Cox proportional HR with 95% CI for the association between childbirth, hospitalization and DP due to mental or musculoskeletal diagnoses during year 2–5 after first childbirth, in twins, where at least one in the pair had their first delivery 1994–2009 and none before 1994.

T0 is defined as year for first childbirth for women who gave birth, and year of twin sister's childbirth for those who did not give birth.
1Twin pairs with one twin having a mental DP diagnosis during the follow-up and the other not (n = 22 pairs).
2Twin pairs with one twin having a musculoskeletal DP diagnosis during the follow-up and the other not (n = 10 pairs).
3Adjusted for birth year.
4Model a, with additional adjustments for delivery year and educational level.
5Model b, with additional adjustments for earlier hospitalization.
6Diagnoses related to pregnancy, childbirth and the puerperium excluded (ICD-10: O80-84).