Background: It is documented that preventive interventions for vulnerable children should start during pregnancy. Clinicians who work with families in the perinatal period must deal with a complexity created by the combination of risk and protection in the child's care system. National guidelines for treatment offered within mental health care in the perinatal period are currently lacking. We present areas of assessment that can inform the clinician working with pregnant women and their partners and show how the use of a biopsychosocial and cultural case formulation can promote therapeutic work in the perinatal period (P-CF). Method: Based upon clinical experience, theory, research, guidelines and knowledge about parents and children in the perinatal period, a selection is made of risk and protective factors that may influence the child’s development and health. Findings: The selected risk and protective factors are systemised in a biopsychosocial and cultural case formulation for the care system in the perinatal period (P-CF). Use of P-CF in clinical settings are exemplified. Implications: The use of P-CF enables the clinician to generate hypotheses about both development and skewed development. Systematic use can increase equality and quality in clinical decision-making processes by making the clinician's assessments less dependent on personal preferences and degree of experience. P-KF can assist the clinician in balancing risk and protective factors and elucidate where therapeutic interventions need to me made and how they should be provided. Keywords: perinatal, mental health, early intervention, case formulation, pregnancy