There is a condition in female dogs and cats known as "ovarian remnant syndrome" in which a spayed female continues to exhibit signs of estrus (heat periods). Usually this occurs because a portion of an ovary was left in the abdomen during the spay surgery. This can happen several ways -- sometimes the surgeon doesn't have a clear view of the ovary for some reason (obesity, not having a big enough incision, bleeding, etc.) and a portion is simply missed, other times it is a piece of the ovary is accidentally dropped as it is removed and reimplants in the abdomen. I have had a couple of patients in our practice that appeared to have small pieces of ovary near the normal ovarian tissue but separate from it. So far we
have seen these pieces and removed them but I tend to think that there are times when there is just extra ovarian tissue. Dogs that have ovarian tissue that is not removed during a spay tend to show all the normal signs of heat -- swelling of the vulva, bleeding, attraction of male dogs and often false pregnancy signs. These usually occur at the same interval as other estrus periods, roughly seven months. Cats tend to have normal estrus behavior for their species, too. This can be crying, rolling on the floor, overly friendly behavior and attraction of male cats at about 3 week intervals. The only treatment I know of is to find the ovarian tissue that remains and to remove it. This is easiest to do when the pet is in estrus because the ovarian tissue is active and easier to find.

In a dog that has already had exploratory surgery once I think it would be a good idea to do hormonal testing prior to considering another surgery. Sometimes it is possible to tell that ovarian tissue remains just by testing progesterone levels in dogs but only if the progesterone levels are over 2ng/ml. A hormonal response test that starts with administration of  HCG (human chorionic gonadotropin) during estrus behavior and then is followed by measurement of progesterone is more accurate. This test works for cats, too.