Table 1.
Characteristics of the study population by average nitrate concentration (mg/L ) in home drinking water during pregnancy.
| Estimated mean nitrate (mg/L NO3–) in household drinking water | ||||
|---|---|---|---|---|
| Characteristic | ≤2 | >2–5 | >5–25 | >25 |
| Total, n (%)a | 529,172 (52) | 318,135 (32) | 124,602 (12) | 37,280 (4) |
| Case status, n (%) | ||||
| Preterm (140–258 days) | 26,616 (51.4) | 16,547 (32.0) | 6,579 (12.7) | 2,005 (3.9) |
| Full term (259–293 days) | 502,556 (52.5) | 301,588 (31.5) | 118,023 (12.3) | 35,275 (3.7) |
| Gestational age (days), n (%) | ||||
| Q1 (≤273 days) | 163,537 (51) | 101,451 (32) | 42,122 (13) | 12,141 (4) |
| Q2 (274–280 days) | 146,194 (52) | 88,684 (32) | 35,756 (13) | 10,173 (4) |
| Q3 (281–287 days) | 147,762 (53) | 87,624 (31) | 33,070 (12) | 10,245 (4) |
| Q4 (288–293 days) | 71,679 (55) | 40,376 (31) | 13,654 (10) | 4,721 (4) |
| Sex, n (%) | ||||
| Female | 258,603 (53) | 154,775 (31) | 60,738 (12) | 17,998 (4) |
| Male | 270,569 (52) | 163,360 (32) | 63,864 (12) | 19,282 (4) |
| Birth order, n (%) | ||||
| 1 | 227,350 (52) | 146,330 (33) | 51,060 (12) | 16,066 (4) |
| 2 | 207,786 (53) | 122,971 (31) | 49,621 (13) | 14,274 (4) |
| ≥3 | 94,036 (54) | 48,834 (28) | 23,921 (14) | 6,940 (4) |
| Short interpregnancy interval, n (%) | ||||
| No | 1,144 (51) | 680 (30) | 355 (16) | 85 (4) |
| Yes | 300,678 (53) | 171,125 (30) | 73,187 (13) | 21,129 (4) |
| Missing (i.e., only child in this period) | 227,350 (52) | 146,330 (33) | 51,060 (12) | 16,066 (4) |
| Urbanicity, n (%) | ||||
| Capital | 55,352 (39) | 83,871 (58) | 4,385 (3) | 18 (0) |
| Suburb of the capital | 52,095 (41) | 58,310 (46) | 17,178 (13) | 163 (0) |
| Provincial cityb | 86,212 (68) | 10,880 (9) | 14,677 (11) | 15,944 (12) |
| Provincial townc | 150,204 (52) | 91,239 (32) | 40,736 (14) | 6,398 (2) |
| Rural areasd | 185,309 (58) | 73,835 (23) | 47,626 (15) | 14,757 (5) |
| Year of birth, n (%) | ||||
| Q1 (1991–1996) | 117,479 (47) | 77,511 (31) | 45,397 (18) | 11,078 (4) |
| Q2 (1997–2002) | 129,740 (53) | 74,448 (31) | 30,621 (13) | 8,974 (4) |
| Q3 (2003–2008) | 133,675 (54) | 79,816 (32) | 26,467 (11) | 8,831 (4) |
| Q4 (2009–2015) | 148,278 (56) | 86,360 (33) | 22,117 (8) | 8,397 (3) |
| