Female dogs are not always ready and able to accept a male dog to reproduce. If you have an unspayed dog and she's at least a few months old, she could be in heat, meaning she'll be interested and receptive to an un-neutered male dog. It's called being "in season" because it only occurs about once every six months, says the American Kennel Club.
Dog periods, or the dog menstruation cycle, last about three weeks. During that time you might notice that your dog's vulva is swollen and she will exhibit a discharge that can be red or yellow. Unless you are interested in breeding your dog, keep her away from other dogs during this time, as she will be very attractive to male dogs. Male dogs can smell the pheromones released by a female dog, and could be likely to be aggressive in wanting to get to her.
Female dog in heat behavior
When a female dog is in heat, her behavior may change. She may be agitated or restless, or as Tractive says, she may even become more affectionate. She may whimper and pant more than usual. She may be aggressive to other pets in the home, or to other dogs she might encounter on her walks, so keep her firmly on leash.
This female dog in heat behavior can be irritating to be around, as it can continue almost constantly. It can also be difficult for the dog to endure, so unless you are planning to breed, consider getting her spayed. Dealing with the discharge is the other challenge. The AKC says that dogs can be as young as four months and as old as 24 months (very large dogs) when they have their first season.
Small dogs are more likely to go into heat earlier and more often, says the AKC, and the heat lasts two to four weeks. The discharge you see will change from red blood to clear fluid over that time. Creating a safe place for her where she can be by herself and you don't have to worry about her discharge getting on your furniture or clothing can be a good idea. For instance, you might use a baby gate to keep her in a bathroom or other space in your home where the floor is easy to clean.
Dog cycle pads
You may want to use some dog cycle pads if you don't want to keep her confined in a small area during her heat. Dog cycle pads are commercially available pads that absorb the discharge. These generally slip into dog "underwear" or "diapers" that slip on over their back legs and stay on their rear end. Look for washable "underwear," and either disposable or washable dog cycle pads.
Homemade dog diapers
Best Dog Central has a tip for making your own homemade dog diapers. Their advice is to acquire some baby diapers in a size that will fit your dog. All the modification that is needed for these homemade dog diapers are to cut a small hole in the diaper to accommodate your dog's tail.
They also suggest that you can make a homemade diaper from a dog pee pad. In this case, you would fold it into a triangular shape in a similar way to folding a cloth diaper for a human baby. Cut a hole for the tail and place it around your dog's rear end. Secure with tape.
You can make a washable diaper in a similar way by cutting a piece of cloth, perhaps from an old sheet or t-shirt, and cutting a hole for the tail. Using some easily removable tape, simply wrap it around your dog's rear end and secure. If the material you chose is not as absorbent as you would like it to be, you could use this piece of cloth as a "holder" and place an absorbent human or dog cycle pad inside it.