Table 3. Age at first sexual intercourse and risk of cervical cancer according to various characteristics.

	Number of cases/controls	Odds ratio (95% CI)a	Odds ratio (95% CI)b	P-trend
Parity
Nulliparous
⩾21 years	14/40	1.00	1.00
17–20 years	7/14	1.88 (0.53–6.66)	1.61 (0.40–6.57)
⩽16 years	7/6	3.60 (0.83–15.52)	1.50 (0.17–13.55)	0.56
Ever parous
⩾21 years	327/614	1.00	1.00
17–20 years	804/645	2.47 (2.07–2.94)	1.97 (1.63–2.36)
⩽16 years	703/387	4.10 (3.36–4.99)	2.59 (2.08–3.21)	<0.001
P-heterogeneity between AFSI and ever parous=0.64
Parity (1–4 births)
⩾21 years	171/399	1.00	1.00
17–20 years	273/287	2.58 (1.98–3.35)	1.99 (1.51–2.62)
⩽16 years	169/115	4.72 (3.41–6.54)	2.71 (1.89–3.87)	<0.001
Parity (⩾5 births)
⩾21 years	156/215	1.00	1.00
17–20 years	531/358	2.01 (1.57–2.59)	1.71 (1.32–2.23)
⩽16 years	534/272	2.88 (2.19–3.78)	2.08 (1.55–2.78)	<0.001
P-heterogeneity between AFSI and parous groups (nulliparous, 1–4 births, and ⩾5 births)=0.90
Oral contraceptive use
Never
⩾21 years	218/400	1.00	1.00
17–20 years	453/364	2.33 (1.87–2.90)	1.77 (1.40–2.25)
⩽16 years	376/184	4.28 (3.29–5.55)	2.45 (1.82–3.29)	<0.001
1–4 years
⩾21 years	64/162	1.00	1.00
17–20 years	117/114	2.64 (1.76–3.94)	1.67 (1.07–2.61)
⩽16 years	111/89	3.94 (2.53–6.13)	1.95 (1.15–3.30)	0.01
⩾5 years
⩾21 years	36/52	1.00	1.00
17–20 years	124/72	3.26 (1.85–5.72)	2.46 (1.36–4.46)
⩽16 years	96/49	4.48 (2.39–8.40)	2.80 (1.38–5.65)	0.006
Smoking
Never
⩾21 years	272/569	1.00	1.00
17–20 years	587/552	2.34 (1.93–2.83)	1.68 (1.36–2.06)
⩽16 years	550/348	3.76 (3.03–4.67)	2.06 (1.61–2.64)	<0.001
Ever
⩾21 years	67/85	1.00	1.00
17–20 years	221/113	2.73 (1.81–4.13)	2.32 (1.50–3.60)
⩽16 years	152/43	5.63 (3.42–9.27)	3.62 (2.10–6.26)	<0.001
Lifetime number of sexual partners
Monogamous
⩾21 years	270/569	1.00	1.00
17–20 years	529/499	2.33 (1.92–2.83)	1.80 (1.46–2.22)
⩽16 years	349/214	3.89 (3.05–4.96)	2.38 (1.83–3.11)	<0.001
Partners >1
⩾21 years	69/74	1.00	1.00
17–20 years	280/151	2.03 (1.37–3.01)	1.74 (1.15–2.63)
⩽16 years	352/156	2.75 (1.84–4.11)	2.14 (1.39–3.28)	0.001
P-heterogeneity=0.36
Education
Never go to school
⩾21 years	54/57	1.00	1.00
17–20 years	255/155	1.68 (1.09–2.60)	1.46 (0.93–2.30)
⩽16 years	402/173	2.41 (1.55–3.75)	2.09 (1.31–3.35)	0.001
Primary school
⩾21 years	164/238	1.00	1.00
17–20 years	404/311	1.99 (1.54–2.56)	1.62 (1.24–2.12)
⩽16 years	236/167	2.51 (1.87–3.39)	1.71 (1.24–2.36)	0.001
Secondary school
⩾21 years	118/359	1.00	1.00
17–20 years	150/198	2.68 (1.95–3.68)	2.29 (1.65–3.20)
⩽16 years	72/55	4.79 (3.07–7.47)	3.36 (2.07–5.47)	<0.001
P-heterogeneity=0.08
Ever have a pap smear 12 months before study enrolment
Never
⩾21 years	183/320	1.00	1.00
17–20 years	434/371	2.20 (1.74–2.79)	2.29 (1.65–3.20)
⩽16 years	416/205	3.65 (2.79–4.78)	3.36 (2.07–5.47)	<0.001
Ever
⩾21 years	158/336	1.00	1.00
17–20 years	379/296	2.65 (2.02–3.46)	1.92 (1.44–2.57)
⩽16 years	294/191	4.49 (3.32–6.06)	2.30 (1.63–3.24)	<0.001

Abbreviations: CI=confidence interval; AFSI=age at first sexual intercourse.

^aAdjusted for age and study centre.

^bAdjusted for age, study country, lifetime number of partners (1, ⩾2), parity (0, 1–4, ⩾5), and education (never, primary, secondary).