Subjects
All women resident in Norway in 31.12.1996 were identified from the Population Register. Women born before 1900, diagnosed with invasive cervical cancer or underwent hysterectomy before 1996, died in 1997, or had inadequate data for estimating the pregnancy duration were excluded (N = 39 707). The final study population consisted of 2175762 females and the personal identification number (PIN), a unique 11 digit code, identified the subjects.
The Norwegian co-ordinated CC screening programme
A thorough description of the CC Screening programme is provided elsewhere [12]. Briefly, the Screening programme in Norway is built around the Cytology Register, which was established in 1992 to administrate the CC screening programme in Norway. It is mandatory to register PIN, date and result of every single Pap smear diagnosed in the country, irrespective of age of the woman, the nature of the health service (private/public) or the indication of the Pap smear. The PIN is used as a key for linkage between the Population and the Cytology Registry in order to identify all 25–69 year old women without a normal Pap smear during a period of three years, which is the recommended screening interval in Norway. From 1995 every women without a Pap smear in the period was identified on a monthly basis and they received personal invitations to participate. In this way, the opportunistic screening was integrated into the organised programme. To date information on more than six million Pap smears is available from the register.
The Medical Birth Registry of Norway
Established in 1967, the Medical Birth Registry of Norway (MBRN) was organised to conduct epidemiological surveillance of birth defects and other perinatal health problems in Norway aiming at prevention, as well as health services related to pregnancy, childbirth and the neonatal period, aiming at quality assurance. MBRN routinely registers the PIN and demographic information on the mother and the father, mother's health before and during pregnancy, including chronic diseases, complications during pregnancy and delivery as well as information on the infant, including birth defects and other perinatal problems.
Linkage between study population, MBRN and the Cytology Register
The PIN was used to gather information about pregnancies from the MBRN and previous Pap smears from the population-based Cytology Register for each subject.
From the MBRN records all 116 810 women who gave birth (or had abortion after the 16th gestation week) in 1996–7 in Norway (N = 121 400) were identified with information about exact date of birth, duration of the pregnancy, details about pregnancy outcome, multiple offspring, birth weight, and information on previous pregnancies. When there was more than one pregnancy registered per women for the period 1996–7 (2615 women had two and four women had three consecutive records during two years period) the first was included. When there was more than one offspring per birth (1904 twins, 58 triples and 2 women had 4 offspring) we used information about the first born. For 105 818, women the date of the last menstruation was available and used as the beginning of the antepartum period. For 10 992 women (9.4%) this information was missing and beginning of the pregnancy period was estimated by subtracting from date of birth 41 weeks if birth weight was > 3400 gram, 40 weeks if 3200–3400 gram, 39 weeks if 2800–3000 gram, 38 weeks if 2600–2800 gram, 37 weeks if 2400–2600 gram, 36 weeks if 2200–2400 gram, 35 weeks if 2000–2200 gram, 34 weeks if 1800–2000 gram, 33 weeks if 1600–1800 gram, 32 weeks if 1400–1600 gram, 31 weeks if 1200–1400 gram, 30 weeks if 1000–1200 gram, 29 weeks if 800–1000 gram, 28 weeks if 600–800 gram, 27 weeks if 400–600 gram.
For each study subject we identified the date and the diagnoses of the Pap smears from the Cytology Register. Subjects who received an invitation letter from the Cancer Registry of Norway were identified, together with the posting date of the invitation letter.
Study cohorts
We categorised the following three mutually exclusive cohorts, within the study population:
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1.
Pregnant women cohort (Cpreg): subjects giving birth during the period ± 3 month from the date 31.12.1996. T0 is the date denoting the start of the follow-up. For subjects within the pregnant women cohort, T0 was the date when the antepartum period started, estimated individually for each subject as described above.
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2.
Reference cohort (Cref): subjects who neither gave birth nor were pregnant during this calendar period respective to the follow-up period of the pregnant women e.g. from 01.01.1996 to 31.12.1997. For the Reference cohort, T0, the date marking the start of the follow-up was randomly chosen for each subject from the same calendar period as for the Cpreg.
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3.
Mixed cohort: all other subjects in the study population who gave birth in 1996–7 but were not included in the Cpreg.
Pregnant women and reference cohort were followed up for one year (since T0) in the Cytology Register for Pap smears. To identify the pattern of attendance to screening for each subject, time since last Pap smear before T0 was identified and stratified as the last Pap smear taken 1, 2, 3 or > 3 years earlier. To evaluate the response to the invitation letter, we identified subjects who received an invitation to screening and stratified analyses by the assessed time since the invitation was mailed in relation to T0, i.e. 24 to 2 months prior to T0, one month prior to 3 months after the T0, more than three months after the T0.
Statistical analyses
The Kaplan-Meier method was used to estimate the cumulative probability of having a Pap smear since T0.[17]
We used unconditional logistic regression to compare the odds of having a Pap smear within 12 month after inclusion in the study for Crefand Cpreg. Odds ratios (OR) with 95% confidence intervals (95% CI) were estimated both crude and adjusted for age, screening history and time since invitation.