowing your exact gestational age, doctors can:
- Compare your baby’s ultrasound measurements to normal ranges
- Identify if growth is on track
- Detect potential developmental concerns early
How to Calculate Due Date from LMP: Naegele’s Rule Explained
Most healthcare providers use Naegele’s Rule to calculate your estimated due date. This proven method has been used for over 150 years.
The Formula:
LMP Date + 280 days (40 weeks) = Estimated Due Date
Simplified Calculation:
First day of LMP + 9 months + 7 days
Step-by-Step Example:
If your LMP was April 10, 2024:
- Add 7 days: April 17, 2024
- Add 9 months: January 17, 2025
- Estimated Due Date: January 17, 2025
Expert Insight: “Remember, this is an ‘estimated’ date. Only about 5% of babies are born on their exact due date. The calculation assumes a regular 28-day cycle with ovulation on day 14.” – Dr. Anuja Dokras
What If You Don’t Remember Your LMP Date?
Don’t panic – this is incredibly common. Many women either can’t recall their exact LMP or have irregular menstrual cycles that make LMP-based calculations unreliable.
Solution 1: Early Pregnancy Ultrasound (Dating Scan)
When LMP is unknown, doctors perform a dating scan – typically between 8-13 weeks of pregnancy. This ultrasound:
- Measures the baby’s size
- Provides accurate gestational age
- Calculates a reliable due date
Solution 2: For Irregular Periods
If your menstrual cycle is significantly shorter or longer than 28 days, LMP-based calculations may be inaccurate because:
- Ovulation doesn’t occur on day 14
- Conception timing varies
- Standard formulas don’t apply
The gold standard solution: First-trimester ultrasound is considered more accurate than LMP for dating pregnancies, especially with irregular cycles.
When LMP Calculation Might Be Inaccurate
Several factors can make LMP-based due dates less reliable:
Irregular Menstrual Cycles
- Cycles shorter than 21 days or longer than 35 days
- Unpredictable ovulation timing
- Hormonal imbalances affecting cycle length
Recent Hormonal Changes
- Recently stopped birth control pills
- Breastfeeding while conceiving
- PCOS (Polycystic Ovary Syndrome)
Implantation Bleeding Confusion
Some women mistake early pregnancy bleeding for a menstrual period, leading to incorrect LMP dates.
LMP vs. Ultrasound Dating: Which is More Accurate?
While LMP dating is the traditional method, first-trimester ultrasound dating is generally more accurate, especially when:
- Performed between 8-13 weeks
- Measurements are taken by experienced technicians
- Multiple measurements confirm consistency
When Doctors Adjust Due Dates
If there’s more than a 7-day difference between LMP and ultrasound dating, most doctors will use the ultrasound date as the official due date.
Tips for Tracking Your LMP
Before Getting Pregnant:
- Keep a menstrual calendar or use period-tracking apps
- Note the first day of each period
- Track cycle length patterns
- Record any irregularities
If Already Pregnant:
- Try to recall your last normal period
- Check calendar apps or written records
- Ask your partner if they remember
- Don’t guess – be honest with your healthcare provider
Common LMP Mistakes to Avoid
- Using the last day of your period instead of the first day
- Confusing implantation bleeding with your actual period
- Not mentioning irregular cycles to your doctor
- Guessing dates instead of admitting uncertainty
Expert Advice on LMP and Pregnancy Dating
“LMP provides the crucial starting point for pregnancy care. If you’re unsure about your LMP, don’t worry. Modern ultrasound technology gives us accurate methods to care for you and your baby. The most important thing is beginning prenatal care as soon as you confirm pregnancy.” – Dr. Anuja Dokras
Key Takeaways About LMP in Pregnancy
- LMP = First day your last period started (not when it ended)
- Due date calculation: LMP + 9 months + 7 days
- Only 5% of babies arrive on their exact due date
- First-trimester ultrasound is more accurate than LMP dating
- Always tell your doctor about irregular periods
- Don’t guess your LMP date – honesty helps ensure better care
When to See Your Doctor
Contact your healthcare provider as soon as you:
- Confirm pregnancy with a home test
- Have questions about your LMP or due date
- Experience any pregnancy symptoms or concerns
Early prenatal care, regardless of whether you know your exact LMP, is crucial for a healthy pregnancy and baby.
Medical Disclaimer: This information is for educational purposes only and should not replace professional medical advice. Always consult with your healthcare provider for accurate pregnancy dating and prenatal care guidance.