Prospective case-control study of premenopausal serum estradiol and testosterone levels and breast cancer risk.

Abstract

Breast cancer is frequently a hormonally dependent cancer, and associations of circulating estrogens and androgens with subsequent breast cancer risk are well established in postmenopausal women. Associations of serum estrogens and androgens with breast cancer risk in premenopausal women are less well studied. The objective of this study was to determine whether estradiol and testosterone levels in serum collected before menopause are associated with subsequent breast cancer risk.
We conducted a prospective case-control study of 266 participants who were registered in the Columbia, Missouri, Serum Bank and not using exogenous hormones at the time of blood collection. Each of 98 in situ or invasive breast cancer cases with prediagnostic serum collected before menopause was matched to two controls by age, date, menstrual cycle day, and time of day of blood collection. Estradiol and testosterone concentrations were quantified by using specific radioimmunoassays, and sex hormone-binding globulin (SHBG) was quantified with a chemiluminescent immunoassay to allow calculation of the non-SHBG bound hormone fractions. Data were analyzed by using conditional logistic regression. All tests of statistical significance were two-sided.
Serum testosterone was strongly and significantly associated with breast cancer risk. The relative odds (OR) for increasing quartiles of total testosterone were 1.0, 2.1 (95% confidence interval (CI) 0.9 to 4.8), 1.5 (95% CI, 0.6 to 3.4), and 3.3 (95% CI, 1.5 to 7.5, P(trend) = 0.006). Comparable ORs for the non-SHBG bound fraction of testosterone that is bioavailable were 1.0, 1.7 (95% CI, 0.7 to 4.2), 1.7 (95% CI, 0.7 to 4.0), and 4.2 (95% CI, 1.6 to 10.9, P(trend) = 0.002). Total and non-SHBG-bound estradiol were not associated with breast cancer, but extreme variation in levels across the menstrual cycle coupled with relatively small numbers, particularly for analyses stratified by cycle phase, limited the power to detect associations.
Results suggest that premenopausal women with elevated serum testosterone levels are at an increased risk of breast cancer.

Full text

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RESEARCH ARTICLEOpen Access
Prospective case-control study of premenopausal
serum estradiol and testosterone levels and
breast cancer risk
Joanne F Dorgan1*, Frank Z Stanczyk2, Lisa L Kahle3, Louise A Brinton4
Abstract
Introduction: Breast cancer is frequently a hormonally dependent cancer, and associations of circulating estrogens
and androgens with subsequent breast cancer risk are well established in postmenopausal women. Associations of
serum estrogens and androgens with breast cancer risk in premenopausal women are less well studied. The
objective of this study was to determine whether estradiol and testosterone levels in serum collected before
menopause are associated with subsequent breast cancer risk.
Methods: We conducted a prospective case-control study of 266 participants who were registered in the
Columbia, Missouri, Serum Bank and not using exogenous hormones at the time of blood collection. Each of 98 in
situ or invasive breast cancer cases with prediagnostic serum collected before menopause was matched to two
controls by age, date, menstrual cycle day, and time of day of blood collection. Estradiol and testosterone
concentrations were quantified by using specific radioimmunoassays, and sex hormone-binding globulin (SHBG)
was quantified with a chemiluminescent immunoassay to allow calculation of the non-SHBG bound hormone
fractions. Data were analyzed by using conditional logistic regression. All tests of statistical significance were two-
sided.
Results: Serum testosterone was strongly and significantly associated with breast cancer risk. The relative odds
(OR) for increasing quartiles of total testosterone were 1.0, 2.1 (95% confidence interval (CI) 0.9 to 4.8), 1.5 (95% CI,
0.6 to 3.4), and 3.3 (95% CI, 1.5 to 7.5, Ptrend= 0.006). Comparable ORs for the non-SHBG bound fraction of
testosterone that is bioavailable were 1.0, 1.7 (95% CI, 0.7 to 4.2), 1.7 (95% CI, 0.7 to 4.0), and 4.2 (95% CI, 1.6 to
10.9, Ptrend= 0.002). Total and non-SHBG-bound estradiol were not associated with breast cancer, but extreme
variation in levels across the menstrual cycle coupled with relatively small numbers, particularly for analyses
stratified by cycle phase, limited the power to detect associations.
Conclusions: Results suggest that premenopausal women with elevated serum testosterone levels are at an
increased risk of breast cancer.
Introduction
Most breast tumors are estrogen dependent, and post-
menopausal women with elevated serum estrogens are
at an increased risk of developing breast cancer [1].
Estrogens are synthesized from androgens in the preme-
nopausal ovary and in extraovarian tissues, and postme-
nopausal women with elevated serum androgens are
similarly at an increased risk of breast cancer [1].
Several lines of epidemiologic evidence indicate a role of
premenopausal ovarian hormones in breast cancer etiol-
ogy. In particular, menarche and menopause delineate
the beginning and end of ovarian function, and younger
age at menarche but older age at menopause are asso-
ciated with an increased risk of breast cancer [2]. Thus,
duration of exposure to premenopausal ovarian hor-
mones is positively associated with risk. Evidence also is
accumulating from prospective cohort studies on asso-
ciations of circulating estrogens and androgens with pre-
menopausal breast cancer risk [3-7]. However, results
are not totally consistent across studies, particularly for
* Correspondence: joanne.dorgan@fccc.edu
1Fox Chase Cancer Center, 333 Cottman Avenue, Philadelphia, PA 19111,
USA
Full list of author information is available at the end of the article
Dorgan et al. Breast Cancer Research 2010, 12:R98
http://breast-cancer-research.com/content/12/6/R98
© 2010 Dorgan et al.; licensee BioMed Central Ltd. This is an open access article distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.

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estrogens, and additional research is needed on the
long-term associations of premenopausal sex-hormone
levels with breast cancer risk. Here we report results of
a case-control study nested in the Columbia, MO,
Serum Bank of the prospective associations of estradiol
and testosterone levels in blood samples collected before
menopause with subsequent breast cancer.
Materials and methods
The Columbia, MO, Serum Bank initially was estab-
lished in 1977 as part of the National Cancer Institute
(NCI) Biological Markers Project to identify serum mar-
kers for breast cancer. Participants were volunteers iden-
tified primarily through the Breast Cancer Detection
Demonstration Project (BCDDP) at the University of
Missouri Hospital and Ellis Fischel Cancer Center in
Columbia. A total of 6,915 women who initially were
free of cancer, other than nonmelanoma skin cancer,
donated blood to the bank on one or more occasions
between December 1977 and June 1989. All women
gave written informed consent before donating serum to
the serum bank, and Institutional Review Boards at the
National Cancer Institute and Fox Chase Cancer Center
approved the research reported herein.
Serum specimens were collected, and clinical data,
including age, height, weight, menstrual and reproduc-
tive histories, smoking, hormone replacement and oral
contraceptive use, and family history of breast cancer,
were obtained by self-report or medical-record review.
Approximately 30 ml of blood was collected from each
woman by using standard procedures. Blood was
allowed to stand at room temperature for at least one-
half hour or until it was thoroughly clotted and then
refrigerated. Within 2 hours of collection, blood was
centrifuged, and serum was separated and aliquoted into
1.1-ml sterile glass vials. Vials were labeled, sealed with
rubber stoppers, and stored at -70°C.
Follow-up of women who donated blood to the
Columbia, MO, Serum Bank was conducted in two
phases. Initial follow-up continued for up to 12 years
until December 1989. A questionnaire was mailed to
participants annually to ascertain information on interim
cancer diagnoses. Women who indicated that they had a
breast biopsy or breast cancer were sent a consent form
for permission to obtain medical records, including
pathology reports. For cancers at sites other than the
breast, date of diagnosis was ascertained. An extended
follow-up of Columbia, MO, Serum Bank participants
was conducted by NCI in 1999 through 2004. Cancer
diagnoses were ascertained by self-report and by search-
ing the Missouri Cancer Registry, BCDDP Cohort files,
and National Death Index (NDI) Plus. Of the 6,720
women included in the extended follow-up because they
had one or more vials of serum remaining, 6,131 (91%)
were located as either alive or dead, 589 (9%) were not
able to be located, 59 (< 1%) refused to participate, and
49 (< 1%) were too ill to participate. In total, 1,751
women (25%) were identified as deceased, with confir-
mation of causes of death provided via NDI.
