The phrase "initial phase" refers to the first part or beginning stage of something. It is the early period where something starts or starts to happen. Full definition
The growing human baby would need the most protein during initial phases of bone development. (womeninneuroscience.org)
During this intense initial phase of getting to know someone, we often selectively focus on his or her best points. (modernlovelongdistance.com)
Topics include human subjects protection, measurement, development of instruments, data collection and management, and initial phases of data analysis. (onlineeducation.net)