Animal sentience is the capacity of an animal to experience different feelings such as suffering or pleasure. The onset of sentience, that is the life stage at which an animal becomes sentient, is a highly contentious topic but it’s important as it forms the basis of decisions regarding animal foetuses.
However, expert bodies considering available scientific literature have “concluded that the stage of development at which there is a risk of poor welfare … is the beginning of the last third of development for mammals; when a fish, amphibian, cephalopod or decapod becomes capable of independent feeding and during the last days before hatching in precocial oviparous species [such as birds]” .
In regards to marsupials which develop in the mother’s pouch, unfurred pouch young are not considered sentient  because they “do not appear to show clear behavioural or EEG [electroencephalogram] signs of conscious awareness for at least the first one-third to one-half of pouch life” . Pouch young are “thought to become sentient at roughly four months” .
Recommendations have been made that “consideration should be given by regulators to the possibility that foetuses may suffer … [and] even if one is convinced that prenatal animals cannot suffer, there is good reason to afford protection to animal foetuses not for their own sake but in order to provide coherent protection to the welfare of the animals which they will become” .
Some codes of practice and guidelines consider the onset of sentience. For example, the Australian Code for the Care and Use of Animals for Scientific Purposes  states “As a guide, when embryos, fetuses and larval forms have progressed beyond half the gestation or incubation period of the relevant species, or they become capable of independent feeding, the potential for them to experience pain and distress should be taken into account”. Accordingly, the Victorian Guidelines for the Euthanasia of Foetal and Neonatal Mice, Rats, Guinea Pigs and Rabbits  states that the literature on the development of pain pathways suggests the possibility of pain perception in mice and rat fetuses 15 days in gestation to birth and “there may be pain perception consistent with development of the functional brain… from 60% gestation in guinea pig and rabbit foetuses.”
Based on decisions about onset of sentience, several jurisdictions including the European Union, NZ and QLD have moved to protect animal foetuses in the last stages of development in legislation.
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 Agriculture Victoria (2017) Guidelines for the Euthanasia of Foetal and Neonatal Mice, Rats, Guinea Pigs and Rabbits http://agriculture.vic.gov.au/agriculture/animal-health-and-welfare/animal-welfare/animal-welfare-legislation/victorian-codes-of-practice-for-animal-welfare/code-of-practice-for-the-housing-and-care-of-laboratory-mice,-rats,-guinea-pigs-and-rabbits/appendix-3-guidelines-for-the-euthanasia-of-fetal-and-neonatal-mice,-rats,-guinea-pigs-and-rabbits (accessed on June 10, 2019).