Depends on breed of ewe and her genes and the same for the ram . My hill ewes with a hill ram always start at circa 140 days ,put a Texel ram on them and it goes to 146 days . Tex x ewes with Beltex or Highlander ram will do 143days , to a Texel ram 147 days when I used a Suffolk it was 148 days , First timers often carry for less time than an adult and a ewe can lamb every year at almost the same time