The number and percentage of deliveries where the female’s average BTS asthma treatment step increased, decreased or remained the same, when compared with the same calendar time in the year before pregnancy, stratified by pregnancy trimester.

1a decrease in average treatment step was considered to be indicative of an improvement in asthma control whilst an increase in average treatment step was considered to represent a worsening in asthma control.2N = 14,141.3N = 14,097 as a small number of deliveries were very premature and born at the very end of the second trimester and therefore did not contribute to the third trimester.