Table 2.

Association of early life factors and risk of uterine leiomyomata, by age, Black Women’s Health Study, 1997–2009.

Characteristic	AGE <30				AGE ≥30				P-value, test for age interaction†
	Person-years	Cases	Multivariable modela		Person-years	Cases	Multivariable modela
			IRR	95%CI			IRR	(95%CI)
Born outside of the U.S.
No	21,484	496	1.00	Ref.	155,608	5,882	1.00	Ref.	0.56
No, but ≥1 parent born outside U.S.	2,563	61	1.10	0.84, 1.44	7,441	275	1.01	0.89, 1.14
Yes	1,830	51	1.26	0.94, 1.69	10,344	426	1.11	1.01, 1.22
Region of residence from birth to age 15
Northern tier	5,127	136	0.95	0.74, 1.21	31,558	1,246	1.00	0.92, 1.08	0.09
Middle tier	6,430	138	0.76	0.60, 0.97	45,556	1,875	1.03	0.96, 1.11
Southern tier	4,452	126	1.00	Ref.	35,376	1,400	1.00	Ref.
Any combination of above	2,223	55	0.86	0.62, 1.18	17,507	687	0.98	0.89, 1.07
Season of birth
Spring	6,173	128	0.96	0.75, 1.22	38,815	1,474	1.00	0.93, 1.07	0.19
Summer	6,933	178	1.20	0.96, 1.50	45,936	1,715	0.98	0.92, 1.05
Fall	6,958	171	1.12	0.89, 1.40	47,018	1,811	1.01	0.94, 1.08
Winter	6,190	136	1.00	Ref.	43,666	1,655	1.00	Ref.
Highest education of either parent, y
<12	1,475	49	1.30	0.94, 1.80	22,831	880	1.00	0.91, 1.08	0.30 [P-interaction = 0.06 if one IX term for education <12 yrs with age]
12	4,383	105	0.93	0.73, 1.18	40,298	1,587	0.99	0.92, 1.06
13–15	6,133	143	0.93	0.74, 1.15	36,203	1,411	0.98	0.91, 1.05
≥16	7,537	191	1.00	Ref.	38,093	1,525	1.00	Ref.
P-trendc				P = 0.58				P = 0.81
Home ownership up to age 11
Rented only	7,998	203	1.01	0.83, 1.22	60,130	2,397	1.04	0.98, 1.10	0.84
Rented and owned	955	27	1.11	0.75, 1.66	4,955	222	1.13	0.98, 1.29
Owned only	10,449	258	1.00	Ref.	72,621	2,820	1.00	Ref.
In utero exposure to cigarette smoke
No	16,219	396	1.00	Ref.	105,976	4,126	1.00	Ref.	0.02
Yes	2,055	37	0.74	0.53, 1.04	19,284	779	1.03	0.95, 1.11
Passive smoke exposure up to age 10
No	12,434	279	1.00	Ref.	72,546	2,674	1.00	Ref.	0.62
Yes	12,343	304	1.10	0.94, 1.30	89,775	3,511	1.06	1.00, 1.11
Mother’s age at participant birth, y
<20	3,276	94	1.08	0.79, 1.47	20,313	846	1.01	0.92, 1.11	0.67
20–24	5,316	142	1.01	0.77, 1.33	35,403	1,413	0.97	0.90, 1.05
25–29	3,866	97	0.96	0.72, 1.28	26,345	1,108	1.03	0.95, 1.12
≥30	3,749	96	1.00	Ref.	30,261	1,229	1.00	Ref.
First-born
No	11,611	267	1.00	Ref.	95,876	3,701	1.00	Ref.	0.83
Yes	8,893	230	1.12	0.92, 1.36	50,348	2,036	1.06	1.00, 1.13
Mother’s age (y) and first-born status
<20 and first-born	2,275	65	1.16	0.84, 1.62	13,208	553	1.05	0.95, 1.17	0.98
<20 and later-born	551	17	1.18	0.70, 2.00	4,761	214	1.11	0.95, 1.28
20–29 and first-born	3,804	102	1.06	0.78, 1,43	20,818	864	1.04	0.94, 1.14
20–29 and later-born	4,345	113	1.03	0.77, 1.37	35,120	1,442	1.01	0.93, 1.09
≥30 and first-born	424	11	1.07	0.57, 2.02	3,155	138	1.12	0.94, 1.34
≥30 and later-born	3,001	77	1.00	Ref.	24,517	1,001	1.00	Ref.
Participant’s birth weight, g
<2,000	810	17	0.93	0.57, 1.52	4,202	148	0.94	0.80, 1.11	0.59
2,000–2,499	2,651	57	0.90	0.68, 1.19	18,334	729	1.05	0.97, 1.13
≥2,500	18,219	434	1.00	Ref.	116,178	4,444	1.00	Ref.
Participant preterm
No	16,314	406	1.00	Ref.	105,387	4,021	1.00	Ref.	0.18
Yes	2,383	47	0.80	0.56, 1.16	11,789	465	1.08	0.96, 1.20
Participant twin or triplet
No	25,516	603	1.00	Ref.	170,405	6,472	1.00	Ref.	0.30 for all twins; 0.19 for sub-categories
Yes:	496	7	0.64	0.30, 1.36	3,300	113	0.91	0.75, 1.10
Identical	124	3	0.99	0.32, 3.11	717	21	0.76	0.50, 1.18
Fraternal	355	3	0.40	0.13, 1.24	2,338	83	0.95	0.76, 1.18
Left-handed
No	17,227	421	1.00	Ref.	122,069	4,748	1.00	Ref.	0.41 for all left-handers, 0.86 with parental handedness
Yes:	2,408	71	1.21	0.94, 1.55	18,082	764	1.08	1.00, 1.17
Yes, as was ≥1 parent	556	18	1.32	0.82, 2.12	3,781	184	1.22	1.06, 1.42
Yes, but parents were not	1,682	47	1.14	0.84, 1.54	12,591	517	1.05	0.96, 1.16
Breastfed as infant
No	12,886	304	1.00	Ref.	82,074	3,134	1.00	Ref.	0.17
Yes	4,994	139	1.18	0.95, 1.45	40,023	1,612	1.04	0.98, 1.11
Duration of breastfeeding, mo.
≤ 3	664	17	1.09	0.66, 1.78	4,065	165	1.05	0.90, 1.23	0.38
4–6	1,075	31	1.21	0.83, 1.77	7,393	284	0.99	0.87, 1.12
7–11	731	26	1.50	1.00, 2.25	6,255	261	1.07	0.94, 1.21
≥12	998	20	0.86	0.54, 1.36	6,553	263	1.03	0.90, 1.17
P-trendc				P = 0.55				P = 0.39
Fed soy formula as infant
No	14,874	357	1.00	Ref.	111,245	4,309	1.00	Ref.	0.19
Yes	1,254	38	1.28	0.91, 1.79	6,030	227	0.99	0.87, 1.13

IRR, incidence rate ratio; 95% CI, 95% confidence interval.

^aAdjusted for age, questionnaire cycle, birth weight, birth order, maternal age at participant’s birth, nativity, and parental education.

^bTest for interaction using cross-product terms for each level of exposure variable and binary age (<30 vs. ≥30); degrees of freedom equal to the number of levels of exposure.

^cP-v alue from Wald test of ordinal variable.