VIA US WEEKLY “They just found out and tweeted it to the world,” a source tells Us Weekly. Duhamel shared the same message via his Twitter and Facebook accounts. The couple, who wed in January 2009, spoke about their desire to start a family during an October 2012 appearance on Oprah’s Next Chapter. Though Duhamel said he hoped to one day have five children, Fergie lowered his expectations. “We negotiated that early on,” the 37-year-old singer said. “Two is our number.” Duhamel, 40, added that because he and Fergie “grew up in very similar ways,” they’re on the same page when it comes to raising a little one. “My mother…