Caesarean section and risk for endometriosis: a prospective cohort study of Swedish registries.

OBJECTIVE

To investigate the association between caesarean section and later endometriosis.

DESIGN

A prospective cohort study.

SETTING

The Swedish Patient Register (PAR) and the Swedish Medical Birth Registry (MBR).

SAMPLE

Women who were delivered in Sweden between 1986 and 2004.

METHODS

Women with the diagnosis of endometriosis, defined as codes 617 (International Classification of Diseases, ninth revision, ICD-9) or N80 (ICD-10), were retrieved from the PAR. Obstetric outcome was assessed through linkage with the MBR. Out of 709,090 women, 3110 were treated as inpatients with a first diagnosis of endometriosis after their first delivery. Women with a diagnosis of endometriosis before their first delivery were excluded. Cox analyses were performed to obtain hazard ratios for endometriosis and adjusted for maternal age at first delivery, body mass index, maternal smoking, and years of involuntary childlessness at study entry. Kaplan-Meier estimates were performed to calculate the risk according to time elapsed.

MAIN OUTCOME

In-hospital diagnosis of endometriosis.

RESULTS

The Cox analyses yielded a hazard ratio of 1.8 (95% CI 1.7-1.9) for endometriosis in women who had had a previous caesarean section compared with women with vaginal deliveries only. The risk of endometriosis increased over time: one additional case of endometriosis was found for every 325 women undergoing caesarean section within 10 years. No increase in risk could be seen after two caesarean deliveries. The risk of caesarean scar endometrioma was 0.1%.

CONCLUSION

In addition to the recognised risk of scar endometrioma, we found an association between caesarean section and general pelvic endometriosis. Further studies are needed to confirm our findings.