Table 1 Characteristics of the study population by nitrate concentration in drinking water (in five categories)

Total pregnancies^a, n (%)

27,413 (29.8)

25,753 (28.0)

24,285 (26.4)

10,449 (11.4)

4,021 (4.4)

Age at conception, mean (± SD)

29.56 (4.29)

29.90 (4.25)

29.86 (4.42)

29.44 (4.36)

29.32 (4.35)

Age at conception, n (%)

  < 20

282 (1.0)

224 (0.9)

261 (1.1)

129 (1.2)

48 (1.2)

  ≥ 20- < 25

3,461 (12.6)

2,744 (10.7)

2,860 (11.8)

1,443 (13.8)

568 (14.1)

  ≥ 25- < 30

11,653 (42.5)

10,616 (41.2)

9,719 (40.0)

4,340 (41.5)

1,705 (42.4)

  ≥ 30- < 35

9,049 (33.0)

9,030 (35.1)

8,364 (34.4)

3,386 (32.4)

1,280 (31.8)

  ≥ 35- ≤ 40

2,704 (9.9)

2,848 (11.1)

2,763 (11.4)

1,056 (10.1)

381 (9.5)

  > 40

264 (1.0)

291 (1.1)

318 (1.3)

95 (0.9)

39 (1.0)

Highest education, n (%)

  Primary school

2,399 (8.8)

1,592 (6.2)

1,838 (7.6)

926 (8.9)

346 (8.6)

  High school or vocational

11,449 (41.8)

8,721 (33.9)

8,833 (36.4)

4,495 (43.0)

1,600 (39.8)

  Basic education

2,228 (8.1)

1,733 (6.7)

1,705 (7.0)

767 (7.3)

270 (6.7)

  Middle education

8,838 (32.2)

8,548 (33.2)

7,732 (31.8)

3,290 (31.5)

1,361 (33.8)

  Higher education or Ph.D

2,451 (8.9)

5,102 (19.8)

4,110 (16.9)

950 (9.1)

433 (10.8)

  Missing

48 (0.2)

57 (0.2)

67 (0.3)

21 (0.2)

11 (0.3)

Occupation, n (%)

  Unemployed^b

2,816 (10.3)

2,330 (9.0)

2,311 (9.5)

1,211 (11.6)

471 (11.7)

  Student

2,036 (7.4)

3,130 (12.2)

2,305 (9.5)

813 (7.8)

378 (9.4)

  Employee unspecified income

2,066 (7.5)

1,671 (6.5)

1,637 (6.7)

779 (7.5)

287 (7.1)

  Employee with low income

10,090 (36.8)

7,344 (28.5)

7,747 (31.9)

3,895 (37.3)

1,390 (34.6)

  Employee with middle income

6,371 (23.2)

6,149 (23.9)

5,613 (23.1)

2,255 (21.6)

887 (22.1)

  Chief executive or high income

3,026 (11.0)

4,221 (16.4)

3,770 (15.5)

1,147 (11.0)

469 (11.7)

  Owner of business

501 (1.8)

496 (1.9)

474 (2.0)

192 (1.8)

58 (1.4)

  Other

507 (1.8)

412 (1.6)

 > 422(1.7)

157 (1.5)

81 (2.0)

  Missing

0 (0.0)

0 (0.0)

 < 5 (.)

0 (0.0)

0 (0.0)

  Population density^c, mean (± SD)

424.88 (488.63)

1215.16 (1382.67)

1137.34 (1306.25)

624.18 (874.46)

592.85 (717.16)

Smoking in pregnancy (cigarettes/day), n (%)

  Non-smoker

18,937 (69.1)

17,905 (69.5)

16,364 (67.4)

7,038 (67.4)

2,781 (69.2)

  Smoked in early pregnancy

2,061 (7.5)

2,460 (9.6)

2,280 (9.4)

879 (8.4)

295 (7.3)

  1–9

2,365 (8.6)

2,027 (7.9)

2,045 (8.4)

965 (9.2)

320 (8.0)

  10–14

1,172 (4.3)

980 (3.8)

1,066 (4.4)

538 (5.1)

210 (5.2)

  > 15

737 (2.7)

576 (2.2)

712 (2.9)

349 (3.3)

129 (3.2)

  Missing

2,141 (7.8)

1,805 (7.0)

1,818 (7.5)

680 (6.5)

286 (7.1)

Weekly alcohol consumption (units/week), n (%)

  0

13,860 (50.6)

12,911 (50.1)

12,677 (52.2)

5,613 (53.7)

2,121 (52.7)

  0.5–3.5

10,866 (39.6)

10,468 (40.6)

9,276 (38.2)

3,940 (37.7)

1,539 (38.3)

  4–7

489 (1.8)

517 (2.0)

447 (1.8)

181 (1.7)

 > 65 (1.6)

  ≥ 7.5

39 (0.1)

36 (0.1)

59 (0.2)

20 (0.2)

 < 5 (.)

  Missing

2,159 (7.9)

1,821 (7.1)

1,826 (7.5)

695 (6.7)

288 (7.2)

Body mass index (kg/m2), n (%)

  < 18.5

1035 (3.8)

1,097 (4.3)

1,098 (4.5)

439 (4.2)

166 (4.1)

  18.5–25

16,262 (59.3)

16,761 (65.1)

15,371 (63.3)

6,206 (59.4)

2,350 (58.4)

  25–30

5,238 (19.1)

4,096 (15.9)

4,011 (16.5)

2,051 (19.6)

797 (19.8)

  > 30

2,360 (8.6)

1,607 (6.2)

1,613 (6.6)

930 (8.9)

369 (9.2)

  Missing

2,518 (9.2)

2,192 (8.5)

2,192 (9.0)

823 (7.9)

339 (8.4)

Gravidity, n (%)

  Primigravidae

9,604 (35.0)

9,988 (38.8)

8,971 (36.9)

3,632 (34.8)

1,517 (37.7)

  Multigravidae

 > 17,803(64.9)

15,765 (61.2)

 > 15,308 (63.0)

 > 6,811 (65.2)

2,504 (62.3)

  Missing

 < 5 (.)

0 (0.0)

 < 5 (.)

 < 5 (.)

0 (0.0)

Parity, n (%)

  Nulliparous

12,607 (46.0)

13,825 (53.7)

12,630 (52.0)

4,901 (46.9)

1,998 (49.7)

  Multiparous

 > 14,800 (54.0)

11,928 (46.3)

 > 11,649 (48.0)

 > 5,542 (53.1)

2,023 (50.3)

  Missing

 < 5 (.)

0 (0.0)

 < 5 (.)

 < 5 (.)

0 (0.0)

Previous spontaneous pregnancy loss, n (%)

  0

21,941 (80.0)

20,707 (80.4)

19,547 (80.5)

8,394 (80.3)

3,212 (79.9)

  1–2

5,283 (19.3)

4,917 (19.1)

4,564 (18.8)

1,977 (18.9)

776 (19.3)

  > 3

 > 183 (0.7)

129 (0.5)

 > 168 (0.7)

 > 72 (0.7)

33 (0.8)

  Missing

 < 5 (.)

0 (0.0)

 < 5 (.)

 < 5 (.)

0 (0.0)

Nitrosatable drug in pregnancy, n (%)

  Yes

4,321 (15.8)

3,726 (14.5)

3,795 (15.6)

1,682 (16.1)

650 (16.2)

  No

23,082 (84.2)

22,027 (85.5)

 > 20,484 (84.4)

 > 8,761 (83.9)

3,371 (83.8)

  Missing

10 (0.0)

0 (0.0)

 < 5 (.)

 < 5 (.)

0 (0.0)