Taking care of the newborn baby has become her main preoccupation lately.

preoccupation 是由前缀 pre-（表示"在...之前"）和 occupation（职业、占据）组成的复合词。字面意思是"事先占据"，引申为"全神贯注的事物"或"最关心的事情"。在句中表示"占据她全部注意力的事情"。