Table 2.

Baseline Characteristics of 786 Control Participants by Quartile of Urinary 6-Sulfatoxymelatonin Concentration in Nurses' Health Study II, 1996–2007

Characteristica	Quartile of Urinary aMT6s Concentrationb
	Q1 (n = 197)		Q2 (n = 196)		Q3 (n = 196)		Q4 (n = 197)
	Mean (SD)	%	Mean (SD)	%	Mean (SD)	%	Mean (SD)	%
Age, yearsc	44.8 (3.8)		44.1 (4.1)		43.2 (4.3)		43.8 (4.2)
Age at menarche, years	12.3 (1.5)		12.5 (1.4)		12.5 (1.3)		12.4 (1.4)
Body mass indexd	26.9 (6.3)		25.9 (6.5)		25.7 (5.1)		24.0 (4.9)
Alcohol consumption, g/day	3.5 (5.6)		3.9 (6.4)		2.8 (5.5)		4.3 (7.0)
Age at first birth, yearse	26.6 (4.5)		26.3 (4.8)		26.3 (4.7)		26.0 (4.3)
Paritye	2.3 (0.9)		2.3 (1.0)		2.3 (0.9)		2.4 (0.9)
Nulliparous		18.7		20.6		17.8		16.5
Caucasian ethnicityc		94.9		98.9		97.6		98.9
History of benign breast disease		17.6		22.1		18.3		19.3
Family history of breast cancer		8.5		10.6		11.3		10.4
Premenopausal at urine collectionc		77.2		74.9		84.7		80.0

Abbreviations: aMT6s, 6-sulfatoxymelatonin; Q, quartile; SD, standard deviation.

^aValues are standardized to the age distribution of the study population. All characteristics except age were adjusted for age.

^bQuartile ranges were as follows: Q1, ≤26.6 ng/mg creatinine; Q2, 26.7–42.5 ng/mg creatinine; Q3, 42.6–61.8 ng/mg creatinine; Q4, ≥61.9 ng/mg creatinine.

^cMatching variables included age, ethnicity, and menopausal status at the time of urine collection.

^dWeight (kg)/height (m)².

^eAmong parous women only.