Table 1 Characteristics of twins and singletons in the Basse HDSS, The Gambia, 2009–2013 (N = 33,519)

Sex

Boys

16386

50.5

504

46.5

1 (reference)

Girls

16049

49.5

579

53.5

1.17

1.02–1.34

0.022

Ethnicity

Serahule

14604

45.0

513

47.4

1 (reference)

Mandinka

6721

20.7

232

21.4

0.98

0.84–1.15

0.828

Fula

10508

32.4

321

29.6

0.87

0.75–1.00

0.053

Others

597

1.8

17

1.6

0.81

0.50–1.32

0.401

Region

Rural

28633

88.3

970

89.6

1 (reference)

Urban

3803

11.7

113

10.4

0.88

0.67–1.15

0.351

Season of birth ^d

Wet

17294

53.3

641

59.2

1 (reference)

Dry

15142

46.7

442

40.8

0.79

0.67–0.93

0.006

Birth order

1

14395

44.4

393

36.3

1 (reference)

2

12461

38.4

450

41.6

1.32

1.09–1.60

0.005

3+

5580

17.2

240

12.2

1.58

1.25–1.98

<0.001

Mother age

<20

4810

15.0

80

7.5

1 (reference)

20–24

10087

31.4

249

23.2

1.48

1.04–2.11

0.024

25–29

7470

23.3

265

24.7

2.12

1.48–3.04

<0.001

30–34

5770

18.0

282

26.3

2.94

2.06–4.19

<0.001

≧35

3978

12.4

196

18.3

2.96

2.04–4.29

<0.001

No

5600

17.3

162

15.0

1 (reference)

Religious

17201

53.0

633

58.5

1.28

1.00–1.64

0.052

Basic

2665

8.2

67

6.2

0.87

0.58–1.31

0.517

Senior

679

2.1

20

1.9

1.02

0.50–2.11

0.948

Unknown

6291

19.4

201

18.6

1.11

0.82–1.50

0.489

≧2 years

29110

90.2

981

91.6

1 (reference)

<2 years

3181

9.9

90

8.4

0.84

0.67–1.05

0.117

Yes

23723

86.5

793

85.6

1 (reference)

No

3700

13.5

133

14.4

1.08

0.82–1.41

0.603

SES index ^g

1^st/Poorest

5,150

20.3

159

18.6

1 (reference)

2nd

6,797

26.8

221

25.8

1.05

0.77–1.43

0.742

3rd

3,250

12.8

127

14.8

1.27

0.90–1.77

0.180

4th

5,129

20.2

195

22.8

1.23

0.90–1.67

0.194

5^th/wealthy

5,061

19.9

154

18.0

0.99

0.71–1.36

0.930