Table 1 Maternal, fetal and delivery characteristics, among nulliparous women with singleton term vaginal births

 0–< 1 h

16972

903

5.3

<.0001

 1–< 2 h

15238

1114

7.3

 2–< 3 h

9424

849

9.0

 3–< 4 h

6217

654

10.5

  ≥ 4 h

4360

530

12.2

Maternal age

<.0001

  ≤ 24

8846

460

5.2

 25–29

17176

1273

7.4

 30–34

18632

1650

8.8

  ≥ 35

7523

665

8.8

 Missing

34

2

-

BMI (kg/m²)

0.014

  < 19.9

7102

550

7.7

 19.9–24.9

30021

2271

7.6

 25.0–29.9

9099

783

8.6

  ≥ 30

3021

241

7.9

 Missing

2968

205

-

Maternal height (cm)

<.0001

 130–154

1321

127

9.6

 155–159

4811

440

9.1

 160–164

12401

977

7.9

 165–169

14832

1127

7.6

 170–200

18309

1343

7.4

 Missing

537

36

-

Cohabiting

0.9

 Yes

47336

3664

7.7

 No

4385

337

7.7

 Missing

490

49

Daily smoking

<.0001

 Non-smoker

49543

3921

7.9

 Smoker

2259

106

4.7

 Missing

409

23

-

Gestational age (weeks)

<.0001

 37

2284

104

4.5

 38

5375

297

5.5

 39

11956

716

6.0

 40

16775

1351

8.1

 41

11702

1124

9.6

 42

4109

448

11.1

Induction of labor

<.0001

 Yes

8828

775

8.8

 No

43383

3275

7.5

Epidural analgesia

< .0001

 Yes

32756

2663

8.1

 No

19455

1387

7.1

Oxytocin augmentation

< .0001

 Yes

31095

2672

8.6

 No

21116

1378

6.5

Mode of delivery

< .0001

 Non-Instrumental

42261

2535

6.0

 Instrumental (vacuum extraction or forceps)

9950

1515

15.2

Episiotomy

< .0001

 Yes

4735

522

11.1

 No

47476

3528

7.4

Occiput posterior position

< .0001

 Yes

1658

229

13.8

 No

50553

3821

7.6

Birthweight more than 4500 g

< .0001

 Yes

767

161

20.9

 No

51412

3888

7.6

 Missing

32

1

-

Head circumference more than 35.5 cm

< .0001

 Yes

14613

1661

11.4

 No

37376

2366

6.3

 Missing

222

23

-