Table 3

Relative risk of ACPA-positive and ACPA-negative RA according to age at first birth in women aged 18–44 years

ACPA statusAge at first birthCases/controlsOR* 95% CIOR† 95% CI
ACPA-negativeNulliparous65/3601.01.0
≤22 years39/1162.5 (1.5 to 4.1)2.7 (1.6 to 4.8)
23–26 years39/1442.1 (1.3 to 3.5)2.4 (1.3 to 4.4)
27–30 years36/1561.8 (1.1 to 3.1)2.2 (1.1 to 4.2)
≥31 years22/1301.5 (0.8 to 2.8)1.8 (0.8 to 4.0)
P for trend‡0.00470.0058
ACPA-positiveNulliparous165/3601.01.0
≤22 years57/1161.0 (0.7 to 1.5)0.9 (0.6 to 1.5)
23–26 years68/1441.0 (0.7 to 1.4)0.9 (0.6 to 1.3)
27–30 years67/1560.9 (0.6 to 1.3)0.8 (0.5 to 1.2)
≥31 years45/1300.7 (0.4 to 1.0)0.5 (0.3 to 1.0)
P for trend‡0.33050.2339
  • *Adjusted by matching variables (age and residential area).

  • †Adjusted by age, residential area and years between last delivered child and the index year. 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.