How Long Is a Dog in Heat? Nine Facts About Dog Reproductive Health

For many dog owners, dog reproduction is something of a mystery. Most of us who adopt a dog do so for companionship and the joy of caring for an animal that loves us unconditionally. And while those who buy a pedigree pup may think about breeding them, after learning a bit about the amount of time, effort and work involved, most folks quickly abandon the idea. So it should come as no big surprise that even the most ardent dog lover might not know the basic facts about canine reproduction, such as how long a dog is in heat or even what “heat” means.

It may seem tempting to let your adorable pure-bred pup reproduce, but make sure you know all that’s involved before you decide.

Still, knowing a bit about a dog’s reproductive system is important for anyone who adopts a puppy and has not yet decided whether to allow them to breed or not. It can also help inform the question of whether to spay or neuter an animal, even if breeding them is not in the cards. Most importantly, it can help prevent unintentional pregnancies and unwanted puppies who are often abandoned to shelters, or worse.

So, with that thought in mind, here are seven facts about dog reproductive health that every dog owner should know.

No. 1. Dogs and humans have very similar reproductive organs

In terms of basic anatomy and physiology, dogs and humans are in many ways very similar. Both species have hearts, lungs, kidneys and livers that perform identical functions, and our neurological makeup is very similar as well. Both humans and dogs have a brain and a spinal cord that interact with a system of sensory and motor nerves that “talk to” each other via chemicals known as neurotransmitters. A dog’s digestive system looks a lot like ours too: with an esophagus, stomach, small and large intestine, liver and gallbladder, all of which function in the same ways as ours. Thus, it’s no surprise that our reproductive systems look and, to some extent, function alike.

Both male and female dogs have reproductive systems very similar to those of humans.

The Male Reproductive System

Like their human counterparts, male dogs have a fairly simple reproductive system, consisting of testicles, which produce and store sperm, a ductus or vas deferens, a prostate gland and a penis. When a male and female copulate, the testicles eject sperm through the vas deferens into the prostate gland, where it mixes with seminal fluids before it travels through the penis and into the female’s vagina and, eventually, to her uterus.

One notable difference between a male dog’s and anatomy and that of a human is the presence of a penile bone or baculum. The exact purpose of this bone is a little unclear, but it is believed to allow for more complete and longer penetration of the female, giving the male’s sperm a better chance of getting where it needs to be.

The Female Reproductive System

Female dogs (aka bitches or dams) are also anatomically very similar to their human counterparts. They have two ovaries, which manufacture and store unfertilized eggs and produce most of the hormones that are responsible for the estrus or heat cycle. From the ovaries, eggs travel through oviducts (known as Fallopian tubes in women) and into the uterus, which consists of two “horns” and a central body. The uterus in both species is where the placenta attaches and nourishes the fetus (or, in the case of most dogs, fetuses).

No. 2. Breeding a too-young dog is a bad idea

Because dogs mature far more quickly than humans, both male and female dogs become sexually mature (e.g. capable of reproducing) at about 6 months of age, although full sexual maturity may not be reached until the dog is 12 to 15 months. Nonetheless, a dog is still a puppy at this age and not yet fully developed physically or emotionally, so it is far too young for a female to be bred. According to AKC guidelines, female dogs should not be allowed to become pregnant until they are at least 18 months old and have completed at least one heat cycle.

Although she may appear mature, a dam less than 1 year old is far too young to breed.

With that being said, puppies can and do become pregnant when they are as young as 5 months of age. But, just like adolescent humans, adolescent dogs are ill-suited for motherhood, and complications often occur. For example, most dogs are not fully grown until they are at least a year old, so the dog’s uterus and birth canal may be too small and immature to accommodate a litter of pups. She may have a miscarriage, or the puppies may be born too early and die. Dystocia, a complication that prevents labor from progressing normally, is also more common in younger dogs. What’s more, even if they are fully vaccinated, young dogs are more prone to infections because their immune systems are immature. It’s not uncommon for a uterine infection to take hold during labor and sicken not only the puppies but the mom. Lastly, even if a young dog manages to have a healthy litter, she may not be emotionally or physically ready to actually raise her pups.

