* Compute additional twins in the last 5 years.  One is counted in totpreg7m already, this just counts the 2nd and 3rd, etc.
* First rename birth history variables to facilitate forvalues loop
rename b*_0* b*_* 
gen twins = 0
* loop through the birth history counting the additional twins in the last 5 years
forvalues i = 1/20 {
  * only count second, third, etc. of twins if born in the last 5 years
  replace twins = twins+1 if b3_`i' != . & b3_`i' >= v008-59 & b0_`i' >= 2
}
replace totpreg7m = totpreg7m+twins
tab totpreg7m [iw=wgt]