Female beagle dogs typically go into heat at 6 months old. Some beagles go into heat as early as 4 months old, while others wait until they are 12 to 15 months old.
Going into heat is part of the normal menstrual cycle for dogs. Once they have their first heat, it lasts two to four weeks and comes around every six months. Signs of a beagle going into heat include a swollen vulva, a pink or red discharge, and other male dogs becoming more interested in her, including sniffing her and becoming excited around her. BeaglePro.com suggests that female dogs, including beagles, may have discomfort and cramping while in heat. It is not uncommon for beagles to rest more often during heat cycles.