I have come across many interesting articles on this topic over the years. I personally have never bred back to back, but I have to say it makes complete and total sense. I have experienced, with my Mastiffs, many of the issues it mentions from skipping heats. -Amanda @ Mountainheel



Breeding Back to Back
Recently at an AKC Dog Breeding Symposium held at Michigan State University with key note speaker Dr. Claudia Orlandi Ph.D. (AKC's Breeder of the year and author of The ABC's of Dog Breeding) shocked many Breeders when it was disclosed that there have been scientific studies and proven evidence to show that it is detrimental for bitches to skip heat cycles. It was shared that once you have started mating a bitch that you should NOT skip any heat cycles until she is completely finished breeding. A bitch is said to be "finished" breeding when her litter size is significantly decreased. The study involved following females that were bred every heat cycle and females that were bred every other heat cycle. After they were "finished" breeding, the bitches were spayed and their uterus's were dissected. Those showing most stress and damage of the uterus were the females that were bred every other heat cycle. Part of the rational that skipping heat cycles is harmful stems from the fact that with consecutive heat cycles there is no flushing action of the uterus that normally occurs by having a litter of puppies. The female will go through estrus, regardless if she is bred or not, and by breeding a healthy bitch back to back can lesson the chances of the female experiencing: pyometra, infections and phantom pregnancies. The choice to breed or not should be contingent upon the goals the breeder has, but most importantly, above all else, the mental and physical health of the female. The important findings to take away from this data is that a breeder with healthy females and honorable policies does have a choice.- Unknown.


I have attached a couple of links on this topic:
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#2: Breeding Back-to-Back: The Truth Behind Breeding Every Heat Cycle by JuliePetersen • It’s a question I sometimes get asked as a breeder, how frequently do you breed your dogs? There are various motivations behind the question, some as simple as trying to determine how long they will need to wait until a certain dog has puppies, others are trying to evaluate you as a breeder. It has been my experience in breeding that no matter what method you choose in anything breeding related, you have to be able to explain your choices. 
I love my dogs and this is why I breed them every heat cycle, it’s in their best interests.  Myth: Females Need a Break to Recover Many people feel that a bitch needs a break after having a litter and therefore, at a minimum, a breeder should skip every other cycle when breeding. In truth, dogs are designed differently than humans. Domestic dogs, as opposed to their wild counterparts, like wolves & coyotes, generally have two heat cycles per year, they average about 6-7 months, but can be longer or shorter. Each dog will have her own spacing in between heats and it is genetic, so using the span between heat cycles of a dam should give you a pretty good idea on her daughter’s timing. Having litters 6-7 months apart is akin to humans have babies every 2 years if you look at the mother’s recovery, when milk production stops, and menstruation begins.
Between the heat cycles the hormones don’t just go stagnate, then continue to move along a rollercoaster of hormones that helps them recover from their previous litter, drying up milk production and generally tucking up, then it moves into a new phase that prepares the body to again attract male attention and conceive a new litter. The hormones during pregnancy fluctuate through the pregnancy identically whether there is a litter of pups to use them on or not, the only difference is the presence of a hormone called relaxin, whose responsibility is to loosen ligaments in the pelvis and other joints in an effort to allow safe passage out for the pups. This process is different than with humans who produce extra hormones on top of relaxin.  Therefore, whether the dog has a litter or not her body will go through the same series of hormones confidently countering the thoughts that skipping heats will ease the hormone rollercoaster that many think causes unnecessary wear and tear on the dog.  Now, dogs with excellent mothering abilities will tend to drain themselves while nursing the puppies. This can be managed by adding more food, or better yet, free choice, and also by increasing calories through other means, like canned food or some meat. I often give my girls a chunk of beef liver a day after they whelp. There are lots of little additives that will help her keep her caloric intake up and keep her body mass up. In regard to managing breeding stock, a dog getting drained from a litter is much more related to the breeder’s management of diet than the frequency of breeding. A well-fed, but not fat, conditioned dog who is healthy at mating will much more easily maintain her weight through the nursing phase and recover quicker. As the puppies get older the process of weaning also comes into play. Interestingly enough, the recovery time after whelping on seasoned dogs is much quicker than first time moms; it’s as if their bodies know the drill and know how to manage calories better. How Litters Improve Reproductive Health You ask me then well okay, just because the hormones are the same, I don’t see how this is actually better for the health of the dog as opposed to not having a litter?
Here is where it is healthier for the dog to have a litter than to skip, it’s all in the uterus and the cervix.  Unlike humans when a dog has a heat cycle and does not conceive, the lining of the uterus is not shed. In fact, the only time a lining is ever shed is with a litter during the whelping process. This is a major factor in conception rates and most importantly in whelping. When a lining is not shed, it is still there during the next heat cycle, conception, gestation, and whelping. Linings can stack on one another, but they weren’t designed to be there more than a series of weeks during a gestation period, so when they build on one another they lose their flexibility and elasticity. A dog will multiple linings in the uterus can have conception problems, because the fertilized eggs can have difficulty attaching to the thicker wall.  The Worst Part: During Whelping During whelping is when the most problems occur with a dog that hasn’t had a litter in a while. The extra linings often harden the uterus over time and it loses its elasticity and often the older linings have become hardened. The loss of elasticity paired with the hardened linings puts a lot of strain on the uterus during contractions and can slow them to near no effect. The same can happen with the cervix, its loss of elasticity can prevent puppies from being able to fit through the birth canal. Some newer breeders won’t know that their dog is even struggling and some dogs are so tolerant of pain they just seem a little off at this time and owners chalk it up to the late pregnancy.
It is at this time that you’ll need to go to the vet for an emergency C-Section. This is emotionally taxing, dangerous for the puppies, as the birthing process prepares lungs and other body functions for life on the outside, and because the dam will not have the opportunity to clean her puppies and get inundated with their scent mixed with hers through amniotic fluid, many recovering dams have a hard time latching onto their puppies. Nursing a full litter of puppies that she may or may not have bonded with, while recovering from surgery is hardly a good time for a mama dog. Often times new breeders won’t know any better and if she is not rushed to the vet in time she and her litter will die. By breeding back-to-back you avoid these problems and your dog’s reproductive system will be healthy and ready to grow a litter. Please don’t breed your older female who has been your pet for the last 8 years and never whelped or only had one litter many years ago and expect that things will go along without a hitch. I have heard so many heart-wrenching stories from owners who loved their dogs so much and just wanted a daughter to keep from her to have her memory live on after passed and who were completely crushed because their wonderful girl died too soon, before even the age of 8 in a whelping complication. 

Here is the link to the article above:

https://bearpawarsenal.com/blog/breeding-back-to-back-the-truth-behind-breeding-every-heat-cycle/


Here is another article on the subject. 

https://www.google.com/amp/s/kentfamilyfarms.wordpress.com/2016/03/23/back-to-back-breeding/amp/