No. 3. A dog’s heat cycle has four stages

Unlike women, who ovulate and menstruate on a monthly cycle, dogs’ estrus cycles occur somewhere around every 6 to 9 months. The cycle has four stages:

• Proestrus — At this stage, the external genitalia (vulva) becomes swollen and blood begins to flow. Bleeding may be rather heavy or very scant: either is normal. This stage lasts about a week to 10 days. During this time, male dogs will be attracted to the female’s scent, but she will not allow them to mate with her.
• Estrus — This is the stage when the female is ready to mate. Blood flow slows and then stops, and the dog will allow herself to be mounted by a male. This is also the stage in which ovulation (the release of one or more eggs into the oviduct) occurs. Under optimal circumstances, the male’s sperm reaches the oviduct and fertilizes the eggs, which then travel to and implant in the uterus.
• Diestrus –The time in which the dog is either pregnant or in a “resting phase.” This stage of estrus can last for 10 to 140 days
• Anestrus — a period of rest between diestrus and the next heat.

No. 4. The normal gestation period for a dog is about nine weeks.

If a dog becomes pregnant after mating, she will normally carry the pregnancy for about nine weeks, or between 58 and 63 days. During this time, physical changes occur at a predictable rate:

• First month: About two weeks after mating occurs, the fertilized eggs are implanted in the uterine wall and begin to grow. At about day 22, they begin to develop into recognizable fetuses. At this early stage of her pregnancy, you may not notice that your dog is pregnant, though you may notice some changes in her behavior, appetite and energy level at about week four.

Early in her pregnancy, your dog may not appear any different from her usual self, although she may be a little more tired than usual.

• Second month: During the second month of pregnancy, the fetuses are growing very quickly. By around day 45-50, their skeletons are fully formed, and they have claws and fur. During this time, you will also notice some definite changes in the mom. Her belly will

get bigger; she will gain weight, and her nipples will be more pronounced. She will almost certainly be eating more and she may have some vaginal discharge. As she gets closer to giving birth, she will begin to exhibit some “nesting” behavior as she looks for a place to deliver her pups. This is a good time to make sure she has a comfy place to deliver and care for her puppies. You may want to invest in a whelping box or build a bed/crate combination yourself.

• Third month: By the time a dog has reached the 58th day of her pregnancy, the puppies are fully formed and have begun to make their way into the birth canal. During this time, the dam will begin to lose her appetite and become more restless as she gets closer to giving birth. Shortly before labor begins, her temperature will drop from a normal of about 102 degrees Fahrenheit to about 99. Once labor begins, she will find a place to lie down as she gathers the energy needed to deliver her pups.

• Labor: Depending on the number of puppies in the litter, labor may last from 2 to 24 hours. The puppies will typically appear every 30 minutes or so. If more than 2 hours elapse between puppies and the dog is still laboring, she may be experiencing a complication known as dystocia. There can be many reasons for this: a puppy may be too large or improperly positioned in the birth canal; uterine contractions may be inadequate, or the mother may be too exhausted to push the puppies out. In any event, the dog will need to be seen by a vet if this occurs.

No. 5. Litter size is determined by breed, age, and overall health.

One question that comes up frequently around dog reproductive health is what determines litter size. We’re all aware that litter sizes differ tremendously — some dogs give birth to only one puppy while others have as many as 12 or more. (The world record is 21, born to an Australian Neapolitan mastiff named Shadow on April 20, 2020.) But what factors affect the size of the litter? According to Oakland Veterinary Referral Services, the most important are:

• Breed: This is the biggest factor in determining the number of puppies a dog will have. Larger breeds tend to have more puppies, simply because their bodies can accommodate a larger litter. A bull mastiff, for example, usually has between five and 13 pups; a German shepherd may have five to nine; and a tiny chihuahua may have two to five. With that being said, there are some exceptions that prove this rule. For example, the average litter size for a Pekingese, is between five and 10.

