Many mammals become heterothermic at times as they “allow” their core temperatures to decrease towards the ambient temperature, a process called torpor.  Decreases in temperature lasting less than 24 hours are generally referred to as daily torpor, since they are often repeated in subsequent days, or as hibernation for longer periods.   Although less frequently than in males or non-reproductive females, some female mammals enter torpor during their reproductive cycle, particularly in unpredictable habitats.  This comes at the cost of a prolonged reproduction period, when development stops or is slowed, but, as Lovegrove(2019) has argued, in the species where this occurs the payoff may be surviving to reproduce at all.
       Torpor during a particular stage of gestation, particularly if there were no effect on the production of viable offspring, would strongly argue against the homeothermic hypothesis, at least for that developmental stage.   I attempted to discern when in development torpor occurs for most of the species listed in latest review of torpor during gestation by McAllen and Geiser (2014) plus whatever more recent reports I could find.