Harington, who will reprise his ''Game of Thrones'' character Jon Snow in a sequel series, made the revelation on the NBC talk show ''The Tonight Show''.

When host Jimmy Fallon asked him about their two-year-old son, the actor said, ''He's about to get the shock of his life, which is that he's about to get a brother or sister.'' Harington, 36, then confirmed that Leslie, 35, is pregnant.

The ''Eternals'' star later talked about the differences in his mindset between expecting the first baby and now preparing for the second.

''With the first baby, you're like walking on clouds and dancing through fields of daisies for nine months — well, the man is, anyway. But this time, the reality check comes much shorter. You get practical real quick,'' he added. Harington and Leslie met in 2012 on the set of ''Game of Thrones'', the HBO megahit series, in which they played on-screen lovers Jon Snow and Ygritte.