Breeding by the Numbers…or Not

by | Dog Breeding

Numbers that help and hinder your breeding success

Dog breeders may not be into math but we toss a lot of numbers around while breeding. Some of these numbers are important, while others are not. Being able to distinguish between those will reduce your frustration and increase your breeding success.

Let’s start with three of the less important numbers:

1. Day she is on:

Breeders often start a conversation with “My bitch is on Day 9.” Heat cycles vary so much from bitch to bitch and even within the same bitch that the day she came in won’t help you accurately determine when to breed her this time. We’ve had successful breedings as early as Day 6 and as late as Day 21, as well as unsuccessful breedings on the “correct” days—Day 9, 11 and 13.

2. She bred on Day ___ for her last litter:

Certainly having your bitch’s history can help you plan but just because your girl bred on Day 11 last litter doesn’t mean she’ll do it again this time. She may progress so quickly through her season that she’ll be out by Day 11 and it will be too late to ship in semen. Or she may move at a snail’s pace and not even ovulate by Day 11, making early shipments a waste of time, money and effort.

3. How long since her last season:

Many breeders begin to worry when a bitch is “late.” Again, heat cycles vary so much that this information is only useful at the extremes. If she had her last cycle less than 5 months ago or more than 18 months ago, you might want to talk to your vet. Otherwise, accept her as normal, put a little weight on her, leave her with a friend whose bitch is in, and plan a big vacation. She’ll come in!

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So what numbers are important?

Those that dramatically improve the success of your breedings, both in numbers of pregnancies and puppies. If you are a seasoned breeder doing a natural breeding at home with your own experienced dogs, you may not need them but they sure are worth it if you are:

  • a novice breeder
  • experiencing missed breedings
  • breeding older brood bitches and/or stud dogs
  • investing in fresh-chilled or frozen semen
  • traveling to do a breeding


Join a community of like-minded dog breeders seeking to breed and raise their pups using the best methods available.

Here are the two numbers I depend on in my breeding program:

1. Reading Vaginal CytologyPercent Cornification:

I don’t know why vaginal cytology has lost favor among breeders in recent years. Although it won’t tell you when to breed, it can allow you to optimize your progesterone testing and, even more importantly, help you pinpoint when your litter is due better than any other method. Best of all, it is a skill you can learn to do yourself, saving you vet visits and bills.

I do my own smears starting the day my bitch comes in season, so I can be sure I caught her first day and that I don’t miss her progression. I continue smears every day or two until she is 80% cornified. At that point, she is in estrus so additional smears won’t help me figure out when to breed so I then turn to progesterone testing. However, using vaginal smears in this way has dramatically cut down on the number of early progesterone tests I run—you know, the $120 ones where she is at 0.2 nanograms per milliliter (ng/ml) for days!

I do a few more smears after I’ve bred my bitch so I can determine the first day of diestrus. In these smears, I am looking for the dramatic drop in cornified cells, from full to less than 20% cornification. The day I see this drop, we know my girl is in diestrus and her pups will be born in 57 days, plus or minus one day.

Tube for Progesterone Testing

Tube for Progesterone Testing

2. Progesterone level:

The number one reason that I have missed breedings over the years is poor timing. Believe it or not, it’s not so easy to figure out when to breed, sometimes even with experienced stud dogs and brood bitches. Researchers are still fine-tuning how we should interpret and use progesterone levels to breed most successfully but my experience and reading have taught me to look for two things before breeding:

  1. progesterone level between 4 and 10 ng/ml (6 and 31.4 nmol/l) to indicate likely ovulation
  2. a minimum of a 3-4 ng/ml jump in progesterone over a 24-hour period to confirm ovulation

Although the typical bitch will ovulate when her progesterone is at or above 5.0 ng/ml, there is a lot of variation so we look for that jump in progesterone that confirms ovulation did occur. Can you tell that variability is a theme in breeding dogs? If your bitches are all typical, you can breed the old way on Days 9, 11 and 13. But if they are not, it’s best to be prepared with another plan that includes vaginal smears and progesterone.

So as dog breeders, we don’t have to know a lot about numbers but focusing on the right ones rather than less important ones will help you have more success with your breedings!

If you would a copy of our Breeding By the Numbers case study, showing how we use these numbers, as well as our BONUS Vaginal Cytology Resource Guide, click here to download your FREE copy.

Click Here to Download


My thanks to Dr. Mary Stankovics and Deb Leach from Straws to Paws, who taught me to do and read vaginal cytology, and my vet, Dr Jim Zgoda, Otterkill Animal Hospital, who always lets me look over his shoulder at the lovely slides he prepares.

Want to know more about raising healthy, stable puppies?

Check out our 8-month, self-paced online course to raising a healthy, stable litter of pups, from before breeding until sending your pups home.


  1. Shirley

    I have a question when doing cytology what do tge reference numbers mean under the letters ,such as P -5. I -5 and S-90???

    • Gayle Watkins

      Shirley, I’m sorry but I don’t know what report you are looking at. Can you post it on our FB page?

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