Minimum effective dose of carbetocin for preventing uterine atony during cesarean delivery:Comparison between patients with preterm versus term pregnancy.

Aim: To evaluate and compare the optimal dose of carbetocin for preventing uterine atony during cesarean delivery in patients with preterm versus term pregnancy.

**Objective:**To evaluate and compare during cesarean delivery in patients with preterm and termpregnancy, the minimum effective dose (defined as ED90 i.e., dose expected to be adequate in 90% of target population) of carbetocin for achieving and maintaining adequate uterine tone.

The bolus dose of carbetocin will be injected over 30 seconds to 1 min immediately after delivery of fetal head. The dose will be 20 mic for the first patient in both cohorts. The dose in subsequent cases for either cohort will be determined by the response to the bolus in previous patient of the particular cohort. After a “failed” dose of carbetocin bolus, the subsequent patient of the group will receive amount greater by 10 mic. In case of a “successful” dose, it will be decreased by 10 mic with a probability of 1/9; otherwise it will remain unchanged. This method of biased-coin allocation method with a probability of 1/9 after a successful dose will be implemented by drawing chits out of an opaque bag. While 9/10 of the chits will read for “same dose” to be repeated, 1/10 will instruct for a “decrease in dose”. The instruction on the randomly pulled out chit will be followed. Determinant of a successful dose will be a satisfactory uterine tone at time of first assessment (2 min after carbetocin) along with no request of any additional uterotonic by obstetrician intraoperatively. Additional uterotonic will be given on request of obstetrician at any time after 2 minutes of carbetocin injection

Statistical analysis: The primary outcome i.e., the ED90 of carbetocin will be calculated using dose-response data for each group by applying log-logistic regression with bootstrapping.