Table 3 Studies assessing the association of birth order and the risk of breast cancer

Case-control studies

[51]

1967

LCC-D

229

229

USA

1

-

The authors measured the mean value of birth weight instead of providing ORs (95% CIs). The mean difference between cases and matched controls was not significant (P > 0.2). They provided the frequency of each case and control in the tables and we calculated crude ORs

2

3

4

5

6

≥ 7

[50]^a

1980

MCC

4339

12,760

USA, Japan, Slovenia, Athens, Taipei

1

Referent

The risks (point estimates) only by birth order were shown in the figure in the original article.

2

0.93

3

1.08

4

0.99

5

1.05

6

1.07

7

1.18

≥ 8

1.02

[42]

1988

PCC

153

461

USA

1

Referent

P for trend = 0.16

2

0.92 (0.55–1.54)

3

0.98 (0.58–1.72)

4

0.69 (0.36–1.32)

≥ 5

1.03 (0.60–1.79)

[46]

1991

MCC

927

2,616

USA/Wales/Japan

All ages

Adjusted for age, study center, parity, age at first birth, age at menarche, height, body mass index, maternal age at birth, and menopausal status

   1

Referent

   2

0.91 (0.73–1.02)

   3

1.11 (0.87–1.27)

   ≥ 4

1.09 (0.81–1.18)

Premenopausal

   1

Referent

   ≥ 2

0.76 (0.60–0.96)

[12]

1992

LCC

458

1,197

Sweden

1

Referent

Adjusted for age and birth date

≥ 2

1.00 (0.76–1.32)

[47]

1994

PCC

2,414

9,138

USA

1

Referent

Adjusted for age at first birth and number of children

2

0.90 (0.78–1.03)

3

0.98 (0.84–1.14)

4

0.86 (0.73–1.02)

5

0.93 (0.78–1.11)

6

1.02 (0.84–1.23)

7

0.91 (0.73–1.14)

≥ 8

0.88 (0.75–1.04)

[14]

1996

PCC

1,129

1,393

USA

1

Referent

Adjusted for age, menopausal status, and maternal smoking; P for trend = 0.06 among both groups

2

1.0 (0.7–1.4)

≥ 3

0.8 (0.6–1.1)

[32]

1997

NCC

1,068

2,727

Sweden

1

Referent

Adjusted for maternal age, socioeconomic status, parity, and preeclampsia or eclampsia, neonatal jaundice, severe prematurity, and twinship

2

1.01 (0.83–1.22)

≥ 3

1.01 (0.81–1.26)

[15]

2000

LCC

481

2,863

USA

1

1.07 (0.84–1.35)

Crude ORs

2–3

Referent

4–5

1.06 (0.81–1.38)

≥ 6

1.50 (1.06–2.13)

[35]

2002

PCC

1,555

1,539

USA

1

Referent

Adjusted for age and residential regions (states)

2

1.07 (0.88–1.30)

3

1.07 (0.85–1.35)

4

1.01 (0.77–1.31)

5

0.66 (0.48–0.92)

≥ 6

0.81 (0.62–1.08)

[18]

2003

LCC

881

3,423

Denmark

1

Referent

Adjusted for mother's marital status, maternal age, and birth order

≥ 2

1.01 (0.83–1.12)

[36]

2004

PCC

854

785

USA

All subjects

Adjusted for age, race and sampling fractions, body mass index, hosehold income, maternal age

   1

Referent

   2–4

0.9 (0.7–1.1)

   ≥ 5

1.0 (0.8–1.3)

Born ≥ 1948

   1

Referent

   2–4

0.9 (0.6–1.4)

   ≥ 5

0.6 (0.3–1.3)

[48]^a

2005

MCC

24

34

Nigeria

≤ 3

Referent

Crude ORs

≥ 4

1.50 (0.25–8.98)

[20]

2005

PCC

1642

1,713

Poland

1

Referent

Adjusted for age, education, age at menarche, menopausal status and age at menopause, age at first full-term pregnancy, number of full-term pregnancies, family history of breast cancer among first-degree relatives, mammography screening, and current body mass index

2

1.07 (0.91–1.24)

3–5

0.99 (0.85–1.15)

≥ 6

0.81 (0.61–1.06)

P for trend = 0.81

[37]

2006

PCC

1,166

2,105

USA

1

Referent

Adjusted for age (years), education (years), race, body mass index, history of breast benign disease, family history of breast cancer, lactation (months), age at menarche (years), age at first full-term pregnancy (years), age at menopause (years), parity

≥ 2

1.27 (0.88–1.85)

Cohort studies

[31]

2006

Cohort

209

1,024

USA

1

Referent

Hazard ratio for breast cancer mortality: adjusted for age at diagnosis, diagnosis year, stage at diagnosis, and birth order, with exception of birth order, which is adjusted for maternal age

2

0.2 (0.2–0.3)

≥ 3

0.2 (0.2–0.3)

P for trend < 0.01

[49]^a

2001

Cohort

-

-

Sweden

Continuous scale

1.05 (1.01–1.10)