Reproductive factors and risk of cervical cancer by cell type. A prospective study.

Relationships between reproductive variables and risk of cervical cancer were examined in a follow-up of 62,079 women in Norway from 1961 through 1980. For the 342 cases classified as squamous cell carcinomas, a higher risk was observed in ever married than in never married women. The risk was especially high among women married more than once and women who were widowed or divorced before start of follow-up. High age at first birth was associated with low risk. The estimated odds ratio for women with first birth at age 35 years or later versus 19 years or earlier was 0.18 (P less than 0.001) in analyses with adjustment for age, urban-rural place of residence and parity. In analyses with adjustment for age at first birth, neither parity or age at first marriage, nor age at menarche or menopause showed significant associations with squamous cell carcinoma. For the 30 cases classified as adenocarcinomas, no significant associations emerged with reproductive factors. The effects of marital status as well as age at first birth differed significantly between adenocarcinomas and squamous cell carcinomas, suggesting dissimilar aetiologies. Although associations between reproductive factors and squamous cell carcinoma may largely be secondary to relationships with sexual habits, there are indications that the association with age at first birth cannot be entirely explained in this way.