In this issue of the Annals of the Rheumatic Diseases Wallenius et al1 report an increase in the onset of rheumatoid arthritis (RA) and other arthritides just after delivery (see article on page 332). They have shown this by linking two national registers—namely, the Norwegian Disease-Modifying Antirheumatic Drug (NOR-DMARD) Register and the Medical Birth Register of Norway. This report confirms previous studies2 3 4 on RA and additionally extends our knowledge to other forms of arthritis. Owing to the low incidence of RA occurring simultaneously or just after pregnancy, the previous studies on this subject were mainly of a matched case–control design.2 4 Since every study design carries the risk of introducing its own type of bias, conformation of this finding in a different setting is important.

In these previous studies it has been shown that the increased incidence of RA after delivery is accompanied by a lower incidence during pregnancy.2 4 This suggests that the increased incidence of RA after delivery is not a true increased incidence, but rather that the occurrence of RA is postponed until after delivery.

How can these data be interpreted? It is clear that they can be seen in the scope of the well-known ameliorating effect of pregnancy on the course of existing RA and the flare-up of disease activity in the postpartum period. Since the first description of this phenomenon by Hench in 1938,5 this finding has been reconfirmed in several studies,6 7 most recently in a large prospective study from before conception and onwards.8 In this first large prospective study using validated instruments to determine disease activity,9 it was shown that almost half of …