Previous studies have identified primiparity (first-time mother) as a significant risk factor for postpartum psychosis and especially bipolar affective disorders.  This is the first study to quantify the risk of psychiatric illness after the first as compared to subsequent pregnancies.

Information was analyzed from a cohort of 750,127 women identified by Danish population registries; the women were followed individually from the date of birth of their first, second, or third child through the following 12 months.  The highest risk for psychiatric hospitalization was found in primiparous mothers at 10-19 days postpartum [relative risk (RR) = 8.65]. After the second birth, the highest risk was at 60-89 days postpartum (RR = 2.01), and there was no increased risk after the third birth. The effect of primiparity was the strongest for bipolar disorder.

Munk-Olsen T, Jones I, Laursen TM. Birth order and postpartum psychiatric disorders. Bipolar Disord. 2014 May;16(3):300-7.