Table 2.

Reproductive history and incidence of uterine leiomyomata, Nurses' Health Study II, 1989–2003

Reproductive characteristic	Cases	Person- years	Age adjusted RR (95% CI)	Multivariate RR (95% CI)	P trend
Parity
Nulliparous	2587	244478	1.00	1.00a
Parous	7002	841730	0.70 (0.68, 0.73)	0.69 (0.66, 0.73)
Number of pregnancies lasting > 6 months (among parous women)
1	1442	171776	1.00	1.00	<0.001b
2	3536	398410	0.96 (0.90, 1.02)	0.97 (0.90, 1.04)
3	1570	201681	0.82 (0.76, 0.88)	0.88 (0.80, 0.96)
≥ 4	454	69863	0.67 (0.60, 0.74)	0.78 (0.69, 0.89)
Age at first birth, years (among parous women)
≤ 25	3844	386148	1.00	1.00c	<0.001c
26–30	2533	360484	0.75 (0.71, 0.79)	0.86 (0.81, 0.91)
≥ 31	805	145375	0.53 (0.49, 0.57)	0.71 (0.64, 0.79)
Time since last birth, years (among parous women)
< 1	83	28085	0.88 (0.70, 1.11)	0.89 (0.70, 1.13)d	<0.001d
1–3	598	173553	1.00	1.00
4–5	523	99238	1.43 (1.27, 1.61)	1.33 (1.18, 1.50)
6–7	603	93575	1.69 (1.50, 1.90)	1.51 (1.33, 1.70)
8–9	702	88914	2.02 (1.79, 2.27)	1.71 (1.51, 1.93)
10–12	1286	117929	2.68 (2.40, 2.99)	2.14 (1.89, 2.41)
13–15	1137	91415	3.02 (2.69, 3.39)	2.24 (1.96, 2.56)
≥ 16	1998	133214	3.75 (3.35, 4.22)	2.48 (2.13, 2.87)
Age at last birth, years (among parous women)
≤25	1321	101680	1.32 (1.23, 1.40)	1.09 (1.00, 1.18)e	0.02e
26–30	2975	325808	1.00	1.00
31–35	2111	305885	0.69 (0.66, 0.73)	0.91 (0.84, 0.98)
≥ 36	556	100673	0.46 (0.42, 0.50)	0.80 (0.70, 0.93)
Total breastfeeding, months (among parous women)
< 1	1470	137699	1.00	1.00e	<0.001e
1–3	453	49318	0.96 (0.87, 1.07)	1.03 (0.92, 1.14)
4–6	656	77864	0.90 (0.82, 0.98)	0.97 (0.89, 1.07)
7–12	1323	153569	0.89 (0.83, 0.96)	1.00 (0.92, 1.08)
13–18	964	114225	0.85 (0.78, 0.92)	0.99 (0.91, 1.08)
19–24	639	83375	0.75 (0.68, 0.82)	0.91 (0.83, 1.01)
25–36	792	101123	0.75 (0.69, 0.82)	0.94 (0.86, 1.03)
≥ 37	419	68487	0.57 (0.51, 0.63)	0.80 (0.71, 0.90)
Exclusive breastfeeding, months (among parous women)
< 1	2783	287392	1.00	1.00e	0.003e
1–3	737	80049	1.00 (0.92, 1.08)	1.05 (0.97, 1.14)
4–6	816	96295	0.92 (0.85, 1.00)	1.02 (0.94, 1.10)
7–12	1292	160233	0.84 (0.79, 0.90)	0.96 (0.90, 1.03)
13–18	534	73153	0.74 (0.68, 0.81)	0.91 (0.82, 1.00)
19–24	161	23494	0.68 (0.58, 0.80)	0.90 (0.76, 1.06)
25–36	44	8046	0.54 (0.40, 0.72)	0.78 (0.57, 1.06)
≥ 37	5	799	0.59 (0.25, 1.43)	0.92 (0.38, 2.23)

^aAdjusted for body mass index, infertility, marital status, race, age at menarche, age at first oral contraceptive use, antihypertensive medication use and diastolic blood pressure.

^bAdjusted for age at first birth and time since last birth as well as all of the covariates listed in footnote a.

^cAdjusted for parity and time since last birth as well as all of the covariates listed above.

^dAdjusted for parity and age at first birth as well as all of the covariates listed in footnote a.

^eAdjusted for parity, age at first birth, and time since last birth as well as all of the covariates listed in footnote a.