Table 2

Relative risk of ACPA-positive and ACPA-negative RA according to postpartum period for last delivered child in women aged 18–44 years

ACPA statusYears between last delivered child and index yearCases/controlsOR* 95% CIOR† 95% CI
ACPA-negativeNulliparous65/3601.01.0
023/592.6 (1.4 to 4.8)2.1 (0.9 to 4.8)
1 year14/571.8 (0.9 to 3.6)1.4 (0.6 to 3.6)
2 years6/491.0 (0.4 to 2.5)0.8 (0.2 to 2.3)
P for trend‡0.00930.1336
ACPA-positiveNulliparous165/3601.01.0
029/591.1 (0.7 to 1.8)0.8 (0.4 to 1.6)
1 year27/571.1 (0.6 to 1.8)0.8 (0.4 to 1.5)
2 years20/490.9 (0.5 to 1.5)0.6 (0.3 to 1.3)
P for trend‡0.91360.6316
  • *Adjusted by matching variables (age and residential area).

  • †Adjusted by age, residential area and age at first birth. Adjustments for smoking (pack-years), body mass index (BMI <25/≥25), use of oral contraceptives (ever/never), breast feeding (<24/≥24 months in total), age of menarche (≤11, 12, 13 and ≥14 years) and formal education (compulsory, vocational/theoretical upper secondary school, university degree, other education) did not substantially change the ORs and were therefore not retained in the final analyses.

  • ‡Wald χ2 test for trend.

  • ACPA,  antibodies to citrullinated peptide antigens; EIRA, Epidemiological Investigation of Rheumatoid Arthritis; RA, rheumatoid arthritis.