Final answer:
The observation that would indicate the need for further assessment related to postpartum depression is option 3: The mother constantly complains of tiredness and fatigue.
Step-by-step explanation:
Of the given options, the observation that would indicate the need for further assessment related to postpartum depression is option 3: The mother constantly complains of tiredness and fatigue. Postpartum depression can often lead to feelings of exhaustion and fatigue, which can significantly impact a new mother's overall well-being and ability to care for her infant. Constant complaints of tiredness and fatigue may indicate that the mother is experiencing symptoms of postpartum depression and should be further assessed and supported.