Tosefta על זבחים 188:10

An animal that died, and he tore her open and found inside her a living nine-month old fetus, [that fetus] requires [ritual] slaughter, and he is liable in the [priestly] gifts, but it is disqualified from being placed on the altar (*because it is a ben pekuah). [*Note: This phrase omitted in MS Vienna.] One who slaughters a tereifah and found in it a live nine-month old fetus, it requires slaughter and is liable in the [priestly] gifts, and if it is dead it is pure from the impurity of carrying because the slaughter of its mother purified it. One who slaughters a sin-offering and found inside her a live four-month old fetus, even after three days (see Minchat Yitzchak), and even if it went "outside the curtains" (i.e., outside the Temple courtyard, see Zev. 94b:13), it is is permitted to eat. If an animal in its first pregnancy miscarried the amniotic sac, it should be thrown to the dogs, but if [the fetus] had been consecrated, [the amniotic sac belonging to the miscarried fetus] must be buried, because its growth was consecrated. The amniotic sac does not disqualify [an otherwise valid] sacrifice, whether [it is eaten] outside the designated time or outside the designated area, nor can the thoughts [of the priest] render it disqualified, and one who offers it [for sacrifice] outside the Temple courtyard is exempt.