local callen = 80
* create separate variables for every month
forvalues i = 1/`callen' {
  gen vcal1_`i' = substr(vcal_1,`i',1)
  * Calculate duration of pregnancy if there is a birth in this month
  gen vcalp_`i' = indexnot(substr(vcal_1,`i'+1,`callen'-`i'),"P") if substr(vcal_1,`i',1) == "B"
  * Note that cases with duration 0 are truncated cases where we do not know the length of the pregnancy
}

* reshape the data file into variables for each month
reshape long vcal1_ vcalp_, i(caseid) j(i)

lab var vcalp_ "Duration of pregnancy"
lab def vcalp 0 "Unknown duration"
lab val vcalp_ vcalp

* Keep only the births
keep if vcal1_ == "B"

* CMC date of birth
gen cmc=v017+80-i