Why might a pregnant woman go into a coma?

[TheCountessofFitzdotterel]

…and is it plausible she would then wake up fine a day later?

This is something that supposedly happened in 1558 when Florence Wyndham fell into a coma and was thought to be dead. She was buried in her coffin in the family vault but woke up when the sexton attempting to steal her rings cut her finger.
Some versions of the story have her going into labour the next day. Either way her son was born and lived to a ripe old age.

What do you think? Likely? Tbh I can believe the part where they think she is dead - after all they have no stethoscopes or other means of detecting a faint pulse- but could she have woken up as described?

Not implausible at that time. Certainly not dead and revived, but unconscious and awakened by pain. Tolerance to pain is still used in the Glasgow coma scale.
opioids were common, and high blood pressure at the end of pregnancy or diabetes. Several conditions of pregnancy lead to coma and potential death, including blood clots.

Thank you Fallfallfall.
That’s an excellent point about opioids - they could easily have been involved but never made their way into the story.