Day 1, Day of Mating (November 24): Sperm migrate up the vagina and through the cervix (entrance to the uterus). They can live inside the bitch up to 7 days.
Days 2-3 (November 25-26): Sperm continue to travel looking for mature eggs, eventually finding them in the oviducts (passages from the ovaries to the uterus). Fertilization occurs in the oviducts.
Days 4-8 (November 27 – December 1): Fertilized eggs move down the oviducts and into the horns of the uterus, rapidly developing all the while. Each fertilized egg divides to form a 2-celled organism, then again to form a 4 celled organism, 16 celled, and so forth. Once the fertilized egg begins to divide, it is called a blastocyst.
Days 14-19 (December 7-12): By day 14 they’re in the 64-cell stage. They continue to grow and divide. By day 19 they are large and complex enough to be classified as embryos.
Days 22-28 (December 15 – 21): The embryos attach to the wall of the uterus, spacing out so that each pup has room to develop. The face, eyes and spinal column develop. Injury or chemicals at this stage can cause deformed embryos.
In some breeds, pregnancy can be verified by palpation during Days 26-32, but I’ve never had any luck with my German Shepherds because their bodies are so deep and muscular.
A blood test can confirm pregnancy by evaluating the amount of the pregnancy-specific hormone Relaxin, which comes from both the uterus and placenta (mainly from the placenta) only when the embryos attach to the wall of the uterus (about 22-27 days after breeding). False negative readings are possible close to the twenty-two-day mark but if repeated at about age 28 in a pregnant bitch will read positive, so I never do the Relaxin test on my bitches before they’re 4 weeks in whelp.
28 days after breeding, an ultrasound will show fetuses, and more importantly, fetal heartbeats. The heartbeats let you know you have live, developing puppies. The bitch is held on her back on an exam table, the hair on her belly is shaved, lubricant is applied and a probe connected to a TV-like screen is pressed firmly against her skin and moved around. The pups are recognizable as puppies and fascinating to watch as they move around and their hearts beat away. Still photos of the pups in utero can be taken. The uterus has two horns, with puppies implanted and developing in each horn. On an ultrasound, which cannot go deep enough to see both horns simultaneously, it’s impossible to see all puppies at once. The ultrasound can only show part of the uterus at a time, so it’s easy to count one puppy several times, thinking you have moved to a different part of the uterus when you actually have just changed angles—and to miss other puppies altogether.
Days 29-35 (December 22 – 29) Toes, claws and whiskers develop, and the fetuses lengthen and begin to look like dogs. Gender is detectable. By the end of this week the fetuses are no longer vulnerable to chemicals.
Days 36-42 (December 15 – January 4): Heartbeats can be heard with a stethoscope. Skin pigment develops.
Days 50-56 (January 12 – 18): The pups are developing fast and beginning to take up more and more room, so the pregnancy begins to be obvious. They don’t fit under the bitch’s rib cage any more so her abdomen distends. It’s usually obvious when they “drop” into the abdomen as they begin to migrate closer to the birth passage. Since some of the pups are resting in the abdomen below the ribcage, they can often be seen moving and kicking when she sleeps stretched out on her side. You can sit beside her and gently hold your palm against her abdomen and feel the babies move. Since the pup’s skeletons are now formed, radiographs can be taken and the number of pups counted. I know approximately how many to expect, so if the bitch quits whelping before she should, I know she could be in trouble and can call the vet, get help, and hopefully save lives.
Days 59-66 (January 21 – 28) The pups should be born during this period.