Why Is Pregnancy Calculated From Last Period

Pregnancy Calculation Method:

\[ \text{Gestational Age} = \text{Current Date} - \text{LMP Date} + 14 \text{ days} \]

Gestational Age:

Estimated Conception Date:

Estimated Due Date:

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1. Why Is Pregnancy Calculated From Last Period?

Pregnancy is calculated from the first day of the last menstrual period (LMP) rather than the actual conception date because LMP is typically more reliably known and documented. This method, known as gestational age dating, adds approximately 2 weeks before conception occurs.

2. How Pregnancy Dating Works

The standard calculation method:

\[ \text{Gestational Age} = \text{Current Date} - \text{LMP Date} \] \[ \text{Conception Date} \approx \text{LMP Date} + 14 \text{ days} \] \[ \text{Due Date} = \text{LMP Date} + 280 \text{ days (40 weeks)} \]

Where:

\( \text{LMP} \) — First day of last menstrual period
\( +14 \text{ days} \) — Approximate time to ovulation and conception
\( 280 \text{ days} \) — Standard full-term pregnancy duration

Explanation: This method accounts for the variable timing of ovulation while providing a consistent starting point for pregnancy dating.

3. Historical and Clinical Basis

Details: The LMP-based calculation has been used for centuries because menstrual dates are often more memorable and documented than conception dates. It provides a standardized reference point for healthcare providers worldwide.

4. Using the Pregnancy Calculator

Tips: Enter the first day of your last menstrual period and the current date. The calculator will determine your gestational age, estimated conception date, and projected due date.

5. Frequently Asked Questions (FAQ)

Q1: Why add 2 weeks before conception?
A: This accounts for the follicular phase of the menstrual cycle when the body prepares for ovulation, providing a consistent starting point.

Q2: How accurate is LMP dating?
A: LMP dating is generally accurate for women with regular cycles. Ultrasound dating may be used for confirmation, especially with irregular cycles.

Q3: What if I don't know my LMP?
A: An early pregnancy ultrasound can provide accurate dating based on fetal measurements, particularly crown-rump length.

Q4: Does this work for IVF pregnancies?
A: For IVF, pregnancy is calculated from embryo transfer date + 14 days for LMP equivalent, or directly from known conception date.

Q5: Why is gestational age different from fetal age?
A: Gestational age includes the 2 weeks before conception, while fetal age starts at conception. Gestational age is always about 2 weeks older.