Why your female dogs fight?
The issue you are seeing is a very common one and unfortunately very difficult to manage. Countless households like yours with two female dogs living together see them get along very well until one or both hits social maturity. Social maturity in dogs is generally reached between the ages of 12 to 36 months, according to the Merk Veterinary Manual. The hormonal changes during heat cycles and pregnancy can exacerbate things, potentially causing very heated fights even between docile females. However, such fights may well endure months after heat/pregnancy because they may be competing over rank and their breeding rights, especially if a breeding male lives in the same household as well.
In nature, two females close to the same age would not live in such close proximity. Because of this unnatural setting you are likely seeing the consequences. Fights between female dogs in the same household are among the most injurious and long lasting. Fatalities can even occur, since many of the worst fights go on so long with neither party doing anything to stop the aggressiveness. In cases in which fights or other aggressive tendencies are occurring between members of the same household, it is essential that steps be taken to prevent any subsequent injuries and to protect and maintain a high quality of life for both dogs and humans. Each case is unique and therefore the specific treatment must be customized, but some general strategies apply. When dogs are having trouble getting along to the point of exhibiting aggressiveness within the household, treatment involves three approaches.
The first strategy is to manage the situation for safety so that there are not opportunities for threats, fights, or injuries. The second strategy is to teach all of the dogs that the way to get what they want in the house is to be polite and patient rather than being pushy and demanding. The third strategy is to work extremely hard so that every dog in the household becomes rock solid at performing basic behaviors on cue.
Hopefully, trainers and behaviorists will continue to try new techniques that will make resolution of the problem of two female dogs fighting in the home a more frequent occurrence.