When it comes to the entire birthing process, female dogs tend to be extremely capable. That doesn't mean, however, that you shouldn't always keep a close eye on their physical condition before, during and after whelping. After parturition, a little discharge from the vulva is pretty commonplace in dogs.
After your dog has given birth to her litter, you might notice a little bit of discharge for a while, usually for between six and eight weeks after the fact. This discharge often starts out somewhat heavy only to eventually become extremely subtle, like spotting. It usually becomes more and more irregular with time, too. The discharge usually is red or brown -- sometimes with small blood clots inside of it at the beginning. The color too tends to darken the further away from labor a female dog gets. The discharge isn't associated with discomfort or feelings of malaise in dogs.
Pay close attention to any vulva discharge just to make sure that everything is OK. If the discharge gives off an unpleasant smell, then it could signify a problem. It should, in normal circumstances, be completely devoid of any odor. The presence of an odor sometimes denotes infection, so take it seriously. Also note if the discharge seems especially thick, or if it looks like it consists of any pus. Pus sometimes points to placental retention, which means that the afterbirth is still inside the mother's uterus. Mother dogs with placental retention also frequently give off discharge that is either black or green. Notify your veterinarian immediately if any of these things are apparent in your pet's discharge.
Discharge at Other Times
Discharge in female dogs doesn't exclusively appear post-whelping. If your pet is close to giving birth and you notice that she has dark greenish discharge, it usually means that the puppies are just about the emerge. Lots of female dogs also have some discharge while they're pregnant -- mucus. This discharge is often slightly pink in color, too.
Other Post-Whelping Concerns
If you ever have any concerns about the discharge your dog gives out before, during or after parturition, consult your veterinarian as soon as possible. Also keep your eyes open to any other hints that something could be amiss with your dog's condition post-whelping, including issues with nursing, seemingly no mothering behaviors, appearance of stomachache, mammary gland inflammation, abnormally low energy, exhaustion, zero appetite and fever. Be attentive to signs of potential problems in the puppies too, whether inadequate growth or nonstop crying. Let the vet know if you see anything worrying at all, period.