Responses
The baby isn't the emperor's, so if the emperor wins the baby dies. If the Prince wins, the baby will be left with no mother (she's accessory to most of the king's crimes) and will be a threat to the Prince's ascension so he will suffer and factions will either use him or want him to die. Either way the empress saved her child from another sad suffocating life of suffering. I guess you can think of it as the only kindness a mother can have.
The empress is pregnant and still drank the poison, damnnn