In a cohort of 500 women attending antenatal clinic, 70 % had ultrasonography (USG). This cohort was followed up at delivery. Of the women who had USG, 70 delivered low birth weight (LBW) babies; whereas of the women, who did not undergo USG, 50 delivered LBW babies. The incidence of LBW babies among women who had USG is:
Calculate Cohort Group Sizes
First, determine the number of women in each group based on whether they underwent ultrasonography (USG).
- Total women in the cohort: 500
- Percentage who had USG: 70%
- Number of women who had USG: $0.70 \times 500 = 350$
- Number of women who did not have USG: $500 - 350 = 150$
Determine Low Birth Weight (LBW) Cases
Identify the number of low birth weight (LBW) babies delivered by each group.
- LBW babies from women who had USG: 70
- LBW babies from women who did not have USG: 50
USG Incidence of LBW Calculation
Calculate the incidence of LBW babies specifically among the women who underwent USG.
The formula for incidence is:
$ \text{Incidence} = \frac{\text{Number of LBW babies among women who had USG}}{\text{Total number of women who had USG}} \times 100\% $Substitute the values:
$ \text{Incidence} = \frac{70}{350} \times 100\% $Simplify the fraction:
$ \text{Incidence} = \frac{1}{5} \times 100\% $Calculate the final percentage:
$ \text{Incidence} = 0.20 \times 100\% = 20\% $The incidence of LBW babies among women who had USG is 20%.
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