As humans have gradually bred dogs specifically for different physical traits, there has been a marked change in the physiology of different breeds. In some breeds this has translated into larger, heavier, stockier or healthier dogs, while in others it may have lead to growth problems, muscular-skeletal problems or even significant dental and respiratory issues.
Changes in how a dog looks when it is fully grown translates in to puppies that have a different appearance when they are born. In some breeds, especially the short-muzzled breeds or brachycephalic dogs, the emphasis has been on larger, wider heads. Think of the difference between the head size of a Bulldog and a German Shepherd or even a Boxer and a Schnauzer. The Bulldog and Boxer both have a much larger head for their body size, not hugely disproportionate but definitely heavier and wider. This, in turn, means that puppies are born with wider and larger heads, potentially causing a problem in the birth canal. You don't even have to go the medium and large breeds, keep in mind the difference in the width and shape of the head of a Pug versus a Chihuahua. Boston Terriers are another breed that can have a great deal of trouble in whelping, again due to the large head compared to the rest of the body of the puppy.
Typically if you are breeding two larger headed or brachycephalic dogs together, either of the same breed or of different breeds, it is very important to check back into the pedigree or family history of both and look for breeding problems. Also know the average litter size for each breed and check to make sure, especially on the female's side, that there are the appropriate number of puppies in each litter. For example, if the typical litter size is 6-9 puppies and the female's history only lists three or four puppies per litter, you need to ask more questions about stillborn puppies or puppies that died shortly after birth. Reputable breeders will be very forthcoming with this information however it is always important to ask. If the breeder seems evasive with this information you may need to seriously consider if you want to risk the chance of breeding your dog, especially if there is a danger to her through the pregnancy or the whelping process.
Generally most dogs that have whelping problems also have trouble conceiving, although this is not always true. The Bulldog is a known difficult breeder and also is prone to relatively small litters of three to five puppies. Since whelping and puppies becoming lodged in the birth canal is such a serious concern both for the puppies as well as the mother they are typically automatically delivered by cesarean section by a vet. In addition natural mating is often rather unsuccessful so artificial insemination or surgical implantation of fertilized eggs is common within the breed.
Another breed that is a wonderful giant sized dog but has a lot of whelping and breeding problems is the Mastiff. These huge dogs have extremely long birth canals and very large puppies, resulting in problems for the female even under the best of circumstances. They often have large litters and the mother may be physically exhausted before all the puppies have been whelped. Female Mastiffs also have long uterine horns that can become twisted, trapping puppies or the placenta and leading to very serious health conditions for the female and death to the puppies. Cesarean sections are very common in this huge breed as well, particularly if the female has had any difficulty in her last whelping.
Toy breeds, especially those bred to be mini, teacup, micro or any other abnormally small size are going to have huge problems in whelping. The smaller the female is the more difficult it is going to be to find a male that is about her size or smaller. If the male is larger than the female, you are likely to have larger puppies. Larger puppies are definitely going to be a problem in a very small mother dog. Toy breeds are also more prone to genetic problems since often inbreeding and breeding relatively genetically unhealthy dogs has been the quickest way to get the unbelievably small sizes that people pay massive sums of money for.
Even if you are fortunately or lucky enough to find a male that is smaller or equal in size to the female, you are then going to have to deal with the female dog being under stress during the pregnancy. Extremely small toy breeds are often less hardy than their standard toy breed counterparts and may need to be very carefully monitored throughout the pregnancy. It is not uncommon for some toy breeds to have only one or two puppies in the litter or to have several puppies that are either stillborn or die shortly after birth.
It is very important to talk to your vet and to completely research your breed to know what, if any, whelping problems are issues that you have to be aware of when whelping starts. Some breeds are prone to having an easy whelping process, but this doesn't mean that your female should be left alone. Each dam needs to be checked every 15 minutes or so until all the puppies are delivered and the placenta are expelled. If the dam is struggling to deliver a puppy for more than an hour, call your vet. If anything doesn't seem right to you, calling your vet. He or she can either provide you reassurance or the information you may need to save both your dam and her puppies.
Breeds that are known problem whelpers or females that have had any history of health problems or previous problems during delivery need additional attention. If you are a first time breeder you need to have your vet on stand-by just in case something goes wrong. Even experienced and trained breeders with some of the more challenging breeds will either take their females to the vet for a c-section or they will alert the vet to the possibility that they may need assistance. In most cases if the female has already started into labor, moving her is not recommended unless it is an emergency so having her overnight at the vet's clinic is often the best and safest option.