The king poisons the queen with the help of his mistress. He told the MC that he accepts their child to ease her doubts, but made her drink tea without her knowing it's poison which leads to her death. Then the MC's soul was transferred to a girl, then she seeks out a duke(?) and introduces herself as a person of the queen that died, and asks him to marry her to get revenge to the king. They got married and she attended her own funeral. That's the last part of the story I've read. I forgot the title.
A Marriage Alliance for Revenge by Heesa Yun and Emma?
Empress Olivia knows happiness is the last thing she deserves, but when she finally becomes pregnant with the emperor's child, she thinks things might finally be taking a turn for the better. Alas, Olivia realizes how naive she was when her husband poisons her tea, killing both her and their unborn child. When Olivia opens her eyes again in the body of an unknown woman in the slums, she vows to take vengeance against the emperor. To do so, she forms a marriage alliance with her former fiancé, Duke Lucas Viterpan, the true heir to the empire who was deposed in a coup. Will this man and woman who had everything taken from them succeed in getting revenge?
Found with a search for
manhwa emperor kills pregnant empress with poison reincarnation, which led to this video, which led to me searching for
empress olivia rhone, which led to this work. I've read through the first chapter (free on TappyToon, the official distributor) and indeed, the emperor claims to accept the child (offering a gift of baby shoes) before she dies from poisoned tea.