• Age: The age of both the dam and the sire play a part in litter size. Older dams tend to have smaller litters; the optimal age for breeding a female dog is between 2 and 5 years. Older males also tend to produce smaller litters because their sperm count diminishes with age. The best age to breed a male dog is thought to be between 1.5 and 5 years.

A healthy 3-year-old German shepherd like this dog may have between 5 and 9 puppies in a single litter.

Additionally, the health of both the dam and the sire play a part in litter size, particularly where nutrition is concerned. According to Breeding Business, at least one study showed that females who were regularly fed a high-quality balanced diet without supplemental protein (often thought to increase litter size) produced the largest litters. In fact, dogs in the study who were fed extra protein had smaller litters, and dogs given high amounts of supplemental protein were more prone to spontaneous abortions.

No. 6. Dogs can have identical twins — but it’s very rare

Anyone who has ever seen a litter of puppies knows that littermates often look very much alike. And so it would seem reasonable to assume that one or more puppies from the same litter might actually be identical twins. But the truth is that, while all puppies from the same litter are technically fraternal twins (products of the same pregnancy but different eggs), identical twins (also called monozygotic twins because they are formed by a single egg that splits in two) are extremely rare in dogs.

That said, rare doesn’t mean impossible. According to a story published online by the BBC, an Irish wolfhound in South Africa gave birth to the first confirmed set of identical twins in 2016. The puppies were delivered by Cesarean section by Dr. Kurt de Cramer, a veterinarian at Rant en Dal Animal Hospital in Mogale City, and were attached to the same placenta by two umbilical cords. Two weeks after the birth, de Cramer called on a team of animal geneticists, Carolynne Joone of James Cook University in Townsville, Australia and Johan Nöthling of the University of Pretoria in South Africa. They drew blood from the puppies, and DNA testing confirmed that they were, in fact, genetically identical twins.

No. 7. New dog moms can have complications, too.

After a dog gives birth, the hard work of raising her puppies begins. Most dams will care for their pups appropriately from the moment of birth: They will tear off the placental sac, clean the pups, and allow them to nurse. The first “milk” produced by the mom is called colostrum and contains antibodies from the mom that will protect the puppies from infection until their own immune systems mature. All of the puppies should ingest this important nutrient within a few hours of birth.

But nursing can also be hard on the mom, and in some cases one or more of her teats will become red, swollen and inflamed. This may be a sign of simple irritation, but it may also indicate an infection in one or more of the mammary glands (mastitis.) Usually bacterial in origin, mastitis can quickly spread, and, if it is left untreated, the dog can become seriously ill.

Fortunately, mastitis is usually very treatable with antibiotics, pain medicine, and warm compresses made from cabbage leaves, which have an uncanny ability to reduce swelling and pain. If the dog is too sore or too ill to nurse, or if the medicine she needs may affect her pups, the babies may need to be fed by hand. (Your vet will tell you the best formula to use.) In either case, the puppies should be weighed daily to make sure they are getting the calories they need.

Nursing puppies can inadvertently injure the dam’s teats, causing an infection known as mastitis.

Yet another complication that can affect a nursing dog is eclampsia or “milk fever.” Unlike eclampsia in humans, which results from severe hypertension during pregnancy, eclampsia in a dam occurs after her puppies are born and is caused by low calcium levels in the blood (hypocalcemia.) Small dogs and dogs who have given birth to large litters are particularly at risk because their bodies can’t replace the calcium lost in their milk quickly enough. Paradoxically, dogs who were given calcium supplements during pregnancy also have a higher incidence of eclampsia because ingesting too much calcium interferes with the function of the parathyroid glands, which regulates the amount of calcium in the blood.

