How Do Dog Fetuses Form?

Published: 10/4/2021

Everyone loves puppies. Even the grr-umpiest among us break down our cuteness barriers when we see a litter of pups rolling and tumbling with each other. We all have our favorite breeds, but every puppy starts out the same: as a single-celled embryo embedded deep in Mom’s womb. 

The story of how a puppy develops in just 9 weeks is fur-cinating. Are you curious about what’s happening inside when a dog Mom is pregnant? Read on to get the scoop!

The best place to begin an exploration of how dog fetuses form into puppies is at the beginning – conception. Like other mammals, dogs are conceived from one female egg and one male sperm. The typical dog pregnancy involves multiple embryos, sometimes ten or more, and each one could have been formed from one mating session or more. Breeders may mate their female dog twice within a couple of days to increase the odds that pregnancy will result.

Dog sperm lives inside the female and remains active for 5 to 7 days, and can fertilize an egg anytime during that period. Because of this extended sperm activity, calculating the exact number of days and weeks it will take to get from embryo to fully formed puppy can be less than pawrfect, but most breeders and pet parents expect to see wiggling, squirming pups in anywhere from 57 to 63 days after fertilization of the eggs. Puppies can safely be born from Day 57 on, but most wait until 61 to 63 days. 

When embryos are first formed, they hang out for a few days high in the uterine body itself, just floating in amniotic fluid. By about 28 to 30 days after fertilization, a heartbeat is present, and an ultrasound performed may show how many fetuses there are. However, a hormone called relaxin, which is only secreted by a placenta, can confirm that the dog is pregnant at this point.

About a week after fertilization, embryos travel up from the uterine body into the uterine “horns'' which join and form the hollow base of the uterus. The horns are where they are implanted in their own nourishing sac until they’re ready to be born. Each embryo has its own umbilical cord and placenta. The developing pup is called an embryo until day 40, when they are called fetuses. 

By Day 35, organs are beginning to form in the little body that’s about the size of a hazelnut. The fetus’s weight increases by 75% during this stage. At 40 days, they “graduate” into fetuses and the development speeds up. The head and spine are present. Also, the eyes and ears emerge, and by the end of the 40th day the fetuses start to look more like puppies. Their skin color emerges and shortly after, their hair. 

Most pups are born with their hair color determined, but certain breeds, like Australian Shepherds who are all white at birth, take on their final colors later.

During the third and final stage of pregnancy, beginning around 42 days, feet with nails are forming, along with whiskers. The organs are fully developed, and the skeleton solidifies further. The sex organs are formed by this time, and an ultrasound may reveal the gender of each pup. Abnormalities may also be detected with an ultrasound. This is a time of rapid weight gain, and the mother is in danger of being undernourished. As her abdomen becomes distended, a few small meals a day will keep her nourished until after the pupsters are born. 

By around Day 50, an X-ray will show that the skeleton is completely formed. An X-ray also can confirm the number of fetuses waiting to be born. Toward Days 57 to 60, Mom’s temperature may drop about 1 degree Celsius (1.8 degree Fahrenheit), indicating that labor is about to start within 48 hours. As the fetuses move toward the Mom’s pelvis, her stomach will become less distended. Whelping, or giving birth, will begin at around Days 61 to 63. Pups usually take up to two hours each to be born, though they can take less time.

Since female dogs go into heat every six months or so that lasts up to 18 days, they may produce as many as 20 or more wiggly, squiggly puppers by the end of the year!

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