Whatever the context of the situation may be, the announcement looks to have hit Toby like a ton of bricks; when Kate is seen storming away from her shocked husband, she’s wearing the same outfit from the birthday scene, so it’s likely they had a talk about the situation in between those edits.
Kate is hoping to try again for a child with Toby, which follows the miscarriage storyline the characters had before their wedding at the end of Season 2.
It’s a bit weird for Kate to be making that announcement in the midst of a birthday party, especially given the superstition that openly talking about one’s wish can lead to it not coming true.
This Is Us Season 3 "Surprises Are Coming" Promo (HD)