1. What is LMP-Based Pregnancy Dating?

LMP-based pregnancy dating is the standard method for estimating gestational age by counting weeks from the first day of the woman's last menstrual period (LMP). This method assumes a regular 28-day cycle with ovulation occurring on day 14.

2. How Pregnancy Weeks Are Calculated

The calculation uses this simple formula:

Where:

• LMP Date — First day of last menstrual period
• Current Date — Today's date or any date you want to calculate for

Explanation: The calculation counts the number of days between the two dates and converts to weeks by dividing by 7.

3. Importance of Accurate Pregnancy Dating

Details: Accurate pregnancy dating is crucial for proper prenatal care, scheduling tests at the right times, monitoring fetal growth, and determining the due date (approximately 40 weeks from LMP).

4. Using the Calculator

Tips: Enter the first day of your last menstrual period and the current date (defaults to today). The calculator will show how many weeks and days pregnant you are.

5. Frequently Asked Questions (FAQ)

Q1: What if my cycles aren't regular?
A: LMP dating may be less accurate if your cycles are irregular. An early ultrasound can provide more accurate dating in these cases.

Q2: Why does pregnancy count include weeks before conception?
A: Pregnancy is traditionally dated from the LMP because this is a definite date, while ovulation/conception dates are often uncertain.

Q3: How accurate is LMP dating?
A: For women with regular cycles, it's accurate to within ±5 days. Ultrasound in first trimester can confirm or adjust the dating.

Q4: When should I see a doctor after a positive test?
A: Most providers schedule the first prenatal visit around 8 weeks from LMP, unless you have medical concerns.

Q5: What if I don't remember my LMP?
A: If you don't know your LMP, an early ultrasound can estimate gestational age based on fetal measurements.