Cases for the current study included premenopausal
women in the Columbia, MO, Serum Bank who were
free of cancer other than nonmelanoma skin cancer and
not taking exogenous estrogens or progestins at the
time they donated blood, who subsequently were diag-
nosed with in situ or invasive breast cancer that was
confirmed by medical records, the Missouri Cancer Reg-
istry, or NDI. Initially, potential cases were selected if
less than 1 year had elapsed since their last menses, or
if they were younger than 50 years (the median age of
natural menopause in the Columbia, MO, Serum Bank)
at blood collection and missing data on the date of their
last menses. For each of 117 potential cases that were
identified, two potential controls who were alive at the
age of the case diagnosis and who remained cancer free
were randomly selected. Controls were matched to the
case on age (± 2 years), date (± 1 year), menstrual cycle
day (± 2 days, or if the potential case was missing data
on the date of the last menses, a potential control with
missing data on the date of her last menses was
selected), and time of day (± 2 hours) of blood collec-
tion. Matching criteria were relaxed to identify potential
controls in some instances (age (n = 6), date (n = 11),
cycle day (n = 13), and time (n = 5)). Final determina-
tion of menopausal status for potential participants who
were missing data on the date of their last menses was
based on age at blood collection and serum follicle-sti-
mulating hormone (FSH) and estradiol concentrations.
Those who were younger than 44 years (the 10thper-
centile of age at natural menopause in the Columbia,
MO, Serum Bank) were presumed to be premenopausal,
whereas those who were 45 to 49 years old and had an
FSH greater than 25 mIU/ml and estradiol less than 25
pg/ml were considered to be postmenopausal and
excluded from analysis. After exclusion of postmenopau-
sal women and one control with serum testosterone of
102.5 ng/dl, 98 cases and 168 matched controls
remained and were included in analyses.
All laboratory analyses were performed at the Repro-
ductive Endocrine Research Laboratory, University of
Southern California Keck School of Medicine. Sera from
each case and the matched control were grouped
together, and matched sets were randomly organized
within batches. The laboratory was blinded to which
samples were from cases and which were from controls.
Estradiol and testosterone were quantified by specific
radioimmunoassays after organic solvent extraction and
Celite column-partition chromatography, as described
previously [8,9]. Sex hormone-binding globulin (SHBG)
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was measured with a chemiluminescent immunoassay
on the Immulite analyzer (Siemens Medical Solutions
Diagnostics, Malvern, PA, USA) to allow calculation of
bioavailable (free plus albumin bound) estradiol and tes-
tosterone [10]. FSH was similarly measured on the
Immulite analyzer. The average coefficients of variation
(CVs) were 8.5% for estradiol, 11.2% for testosterone,
7.0% for SHBG, and 6.9% for FSH.
Statistical analysis
The association of serum estradiol and testosterone
with breast cancer risk for matched sets was evaluated
by using conditional logistic regression. Univariate
associations of individual characteristics shown in
Table 1 with breast cancer risk also were evaluated by
using conditional logistic regression to retain the
matching. Geometric mean concentrations for each
hormone in cases and controls were calculated and
compared by testing the statistical significance of the
trend of the log-transformed variable entered as a con-
tinuous term in conditional logistic regression models.
Women were stratified into quartiles of hormone con-
centration, and a set of categoric indicator variables
was included in conditional logistic regression models
to estimate odds ratios (ORs). Models were fit by using
natural log-transformed hormone concentrations to
test for trend. Adjusted models included terms for
height (continuous), body mass index (BMI, continu-
ous), age at menarche (continuous), number of term
pregnancies (continuous), smoking (never, former, or
current), and history of breast cancer in a first-degree
relative (yes or no). Although controls were matched
to cases on age and menstrual cycle day at blood col-
lection, to adjust for possible residual confounding, age
(continuous) was included in all models, and menstrual
cycle day (cubic spline) was included in models for
estradiol. Interactions were evaluated by testing the
statistical significance of cross-product terms included
in models with main effects. Because estradiol is
secreted primarily by ovarian follicles during the folli-
cular phase of the menstrual cycle but by the corpus
luteum in the luteal phase, we also conducted estradiol
analyses stratified by cycle phase at blood collection
(follicular, 0 to 13 days since start of last menses;
luteal, 14 to 33 days since start of last menses).
