Question

You were interested in examining the association between substance use and timing of first prenatal care visit. You interviewed 1,500 women visiting MCV for their prenatal care visit. Of the women interviewed, 500 reported using illicit drugs. Only 50 reported their first prenatal care visit during the first trimester of pregnancy. Amoung the non drug users, 700 reported their first prenatal care visit during the first trimester of pregnancy. You analyzed the measure of association between substance use and timing of first prenatal care visit. What kind of study did you conduct?

epidemiology

Answer 1

We are trying to study the association between substance use and timing of first prenatal care visit

Exposure - substance abuse

Outcome - first prenatal visit in first trimester

the fact that the woman are coming for their prenatal visit, the outcome has occured, and the history of exposure to substance abuse is found out by interview.

Therefore this is a case-control study

let us plot the information in the 2 x 2 table

	Case (First prenatal visit in the first trimester )	Control(First prenatal visit not in the first trimester )	Total
history of drug abuse (Exposure)	50 (a)	450 (b)	500 (a+b)
No history of drug abuse (Non-exposure)	700 (c)	300 (d)	1000 (c+d)
total	750 (a+c)	750 (b+d)	1500

We can study the association between substance use and timing of first prenatal care visit by calculating the odds ratio

Odds ratio (OR) is the odds that an event (case) will occur in the presence of exposure compared to the odds that an event will occur in the absence of that exposure

Formula = OR = odds that the case was exposed / odds that the control was exposed

OR = (a/c) / (b/d)

=a x d / c x d

= 50 x 300 / 450 x 700

= 15,000 / 3,15,000 = 0.04

Rules of interpretation of Odds ratio

• If the odds ratio is 1 = exposure doesn't affect the outcome
• If the odds ratio is > 1 = there are a higher odds of the outcome occurring after exposure
• If the odds ratio is <1 = Lower odds of the outcome occurring after exposure

As in this case, the odds ratio is <1, it means the odds of first prenatal visit in the first trimester lower in women who do drugs