When do border terrier's first come into season we have a male and female from the same litter nearly 6 months old so we need to know when they first come into season thanks?

Female Border Terriers typically come into season for the first time between 6 to 12 months of age, although it's not uncommon for some to start as early as 5 months or as late as 18 months. Signs of the first heat cycle include behavioral changes and swelling of the vulva. It's important to monitor her closely during this time, as she will be fertile and can potentially mate with your male. If you are not planning to breed, consider spaying to prevent unplanned litters and health issues.