Women whose blood was collected more than 33 days
after the last menses or who were missing data on
time since the last menses were excluded. Women
were categorized into cycle phase-specific tertiles of
estradiol for these analyses, because small numbers did
not allow finer gradation. All tests of statistical signifi-
cance were two-sided, and a cut-off of P less than 0.05
was used to determine statistical significance. All ana-
lyses were performed by using SAS 9.2 (Cary, NC,
USA) and STATA10.1 (College Station, TX, USA).
Results
Participant characteristics are summarized in Table 1.
All women were premenopausal and not using oral con-
traceptives or other estrogens at the time of blood col-
lection. All subjects were white. Cases’ and controls’
ages averaged 44.7 years (range, 31.4 to 56.1 years).
Cases did not differ from controls in terms of anthropo-
metric or reproductive characteristics, except cases
experienced menarche at a slightly younger age com-
pared with controls (12.5 versus 12.8 years; P = 0.08).
More cases had a history of breast cancer in a first-
degree relative compared with controls (17.4% versus
8.3%; P = 0.03). Data on date of last menses were miss-
ing for 14.3% of cases and 11.3% of controls, and 9.2%
of cases and 7.7% of controls had blood collected 34 or
more days after their last menses. The median number
of days from last menses at blood collection for these
participants was 52 days (range, 35 to 81 days). Cases’
mean (± SD) age at breast cancer diagnosis was 58.7 ±
8.4 years (range, 37.7 to 74.0 years). The mean (± SD)
time from blood collection to diagnosis was 14.0 ± 6.6
years. Eighty-six tumors (88%) were invasive.
Table 1 Participant characteristics at blood collection
CharacteristicCases
(n = 98)
Controls
(n = 168)
Continuous variables
Mean ± SD
Pa
Age, years
Height, cm
BMI, kg/m2
Menarche, years
Age at first pregnancy, yearsb
Number of term pregnancies
44.7 ± 4.8
163.4 ± 6.0 163.5 ± 5.8 0.95
25.7 ± 5.1 25.2 ± 4.8
12.5 ± 1.3 12.8 ± 1.4
22.2 ± 4.322.1 ± 4.1
2.7 ± 1.62.9 ± 1.7
44.6 ± 4.50.10
0.35
0.08
0.92
0.50
Categoric variables
Frequency,
number (%)
Pa
Nulliparous
Smoking status
Never
9 (9.2%)13 (7.7%) 0.82
0.50
61 (62.2%) 103
(61.3%)
26 (15.5%)
39 (23.2%)
14 (8.3%)
Former
Current
History of breast cancer in first-
degree relative
Menstrual cycle day at blood
collection
Days 0-13
Days 14-33
Days ≥34
Unknown
19 (19.4%)
18 (18.4%)
17 (17.4%) 0.03
0.45
35 (35.7%)
40 (40.8%)
9 (9.2%)
14 (14.3%)
68 (40.5%)
68 (40.5%)
13 (7.7%)
19 (11.3%)
BMI, body mass index; SD, standard deviation.aP values (two-sided) were
calculated by using conditional logistic regression with a Wald test.bIncludes
89 parous cases and 155 parous controls.
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As shown in Table 2 cases had statistically signifi-
cantly higher geometric mean total testosterone (28.1
versus 24.7 ng/dl; P = 0.005) and bioavailable testoster-
one (11.8 versus 10.1 ng/dl; P = 0.002) concentrations
than did controls. Total and bioavailable estradiol con-
centrations did not differ between the groups. Breast
cancer risk also was not associated with total or bioa-
vailable estradiol levels overall or in analyses stratified
by menstrual cycle phase (Table 3). In contrast, total
and bioavailable testosterone levels were strongly and
significantly associated with risk (Table 4). The associa-
tion was strongest for bioavailable testosterone in which
the multivariable adjusted odds ratios (OR) for increas-
ing quartiles were 1.0, 1.7, 1.7, and 4.2 (95% CI, 1.6 to
10.9; Ptrend= 0.002). Results were not substantially dif-
ferent when analysis was restricted to women who were
within 33 days of last menses at the time of blood
collection.
