The most delicate period of the canine pregnancy is the first month during which organogenesis takes place.
Prior to day 20-22 following ovulation (when implantantion occurs and placental development starts) canine embryos are surrounded by “uterine milk”, a protein endometrial secretion which is in homeostatic equilibrium with the blood compartment, i.e. any substance that arrives in the bloodstream reaches the endometrium. Therefore, use of any substance during this time carries the potential risk of harming foetal development even though there is no risk associated for the mother.
After placental development foetuses become more resistant to toxic insults. Although no real “placental barrier” exist, most substance cannot reach the placental circulation unless they R Close window to return to IVIS 2006 World Congress WSAVA/FECAVA/CSAVA 677 are present in high concentration and for a long time in the bloodstream. However, any drug that reaches the foetal circulation must be metabolised by the foetal kidney (in carnivores the foetal liver is not metabolically active) which in itself might threaten foetal survival. Aspirin, dexamethasone, bromocriptine, carbaryl, estradiol benzoate and cypionate, prostaglandin F2a and antiestrogen drugs are widely described as capable of causing embryonic/foetal death in the dog.
The effect of various drugs on the canine pregnancy is reported in details by Papich (1989). Table n° 2 shows a brief summary of drugs which have been either tested in pregnant dogs and proven safe or used in pregnant laboratory animals and pregnant women without any side effect.