Objective To investigate associations of giving birth with morbidity in terms of hospitalisation and social consequences of morbidity in terms of sickness absence (SA), while taking familial (genetics and shared environmental) factors into account.

Design Prospective register-based cohort study. Estimates of risk of hospitalisation and SA were calculated as HRs with 95% CIs.

Setting All female twins, that is, women with a twin sister, born in Sweden.

Participants 5118 Swedish female twins (women with a twin sister), born during 1959–1990, where at least one in the twin pair had their first childbirth (T₀) during 1994–2009 and none gave birth before 1994.

Main outcome measures Hospitalisation and SA during year 3–5 after first delivery or equivalent.

Results Preceding the first childbirth, the mean annual number of SA days increased for mothers, and then decreased again. Hospitalisation after T₀ was associated with higher HRs of short-term and long-term SA (HR for short-term SA 3.0; 95% CI 2.5 to 3.6 and for long-term SA 2.3; 95% CI 1.6 to 3.2). Hospitalisation both before and after first childbirth was associated with a higher risk of future SA (HR for long-term SA 4.2; 95% CI 2.7 to 6.4). Familial factors influenced the association between hospitalisation and long-term SA, regardless of childbirth status.

Conclusions Women giving birth did not have a higher risk for SA than those not giving birth and results indicate a positive health selection into giving birth. Mothers hospitalised before and/or after giving birth had higher risks for future SA, that is, there was a strong association between morbidity and future SA.