The fertilization of an ovum by a spermatozoon forms an egg that must undergo several divisions and travel to the uterus before becoming implanted in the uterine mucosa. In bitches, implantation does not occur until 17 days after fertilization, and results in the formation of embryonic vesicles that cannot be detected by ultrasound until the third week of pregnancy (18 days at the earliest).
After the third week, experienced hands can sometimes detect a "beading" in the uterus through transabdominal palpitation, as long as the bitch is not too overweight and the abdominal wall is relaxed. Between weeks five and six of gestation, the uterus reaches the same diameter as an intestinal loop. It is therefore difficult at this stage to distinguish a pregnant uterus from an intestinal loop containing hard stools. Generally, the "window of palpitation" therefore occurs from twenty-one to thirty-five days of gestation, depending on the breed.
Vague vulvar discharge can be physiologically observed by the end of the first month of gestation; however, this is not necessarily the sign of an embryonic resorption of an early abortion.
There is no point in taking X-rays until the end of pregnancy, since the fetus'skeleton does not become calcified (and thus, opaque to X-rays) until after the forty-fifth day.
Other techniques for diagnosing pregnancy occur too late or are too uncertain to be reliable in canine breeding. These include behavioral changes, detection of fetal heartbeats (audible in some bitches during the last two weeks of pregnancy) by auscultation, changes in the blood (sedimentation rate, hematocrit), or mammary development.
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