Piotr is aware that it is not the tactful thing to inform your pregnant girlfriend that you have zero desire to investigate the scientific possibilities of mpreg.

He might briefly think it, and Kate can undoubtedly guess, but he doesn't say it.

Instead, he smiles. (He doesn't know how it makes his face light up, no matter how many times she says it, but he wouldn't be too surprised to learn.) "Da. We are."