### Description

Smith et al used probability models for estimating the risk of cesarean section for a pregnant woman. The authors are from Cambridge University, Austin Community College (Texas), and NHS Board Glasgow.

Patient selection: pregnant woman with singleton fetus

Parameters (Table III, page 2032).

(1) maternal age

(2) maternal height in centimeters

(3) week gestation

(4) gender of fetus

 Maternal Age Likelihood Ratio < 16 years NA 16 to 26 years (0.003298 * ((age)^2)) - (0.07917 * (age)) + 0.8150 26 to 48 years (0.000081 * ((age)^2)) + (0.09836 * (age)) - 1.653 > 48 years NA

 Maternal Height Likelihood Ratio < 143 cm NA 143 to 152 (0.02981 * ((hgt)^2)) - (9.225 * (hgt)) + 715.8 152 to 164 (0.006758 * ((hgt)^2)) - (2.247 * (hgt)) + 187.6 164 to 176 (0.001783 * ((hgt)^2)) - (0.6303 * (hgt)) + 56.24 176 to 183 0.54 > 183 cm NA

 Parameter Finding Likelihood Ratio week gestation < 37 NA 37 weeks 1.23 38 weeks 0.91 39 weeks 0.84 40 weeks 0.89 41 weeks 1.01 42 weeks 1.42 43 weeks NA gender of fetus unknown 1 male 1.20 female 0.81

Step 1: Estimate pretest probability of the woman having a cesarean section, in percent.

Step 2: Determine background odds.

background odds of cesarean section =

= (percent cesarean section) / (100 - (percent cesarean section))

Step 3: Calculate posterior odds for the cesarean section.

posterior odds =

= (background odds) * (LR maternal age) * (LR maternal height) * (LR week gestation) * (LR gender fetus)

Step 4: Convert posterior odds to estimated risk.

estimated risk =

= (posterior odds) / (1 + (posterior odds))