Interbirth spacing and offspring mental health outcomes

The perinatal or early life environment may influence the development of mental illness in adulthood. It is not clear how, or when, any such influences might be mediated. Foetal exposure to maternal stress in the intrauterine environment has been suggested as a possible mediator of foetal origins of mental illness but the postnatal environment may also be of importance. This study aimed to test the foetal origins hypothesis by using retrograde and antegrade interbirth intervals (time to mother's most recent and next deliveries respectively) as proxy measures of antenatal and postnatal maternal stress.

Linked datasets of the Scottish Morbidity Record (SMR) were used to identify a birth cohort. Where applicable, the dates of each mother's most recent previous and/or next subsequent delivery were noted, allowing birth intervals to be calculated. The cohort was followed up into young adulthood, using self-harm, substance misuse, psychotic disorder and affective disorder as outcome measures. Data were analysed using Cox regression.

No significant relationship was observed between affective disorders and interbirth interval, neither retrograde nor antegrade. Short (<18-month) antegrade birth intervals were independently associated with increased risk of psychotic disorder and self-harm. Long (>72-month) retrograde intervals were associated with increased risk of self-harm and substance misuse.

The data do not provide evidence for the foetal origins of mental disorders but, in the cases of psychotic disorders, and of self-harm, suggest that the early postnatal rather than the antenatal environment may be of greater importance.