Derivation
variable: pregmthnow [months pregnant] (table[s]: tb_tbxto2007)
variable: prgmthsnow [months pregnant] (table[s]: tbold_part1/2, tbold_control)
variable: pregmthsnow [months pregnant] (table[s]: tb_tboto2007)
replace pregmthnow=prgmthsnow if pregmthnow==. & prgmthsnow~=.
replace pregmthnow=pregmthsnow if pregmthnow==. & pregmthsnow~=.