Here's the author's announcement on Twitter.
And here's what Atwood has to say on her websiteYes indeed to those who asked: I’m writing a sequel to The #HandmaidsTale. #TheTestaments is set 15 years after Offred’s final scene and is narrated by three female characters. It will be published in Sept 2019. More details: https://t.co/e1umh5FwpX pic.twitter.com/pePp0zpuif— Margaret E. Atwood (@MargaretAtwood) November 28, 2018