Prepare to clear your schedules this Christmas, as filming has officially wrapped on the upcoming Call the Midwife spin-off, Sisters in Arms. The three-part prequel special, which comes to BBC iPlayer and BBC One this festive period, follows a young Sister Monica Joan at Nonnatus House in 1939. WATCH: The trailer for Call the Midwife series 14As we count down the days until Christmas, find out all we know about the upcoming spin-off below. © BBC / Neal Street Productions / Nicky Johnston The spin-off is set 30 years before the original series"But I was surprised how many new emotions Call the Midwife: Sisters in Arms sparked," she continued. Call the Midwife is available to stream on BBC iPlayer now.