Objective: Maternal cancer may be associated with offspring mental and behavioural disorders through various biological pathways. When postnatally diagnosed, it may cause stress and changes in care, potentially influencing mental health. Prenatally diagnosed cancer could lead to maternal stress and treatment, or influence foetal neural development. This study investigates associations between prenatally or postnatally diagnosed maternal cancers and mental and behavioural disorders in children.

Methods: The study composed of 2 158 430 children born in Denmark (1978‐2012). Children were exposed if their mother received a cancer diagnosis prenatally (2 years prepartum, until birth) or postnatally (birth, until 18 years postpartum). Further analyses considered cancer types and diagnostic delays. Children were followed until 18 years of age or the first of the following: diagnosis of a mental or behavioural disorder, emigration, death, end of follow‐up.

Results: During follow‐up 79 682 (3.7%) children were diagnosed with mental or behavioural disorders. There was an increased risk among offspring exposed to postnatally diagnosed cancers (HR 1.05; 95% CI, 1.00‐1.11); for prenatally diagnosed cancers HR was 1.07 (0.87‐1.31). The strongest associations for disorder types were for prenatal diagnoses with mood/affective disorders (HR 2.45; 1.02‐5.89) and postnatal diagnoses with mood/affective disorders (HR 1.43; 1.14‐1.79).

Conclusions: The results indicate a link between maternal cancer occurrence during pregnancy or early postnatal life, and mental and behavioural disorders in offspring. This association could be driven by common factors in the two periods, such as psychological stress or genetic factors. No specific foetal programming was identified.