Scarlett Johansson is a mom to two now. The “Black Widow” star recently gave birth to a son, Cosmo, with husband Colin Jost, the “Saturday Night Live” star wrote on Instagram Wednesday. This is the first child for the couple, who were married last October. “Ok, OK, we had a baby. His name is Cosmo. We love him very much,” was Jost’s brief note. People magazine first reported the birth of the baby, which came as a shock to many since news of the pregnancy came out only days earlier after Jost reportedly mentioned it in a stand-up set in Connecticut. No other details were provided. It’s the first child for Jost, 39, who is known for hosting “Weekend Update” on “Saturday Night Live.” Johansson, 36, also has a 6-year-old daughter, Rose, from a previous marriage to Romain Dauriac. Jost wrote on his Instagram page that privacy would be greatly appreciated and …