The unparalleled development of the infant brain occurring within and simultaneously with the forming of a primary attachment bond challenges clinicians to incorporate developmental neuroscience into infant/parent psychotherapy. There is growing evidence that evolution has selected the last trimester of pregnancy continuing through the first two to 2.5 years of life as a Primary Biological Entrainment Period when the quality of infant/parent care and attuned communication (or its lack) affects brain development, secure attachment, and emotional regulation. The manuscript is primarily a theoretical contribution arguing for the need of deeper biological work in infancy. The author suggests the use of an evidenced informed neurobiological scaffold for Infant Parent Psychotherapy called Integrative Regulation Therapy during the Primary Biological Entrainment Period.