When we restricted analysis to invasive breast cancer
(n = 86), results did not differ materially from those for
all cases. However, a pattern of increasing risk with
increasing total estradiol concentration in the follicular
phase of the menstrual cycle emerged that was not
apparent previously. Adjusted ORs for invasive breast
cancer with increasing tertiles of follicular-phase total
estradiol were 1.0, 1.8, and 2.0 (95% CI, 0.4 to 10.3), but
the test for trend remained nonsignificant (Ptrend=
0.40). In contrast, associations of follicular-phase bioa-
vailable estradiol with invasive cancer remained erratic,
with adjusted ORs decreasing to 0.7 (95% CI, 0.1 to 3.8)
and then increasing to 3.2 (95% CI, 0.6 to 16.5) across
increasing tertiles of concentration.
We previously showed a strong positive association of
serum müllerian-inhibiting substance (MIS) with breast
cancer risk. Adjustment for MIS attenuated only slightly
the associations of total and non-SHBG-bound testoster-
one with breast cancer risk. In analysis adjusted for MIS
in addition to other covariates, ORs for increasing quar-
tiles of total testosterone were 1.0, 2.1, 1.2, and 3.0 (95%
CI, 1.2 to 7.3; Ptrend= 0.03). Comparable ORs for non-
SHBG-bound testosterone were 1.0, 1.6, 1.3, and 3.6
(95% CI, 1.3 to 9.9; Ptrend= 0.02). Adjustment for MIS
did not materially change results for total estradiol or
non-SHBG-bound estradiol. Tests for interaction
between MIS and total or bioavailable estradiol and tes-
tosterone in relation to breast cancer risk were not sta-
tistically significant.
Because we did not have data on eventual age at
menopause, we could not perform an analysis stratified
by menopausal status at diagnosis. We, therefore, per-
formed analysis limited to the 67 cases who were 55
years or older at diagnosis and their matched controls
as a proxy for postmenopausal status at diagnosis.
Serum total and bioavailable testosterone remained sig-
nificantly associated with breast cancer risk in these
analyses. Covariate adjusted ORs for increasing quartiles
of total testosterone were 1.0, 2.4, 1.6, and 4.5 (95% CI,
1.6 to 13.0; Ptrend= 0.009) and for bioavailable testoster-
one were 1.0, 2.1, 2.1, and 7.1 (95% CI, 2.1 to 24.3; Ptrend
= 0.002). Few cases (n = 31) were younger than 55 years
old at diagnosis, and results are not shown for this
subgroup.
The time from blood collection to diagnosis ranged
from 2 months to 23 years. To evaluate the potential for
reverse causality as an explanation for the observed
association between serum testosterone and breast can-
cer risk, we repeated analyses restricted to cases whose
blood was collected longer in time before diagnosis.
Results were similar to those shown in Table 4 for ana-
lysis restricted to 83 cases diagnosed at least 5 years
after blood collection and when restricted to 73 cases
diagnosed at least 10 years after blood collection. In
analysis restricted to cases diagnosed at least 10 years
after blood collection, ORs for increasing quartiles of
total testosterone were 1.0, 1.7, 1.6, and 3.7 (95% CI, 1.3
to 10.3; Ptrend= 0.02). For non-SHBG-bound testoster-
one, comparable ORs were 1.0, 2.1, 2.3, and 6.2 (95%
CI, 1.9 to 20.7; Ptrend= 0.003).
Discussion
The positive associations that we observed in the
Columbia, MO, cohort between premenopausal serum
Table 2 Geometric mean (95% confidence interval) hormone concentrations in prediagnostic serum from casesaand
matched controls
Cases Controls
n
Mean 95% CI
n
Mean95% CI
Pb
Estradiol, pg/ml
Bioavailable estradiol, pg/ml
Testosterone, ng/dl
Bioavailable testosterone, ng/dl
SHBG, nmol/L
98
98
98
98
98
85.8
49.9
28.1
11.8
55.2
72.5-101.5
42.1-59.0
26.2-30.1
10.8-12.8
50.4-60.4
168
168
168
168
168
80.3
45.7
24.7
10.1
57.7
70.5-91.6
40.2-51.9
23.3-26.1
9.5-10.7
53.7-62.0
0.62
0.46
0.005
0.002
0.34
All variables are continuous. CI, confidence interval; SHBG, sex hormone-binding globulin.aMean (± SD) time from blood collection to diagnosis was 14.0 ± 6.6
years.bP values (two-sided) were calculated by using conditional logistic regression with log-transformed hormone and SHBG levels entered as a linear term to
test for trend by using a Wald test.
