1. What is LMP Calculation?

The LMP (Last Menstrual Period) calculation is a method to determine the start date of a woman's last menstrual period based on the estimated due date (EDD). This calculation is fundamental in obstetrics for estimating gestational age and tracking pregnancy progression.

2. How Does the Calculator Work?

The calculator uses the standard obstetric formula:

Where:

• \( LMP \) — Last Menstrual Period (date)
• \( EDD \) — Estimated Due Date (date)
• \( 280 \) — Standard duration of pregnancy in days (40 weeks)

Explanation: This calculation is based on Naegele's rule, which assumes a 28-day menstrual cycle and ovulation occurring on day 14. The 280 days represent 40 weeks of gestation from the first day of the last menstrual period.

3. Importance of LMP Calculation

Details: Accurate LMP calculation is crucial for determining gestational age, scheduling prenatal tests, monitoring fetal development, and establishing the estimated due date. It serves as the primary reference point for all pregnancy dating.

4. Using the Calculator

Tips: Enter the estimated due date in the date field. The calculator will automatically compute the corresponding last menstrual period by subtracting 280 days (40 weeks) from the provided due date.

5. Frequently Asked Questions (FAQ)

Q1: Why is LMP used instead of conception date?
A: LMP is more reliably remembered by most women than conception date, and it provides a standardized starting point for pregnancy dating that aligns with clinical practice.

Q2: What if my menstrual cycle is longer than 28 days?
A: For women with longer cycles, the calculated LMP may need adjustment. Typically, add the number of days beyond 28 to the calculated LMP date for more accurate dating.

Q3: How accurate is this calculation?
A: This calculation provides an estimate. First-trimester ultrasound is more accurate for dating pregnancy, especially if there's uncertainty about LMP or irregular cycles.

Q4: What if I don't know my exact due date?
A: If you're unsure of your due date, consult with your healthcare provider who can estimate it through physical examination, ultrasound, or other clinical methods.

Q5: Can this calculator be used for IVF pregnancies?
A: For IVF pregnancies, the calculation differs since the conception date is known precisely. Use the embryo transfer date plus 266 days (38 weeks) for a more accurate due date.