Female dog can get pregnant during their heat cycle which is also called estrous cycle.
Adult female dog will usually experience the heat cycle every 9 to 12 months, one or three cycle a year depend on the breed and type of your dog.
The four stage of estrous cycle and Dog heat symptoms
The first stage is proestrus which has the average of 9 days for most female dogs. This period is often called "pre-heat". The symptoms for this period is vulva swells and bloody vaginal discharge. Female dog will produce hormone (estrogen) which attracts male dog but she will still reject the male during this period.
The second stage is estrus which last about 9 to 12 days depend on the type and size of your dog. Female may still have a discharge and will accept male dog at this stage. There will be significant increase in progesterone and luteinizing hormone which cause ovulation. The eggs that was released from ovulation will go into oviduct. Eggs can be fertilized if it is healthy enough and sperm from male dog are present.
The third stage is diestrus which range from 2 to 3 months. During this stage female dog will no longer accept male. The symptoms for this period is pregnancy, mammary glands swell and some dog may experience false pregancy. Progesterone hormones wii still remain high throughout this period.
The final stage is anestrus. This is the resting period preparing for the next heat cycle which last about 5 months. Hormone is reduce at this period.
If you are planning on breeding your female dog then you should talk with your veterinarians to determine the estrus stage, measuring progesterone hormones level etc. The reasons for doing analysis is to help timing the mating period.