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testosterone levels and breast cancer risk are consistent
with those reported previously in prospective studies. In
the study of Hormones and Diet in the Etiology of
Breast Tumors (ORDET) [5], the breast cancer OR for
premenopausal women in the highest versus lowest ter-
tile of free testosterone was 2.8 (95% CI, 1.1 to 7.3). The
excess risk of breast cancer associated with elevated tes-
tosterone levels was less in the European Prospective
Investigation into Cancer and Nutrition (EPIC) but
remained statistically significant; the OR for women in
the highest versus lowest quartile of total testosterone
was 1.7 (95% CI, 1.2 to 2.6) [4]. Premenopausal testos-
terone levels were not significantly associated with all
breast cancers in the Nurses Health Study II (NHS-II)
[3]. However, testosterone in plasma collected during
the luteal phase of the menstrual cycle was significantly
associated with invasive and estrogen/progesterone-
receptor positive breast cancer; ORs for women in the
Table 3 Odds ratios (ORs) and confidence intervals (CIs) of breast Cancer by prediagnostic serum estradiol
concentration stratified by menstrual-cycle phasea
HormoneNumber of cases/number of controls UnadjustedAdjusted
OR(95% CI)
Pb
OR(95% CI)
Pb
All participants
Estradiol
5-54 pg/ml
55-94 pg/ml
95-138 pg/ml
139-497 pg/ml
0.620.48
22/45
28/37
24/43
24/43
1.0
1.7
1.2
1.1
1.0
1.9
1.1
1.4
(0.8 to 3.6)
(0.6 to 2.5)
(0.5 to 2.2)
(0.8 to 4.6)
(0.5 to 2.5)
(0.6 to 3.4)
Bioavailable estradiol
1.5-31.3 pg/ml
31.6-54.9 pg/ml
55.0-79.2 pg/ml
79.3-302.4 pg/ml
0.46 0.38
23/43
25/42
26/41
24/42
1.0
1.2
1.2
1.1
1.0
1.2
1.2
1.3
(0.6 to 2.5)
(0.6 to 2.6)
(0.5 to 2.2)
(0.5 to 2.8)
(0.5 to 2.8)
(0.5 to 3.1)
Follicularc
Estradiol
5-54 pg/ml
56-108 pg/ml
110-480 pg/ml
0.52 0.69
10/22
13/21
12/21
1.0
1.5
1.4
1.0
2.4
1.4
(0.5 to 4.3)
(0.4 to 4.2)
(0.6 to 10.6)
(0.3 to 6.5)
Bioavailable estradiol
1.5-31.7 pg/ml
32.2-62.9 pg/ml
63.5-264.2 pg/ml
0.510.70
12/21
8/25
15/18
1.0
0.6
1.8
1.0
0.7
2.0
(0.2 to 1.9)
(0.6 to 5.8)
(0.1 to 3.1)
(0.5 to 8.3)
Luteald
Estradiol0.490.62
6-78 pg/ml
80-114 pg/ml
115-299 pg/ml
10/24
16/18
11/23
1.0
2.0
1.2
1.0
2.3
1.3
(0.7 to 5.8)
(0.4 to 3.2)
(0.6 to 9.2)
(0.4 to 4.3)
Bioavailable
estradiol
3.5-48.4 pg/ml
48.8-67.9 pg/ml
68.2-184.6 pg/ml
0.290.51
9/25
15/19
13/21
1.0
2.1
1.6
1.0
2.5
1.8
(0.7 to 6.0)
(0.6 to 4.4)
(0.6 to 10.5)
(0.6 to 5.8)
aCases and controls were matched on age, date, hour, and days since last menses at blood collection. All adjusted models include terms for age and menstrual-
cycle day at blood collection, height, BMI, age at menarche, number of term pregnancies, smoking status, and history of breast cancer in first-degree relative.
Hormone cut points define quartiles or menstrual-cycle phase-specific tertiles estimated from all participants.bP values (two-sided) were calculated by using
conditional logistic regression, with log-transformed hormone concentrations entered as a linear term to test for trend by using a Wald test.cThe case and
matched controls were in the follicular phase of the menstrual cycle at blood collection (0 to 13 days since last menses).dThe case and matched controls were in
the luteal phase of the menstrual cycle at blood collection (14 to 33 days since last menses).
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