How to compute age of gestation

In this guide, we'll walk through how to compute the age of gestation (AOG) and expected delivery date (EDC) manually.

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How to compute age of gestation

AOG refers to the length of time a fetus has been developing inside the mother's uterus. It is calculated in weeks and days, starting from the first day of the mother's last menstrual period (LMP).

The EDC, also known as the estimated due date (EDD), is the projected date when a pregnant woman is expected to give birth. Accurately estimating the EDC helps healthcare professionals monitor the progress of pregnancy and plan appropriate care.

How to Compute AOG and EDC Manually

There are a few different methods and formulas for computing AOG and EDC manually, including Naegele's Rule, McDonald's Rule, and LMP-based calculations.

Naegele's Rule

Naegele's Rule is a standard method for estimating the EDC based on the mother's LMP. Follow these steps:

  1. Determine the first day of the LMP.
  2. Add 7 days.
  3. Subtract 3 months.
  4. Add 1 year.

Example: If the LMP is January 10, 2023:

  1. Add 7 days: January 17, 2023
  2. Subtract 3 months: October 17, 2023
  3. Add 1 year: October 17, 2024 (EDC)

McDonald's Rule

McDonald's Rule estimates AOG using fundal height (the distance between the pubic bone and the top of the uterus). Here's the steps:

  1. Measure fundal height in centimeters (cm).
  2. Divide the fundal height by 4.

Example: If the fundal height is 24 cm:

  • 24 cm ÷ 4 = 6 (AOG in months)

LMP-Based Calculations

To compute AOG using LMP (last menstrual period), follow these steps:

  1. Calculate the number of days between the current date and the first day of the LMP.
  2. Divide the total number of days by 7 to find the AOG in weeks.

Example: If the LMP is January 10, 2023, and the current date is June 10, 2023:

  1. Days between: 151
  2. 151 ÷ 7 ≈ 21.6 (AOG in weeks)
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