Although easily treatable if caught early, eclampsia in dogs is a very serious condition that needs immediate attention by a vet. Signs of eclampsia include any of the following:

• Muscle weakness
• Stiff muscles, muscle spasms (tetany)
• Nervousness or agitation
• Tremors
• An unsteady gait
• Disorientation
• Excessive salivation
• Lethargy, unresponsiveness
• Seizures

If your lactating exhibits any of these symptoms, she should be evaluated by your veterinarian right away. If hypocalcemia is confirmed by a blood test, your vet will give the dog intravenous calcium to correct the deficiency. This usually requires hospitalization, since IV calcium must be given slowly to prevent heart complications that could be lethal for the dog. Once she is stable,the dog will be sent home with oral calcium supplements. Your vet may also advise you to prevent her puppies from nursing for a day or two.

No. 8. Puppies should be weaned by 6 weeks of age.

Unlike human babies, who should stay on mother’s milk for at least the first year, puppies should be weaned rather quickly — definitely by six weeks old. In the wild, this happens naturally as the dam begins to move away from her pups at about 3 to 4 weeks of age, when they start to develop teeth. This is also the age when pet parents should start weaning the pups.

To wean your new puppies, start by offering them a milk-replacement formula recommended by your yet in a small bowl or saucer several times a day. You can entice the pups to drink by using your finger to wet their noses and mouths with the formula and allowing them to lick it off. Continue doing this until the puppies are drinking the formula on their own. This generally takes one to four days at most.

Next, start mixing the milk replacer with a little bit of wet puppy food and offering it to the puppies several times a day. As they begin to eat the solid food, decrease the amount of milk replacer until they are eating the recommended amount of solid food to meet their nutritional needs. You may want to weigh the puppies a few times during this process to ensure that they are not losing weight.

By six weeks of age, your puppies should be fully weaned and eating solid food. Feed them a high quality food formulated for puppies — either kibble or wet food or a combination of the two. Unless your vet recommends a supplement, the food should provide adequate nutrition and allow the pups to grow into strong, healthy young adults.

No. 9. Puppies shouldn’t be taken from their mom too soon.

Although puppies can technically be sent to their new homes as soon as they are weaned, most dog behaviorists and veterinarians believe it’s best to keep them with their mother and littermates until they are at least 8 and preferably 10 weeks old. The reason for this is simple. Those early weeks with their canine family is when puppies learn important lessons that help them develop into socially adept, well-adjusted dogs.

What sort of lessons do puppies learn from their mothers and littermates that human owners can’t provide? Some of the most important are the body postures and other signals that dogs use to communicate nonverbally. A dog who doesn’t learn these postures early may have trouble understanding canine body language and be shy or aggressive around other dogs later in life.

Puppies learn essential lessons from their moms and their littermates during the first 10 weeks of life. Staying with their canine family during this time helps them become well-adjusted, happy dogs.

Puppies also learn bite inhibition — the ability to gauge the pressure they are applying with their teeth — by playing with their littermates. This usually occurs as their teeth begin to develop at about 5 to 7 weeks of age. Dogs who don’t learn this skill early on may injure other dogs or humans unintentionally when they simply want to play.

Perhaps most importantly, puppies also learn to modify unacceptable behavior during their first few months with their mom. When one of her pups does something she doesn’t like, the dam will often snarl at them, bark, or even grab them by the scruff of the neck and toss them roughly across the room. This teaches the pups where their boundaries are and also serves to prepare them to “take orders” later in life. A dog that fails to learn this lesson may be difficult to train and overly assertive as time goes on.

The Bottom Line

There’s a lot to know about canine reproduction, from how long a dog is in heat to how long she’s pregnant, to how to care for puppies after they’re born. Most dog owners don’t need to know all of this: If you don’t plan to breed your dog, you may think it’s not important to pay attention to your dog’s reproductive health at all. But because our dogs are our family, most of us want to know as much as possible about how to keep them happy and healthy no matter what stage of life they are in. Hopefully, this short primer on canine reproduction will help you do just that.