Warning: the following post contains spoilers for Game of Thrones, so proceed with caution.
Oh man, that Tyrion Lannister! As every Game of Thrones fan knows, his most valuable trait is his mind — it's so valuable, even he doesn't realise the extent of it. During episode four, "The Last of the Starks," Cersei Lannister plans her next steps in Seven Kingdoms domination, and it's with Euron Greyjoy by her side. The evil couple appears to be celebrating the news that she's pregnant, but fans all know Cersei began carrying that child long before Euron got in the picture.
Let's go back to season seven, when Cersei reveals to Jaime that after the loss of their three children, they have another baby on the way. During a conversation between Tyrion and Cersei in the season seven finale, Tyrion realises that the shift in her typically sinister (and wine-guzzling) demeanour can only be explained by a pregnancy. At this point, the only people who know about Cersei's pregnancy are Jaime, Tyrion, and Qyburn. Until episode four of season eight, the pregnancy is not mentioned. Cersei sure does a good job at pretending she's not bearing a child with all the red wine she's consuming around Euron, but we all know Jaime's baby is growing inside her.
During season eight's fourth episode, "The Last Stark," Cersei tells Euron she is pregnant and he assumes it's his baby, even though they didn't have sex until the season eight premiere. Minutes after her celebratory conversation with Euron, Cersei and Tyrion confront each other begging one another to surrender. Because we know nothing is more important to Cersei than her children, Tyrion uses this to ask her to surrender "for her child." One Reddit user suggests that saying this out loud could lead to Euron betraying Cersei.
"Euron was standing right behind Cersei, and he just found out about 'his' child moments before this scene. From Euron's point of view, Tyrion should have no knowledge that Cersei is pregnant — Tyrion was up in the North when he slept with Cersei for the first time," grantvoyager wrote. "It might not have been intentional on Tyrion's part, but I think that Euron might realise that Cersei's baby isn't his, turn on Cersei, and potentially kill her."
Whether Tyrion's move to announce her pregnancy publicly is calculated or not, it's a very smart one. And if Euron is quick enough to piece it together and question how Tyrion knows she's pregnant, it could lead to him betraying Cersei. Now with Jaime on his way back to King's Landing, and the public knowing Cersei's bearing a child, we can only hope that the few people who still support Cersei will start to turn their backs on the evil queen.