Sienna Miller is now a mom of three after welcoming her second baby with boyfriend Oli Green. "It's happened," Miller, 44, confirmed in an interview with E! News published Monday, May 11. "I have a tiny baby next door." While gushing over motherhood, Miller admitted that the transition from two kids to three has been an adjustment. "It feels like stringing sentences together is a bit challenging," she teased. "I'm on very little sleep, but I'm madly in love with my baby." Miller and Green, 29, are already parents of a baby girl, whose name they have kept private.