Dog pregnancy test: ultrasound, blood test and X-ray, and when each works

Which test works on which day

Merck describes only veterinary methods for confirming a dog’s pregnancy, and which one works depends on how many days have passed. These are its windows, counted in days of gestation.

A test done too early does not rule pregnancy out. Merck notes that false-negative ultrasound results may occur before 21 days.

• About day 21: a vet may feel small uterine swellings by palpation.
• Days 25 to 35: ultrasound is the most useful single test, and shows whether the puppies’ hearts are beating.
• Days 22 to 35: a relaxin blood test can detect pregnancy, depending on the source.
• After about day 45: X-rays show fetal skeletons.
• After day 55: X-rays are the best way to count the puppies.

Count from ovulation, not just from the mating date

A dog’s pregnancy lasts about 62 to 64 days. Merck says dogs ovulate about 2 to 3 days after the luteinizing hormone peak, so the day of mating and the day of ovulation can differ.

Merck says timing ovulation, which a vet can do with a physical exam, hormone testing and vaginal cytology, helps determine the length of pregnancy and the expected delivery date. If you did not time ovulation, ask your vet to choose a test window that allows for the uncertainty, or to repeat the test a week later.

Palpation and the relaxin blood test

Palpation means a vet feeling the abdomen. Merck says it can detect pregnancy at about day 21, when small swellings along the uterus are palpable in a cooperative dog. Those swellings double in diameter every 7 days, then become less distinct after day 35 to 38, and palpation is hard again until late pregnancy. It cannot count puppies, and it is a skill for a vet.

The relaxin test measures a hormone made by the placentas. The owner-facing Merck page says it can detect pregnancy by day 30 to 35, while the professional page says a commercial assay is sensitive after 22 to 27 days. Ask your clinic which day its test needs. The test shows that there are placentas, not how many puppies there are.

Ultrasound: the usual first confirmation

Merck says ultrasound reliably detects pregnancy at 25 to 35 days and is best performed in that window. It also lets the vet check that the fetuses are alive: Doppler instruments let a vet hear the fetal heart, which beats 2 to 3 times faster than the mother’s. It also helps separate pregnancy from other causes of a swollen uterus such as pyometra.

Merck names the late X-ray, not ultrasound, as the best way to count puppies. The scan has another use: Merck gives formulas that turn scan measurements into a gestational age within about 3 days, which helps when the breeding dates are uncertain.

X-ray: the puppy count

Fetal bones begin to calcify as early as day 28, but routine X-rays do not show them until about day 42 to 45 and they are prominent by day 47 to 48. Merck says radiography at that stage is not teratogenic, and that X-rays taken after day 55 are the best way to determine litter size.

Knowing the count helps at whelping: you know how many to expect, and whether a puppy may still be inside. Merck adds that fetal teeth become visible at term, which can help a vet confirm the puppies are developed enough for a planned cesarean section when breeding dates are vague.

What to do with the result

A negative result early does not close the question. If the dog was bred on known dates and the first test comes back negative or unclear, ask whether to repeat it a week later. Keep a note of the day count each time, since the same scan means different things at day 22 and day 32. Tell your vet the mating dates, any ovulation timing results, and how the dog is eating and behaving, then ask which test fits the day count. Costs vary by clinic and region, so ask for an estimate when you book.

See a vet promptly if your dog seems unwell, has an unusual discharge or stops eating. Write down the test dates and results so the whelping window can be adjusted if a later scan changes the picture.