This Is Us Season 4 Episode 18 revolves around baby Jack's first birthday, while in the past Rebecca and Jack have a bittersweet celebration for the Big Three's first birthday.

While the Pearsons are getting ready to go to LA for Jack's birthday, Beth lets it slip that she doesn't quite approve of how Randall handled the Rebecca situation. At the party, Rebecca decides to announce she has decided to do the clinical trial, shocking Kate and Kevin.

Randall tells Kevin that maybe Rebecca wanted an investment in her future, which the other two think is nerdy. Later, Rebecca parrots the same words to Kevin and he realizes what happened.

Kevin confronts Randall, but their conversation is interrupted with Madison, who has news of her own: she's pregnant with Kevin's child. Kevin walks outside to process this and Randall continues the fight, leading to both of them saying horribly hurtful things to each other.

Meanwhile, Kate and Toby discuss adopting a second child after visiting the NICU to put up a photo of Jack's process, while in the past Rebecca is sad about the child she lost during the Big Three's first birthday and in the future, Jack D. and Lucy have a baby of their own.

To find out what happens, watch This Is Us Season 4 Episode 18 online.