When it comes to estimating gestational age in pregnancy, calculating the Period of Amenorrhea (POA) is a vital skill for healthcare professionals and expecting parents. The POA represents the time elapsed since the first day of a woman’s last menstrual period (LMP), providing a straightforward way to estimate how far along a pregnancy is. In this article, we’ll explore how to calculate the POA with examples, building on the foundation of Naegele’s rule, which we’ve discussed in a previous article.

What is POA?

The Period of Amenorrhea (POA) is the duration from the first day of the last menstrual period to the present day. It’s commonly expressed in weeks and days. POA helps healthcare providers track pregnancy progress and anticipate key milestones.

How to Calculate POA

To calculate the POA:

  1. Identify the LMP: Note the first day of the woman’s last menstrual period.
  2. Calculate the number of days between the LMP and today’s date.
  3. Convert the total days into weeks and days.

Example 1: Calculating POA from LMP

Let’s say today is August 26, 2024, and a patient’s LMP was February 1, 2024. Here’s how you would calculate the POA:

  • Determine the total number of days from LMP to today:
    • February 1, 2024, to February 29, 2024 (leap year): 29 days
    • March 1, 2024, to March 31, 2024: 31 days
    • April 1, 2024, to April 30, 2024: 30 days
    • May 1, 2024, to May 31, 2024: 31 days
    • June 1, 2024, to June 30, 2024: 30 days
    • July 1, 2024, to July 31, 2024: 31 days
    • August 1, 2024, to August 26, 2024: 26 days
  • Total days: 29 + 31 + 30 + 31 + 30 + 31 + 26 = 208 days
  • Convert days into weeks:
    • 208 days ÷ 7 days/week = 29 weeks and 5 days

So, as of August 26, 2024, the POA would be 29 weeks and 5 days.

Example 2: Calculating POA from EDD

You can also estimate the POA using the estimated due date (EDD) by counting backward. Let’s assume an EDD of September 9, 2024. Here’s the calculation:

  1. Calculate days remaining until EDD:
    • August 26, 2024, to August 31, 2024: 5 days
    • September 1, 2024, to September 9, 2024: 9 days
    • Total days remaining: 5 + 9 = 14 days
  2. Calculate the gestational age:
    • Full-term is considered 280 days (40 weeks).
    • Days since conception = 280 – 14 = 266 days.
  3. Convert days into weeks:
    • 266 days ÷ 7 days/week = 38 weeks

Thus, on August 26, 2024, the gestational age is 38 weeks.

Example 3: Calculating POA with Naegele’s Rule

For a quick estimation of gestational age using Naegele’s rule, remember:

  1. Add 7 days to the LMP.
  2. Count forward 9 months to estimate the due date.

For instance, if the LMP is February 1, 2024:

  1. Add 7 days: February 8, 2024
  2. Count forward 9 months: November 8, 2024

From there, calculate the POA using the number of weeks and days since February 1, 2024.

Conclusion

Calculating the POA is essential for tracking pregnancy progress, understanding gestational age, and anticipating key milestones. Whether using the LMP, EDD, or Naegele’s rule, these methods provide valuable insights into fetal development and pregnancy management. By mastering these calculations, healthcare professionals and expecting parents can feel more informed and prepared throughout the pregnancy journey.

By Dr. Yomal Amarathunge

Dr. Yomal Amarathunge is a young and promising doctor who is making a difference in the world of medicine. He graduated from the University of Kelaniya in Sri Lanka, where he earned his Bachelor of Medicine, Bachelor of Surgery (MBBS) degree. In addition to his work as a doctor, Dr. Amarathunge is also a software developer. He is the creator of DewMal’s Health Blog, a website that provides information on health and wellness to the people of Sri Lanka. He is also the developer of DewMal’s Health App, a mobile app that provides users with access to health information and resources.

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