An attempt to estimate the incidence of cervical dysplasia in a group of New Zealand women using contraception. The New Zealand Contraception and Health Study Group.

We attempted to estimate incidence rates of cervical dysplasia and carcinoma in a cohort of 7,199 New Zealand women using contraception. The cohort was followed prospectively with periodic cervical smears, intended to be annual, over a 6-year period. The principal outcome investigated was a diagnosis of cervical dysplasia of any degree, from mild to severe dysplasia and including carcinoma in situ or invasive carcinoma, made cytologically by a central study laboratory and confirmed by histology or deoxyribonucleic acid (DNA) evaluation. These diagnoses are jointly referred to as "dysplasia." Two successive negative (nondysplastic) smears were required before a woman was considered eligible for the analysis of incidence. Even after these two negative smears, the estimated "incidence" of dysplasia declined markedly in each of the 5 years of the study, particularly among women who provided negative smears in each prior year. This suggests that prevalent cases were being diagnosed even after five or more negative smears. Assuming that nearly all of the prevalent cases were removed after five negative smears, our estimate of the annual incidence of cervical dysplasia in this population during this time period would be of the order of 5 per 1,000 per year. We conclude that the sensitivity of cervical testing for identifying cervical dysplasia is quite low in this population but is consistent with values reported from some other populations. Age at first intercourse, age at first pregnancy, number of sex partners, and current cigarette smoking were strongly associated with risk of dysplasia. Our data are equivocal on the question of whether age at first intercourse is a risk factor independently of the closely associated variable, number of sex partners.