Gestational Age calculation formula method.pdf

Three formular to help you calculate gestational Age of a pregnancy very fast

1. Dr Musa

GESTATIONAL AGE CALCULATION
It is well known that medical Community is rather reluctant to accept such mathematical formulae.
But with repeated use, when their simplicity is realised, such formulae become part of the clinical
practice as shown in past by formulas for calculating Total Dose Iron (TDI) requirement; electrolytes
(Na+, K+, etc) requirement in cases or their deficiencies - etc.
If the period of amenorrhoea involves the month of February, for the sake of accuracy 2 days (3 if it is
leap year) should be subtracted from the above calculation.
CONCLUSION
The formula described for most accurate calculation of Gestational Age (GA) i.e

Gestational Age G.A. = 4(M) weeks + 5(M)/2 (or 5(M) x 1/2) days

where 'M' is the period of amenorrhoea in months appears to be more accurate than previously
existing methods and being a simple mental calculation, the method spares the obstetrician of a
calculating device
Ex. 1 - LMP on 20.12.2023, to calculate G.A. on 10.08.2024
for case of calculation, we calculate the period from 20.12.2023 to 10.08.24
number of months = (current date - LMP)
= 10.08.2024 - 20.12.2023
= 21days + 7 months
so, M = 7
Gestational Age G.A. = 4(M) weeks + 5(M)/2 days +21days
= 4(7) weeks + 5(7)/2 days +21days
= 28weeks + 35/2days + 21days
= 28weeks + 17.5days + 21days
= 28weeks + 38.5days
= 28weeks + 5weeks + 3.3days
= 33weeks 3.3days

Or
Calculating from 20.12.23 to 20.08.24 to achieve complete month
Month elapse = (Current date - LMP)
= 20.08.24 - 20.12.23

2. Dr Musa

= 8month

Gestational Age GA = 4(M) weeks + 5(M)/2 days
= 4(8) weeks + 5(8)/2 days
= 32weeks + 40/2days
= 32weeks + 20days
But Current date 10.08.24 is 10days before 20.08.24 hence subtract 10day
Gestational Age GA = 32weeks + 20days - 10days
= 32weeks + 10days
= 32weeks + 1week + 3days
= 33weeks 3days

METHOD 2
Using 4 ⅓ Formula
Besides the Naegele formula, there is one more formula that you can use to calculate the age of the
foetus in the womb, namely the 4 ⅓ formula. This is an example of applying the formula.

Gestational Age G.A. = {(Current date – LMP) x (4 ⅓)}

An example of a calculation scenario is as follows:
Today is August 10th, 2024. Mother-to-be’s LMP falls on December 20th, 2023
That means:
Gestational Age = {(10.08.2024 - 20.12.2023) x (4 ⅓)
= 21days + 7months x (4 1/3) + 0years
= 21days + 91/3weeks
= 21days + 30weeks + 2.3day
= 3weeks + 30weeks + 2.3days
= 33weeks 2.3days
Days are subtracted from days; months are subtracted from months, and years from years

Multiplication is only done in months. Days don’t need to be multiplied by 4 ⅓

3. Dr Musa


So the gestational age in the data above is 33 weeks 2days

Or
Calculating from 20.12.23 to 20.08.24
Months = (Current date - LMP)
= 20.08.24 - 20.08.23
= 8months

Gestational age GA = 4 1/3 x (M) weeks
= 4 1/3 x (8) weeks
= 104/3 weeks
= 34. 7weeks
1week = 7days
0.7weeks = 0.7weeks x 7days/1weeks
= 4.9days
= 5days
Gestational Age = 34weeks + 5days
Current date 10.8.24 is 10days before 20.08.24 ie 1weeks 3days
True gestational age = (34weeks +5days) - (1weeks + 3days)
= 33weeks 2days
It is undeniable that the formula for calculating gestational age using the 4 ⅓ methods is a little more
complex than the previous method. However, this method can also be used as a benchmark for
obtaining EDD

METHOD 3
Calculate gestation age within 15-30sec off head

1month Pregnancy - 4weeks 3days
2month pregnant - 8weeks 5days
3months pregnant - 13weeks 0days
4months pregnant - 17weeks 3days

4. Dr Musa

5months pregnant - 21weeks 5days
6month pregnant - 26weeks 0days
7month pregnant - 30weeks 3days
8months pregnant - 34weeks 5days
9months pregnant - 39weeks 0days

LMP 20th December 2023. Current date 10th August 2024
From 20.12.23 to 20.08.24 is 8months
8months pregnancy is 34weeks 5days
But Current date is 10.8.2023 which is 10days less than 20.08.24
Therefore
Gestational age = 34weeks + 5days - 10days
= 33weeks 2days