Calculating Odds Ration

March 14, 2016

Question

A client had a cesarean delivery in a hospital setting for breech presentation with her first pregnancy. She is pregnant again and after exploring her delivery options, has decided she wants to attempt a vaginal birth after cesarean (VBAC). She has had an uncomplicated pregnancy this time and the fetus is not breech. The same OB-GYN will be assisting in her delivery. The OB-GYN performs a systematic review of the literature to assess the benefits and harms of VBAC versus repeat cesarean delivery.

Case: Calculating Odds Ratio
In planning for her delivery, the client reads about birthing centers and asks the midwife if it is safe to have a VBAC in a freestanding birthing center. The midwife reviews the data from national studies of VBACs in birthing centers compared to VBACs in hospital settings and obtains the following statistics to aid her in clinical decision making:

N= 1913 Birthing Center based VBAC Rates
• 87% delivered vaginally
• 24% of women were transferred to the hospital prior to delivery
• There were 25 women who experienced a serious adverse outcome (of which 6 were uterine rupture)
• There were 7 perinatal deaths (0.5%)
• There were 15 infants with low apgar scores (below 7) after 5 minutes of life (1.0%)

N= 1913 Hospital based VBAC Rates (Control)
• 76% delivered vaginally
• There were 32 women who experienced a serious adverse outcome (of which 15 were uterine ruptures)
• There were 3 perinatal deaths
• There were 2 infants with low apgar scores (less than 7) after 5 minutes of life

(Part 2 of 2): Construct the following for 1 and 2 and answer question 3

1. Construct a 2 x 2 table, calculate, and interpret the odds ratio of women who suffered a serious adverse outcome from attempting a VBAC delivery in order to estimate the relative risk to a mother delivering VBAC in midwifery based freestanding birthing centers. Cases are those with a serious outcome, controls are those without. The exposure is treatment in a birthing center. The not exposed group is treatment in a hospital.

Exposure

Cases

Controls

Birthing Center

Hospital

2. Construct a 2 x 2 table, calculate, and interpret the odds ratio of infants who suffered a serious adverse outcome (including death) from attempting a VBAC delivery in order to estimate the relative risk to an infant delivered VBAC in midwifery based freestanding

Cases

Controls

3. What does the midwife conclude regarding the safety to mother and baby by attempting a VBAC in midwifery based birthing centers? What clinically is the best decision for this client and her unborn baby?

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