On December 19, 2019, at approximately 1020 central standard time, United Airlines flight 2390, a B737-900ER, N68802, made an abrupt stop on the taxiway to avoid ground traffic at Chicago O'Hare International Airport (KORD), Chicago, Illinois. One flight attendant suffered a serious injury, and there were no injuries to the remaining 5 crewmembers and 179 passengers onboard. There was no damage to the airplane. The flight was operating under 14 Code of Federal Regulations Part 121 as a regularly scheduled passenger flight from KORD to Portland International Airport, Portland, Oregon. According to the operator, the airplane was pushed back from the gate and was given instructions from Outbound Ground Control (OGC) to taxi to runway 28R by joining taxiway A. As UAL 2390 was proceeding to taxiway A, an Embraer 175 had landed and was in communication with the Inbound Ground Control (IGC), and was also instructed to proceed to taxiway A. The UAL 2390 flight crew stated they were given no instructions to follow or give way to other airplane. As UAL 2390 approached taxiway A, the flight crew noticed the E175 approaching from behind and to the left and stopped abruptly to avoid it. During the abrupt stop, a flight attendant preparing the cabin for departure fell and sustained a serious injury. UAL 2390 returned to the gate and medical personnel met the airplane to treat the injured flight attendant.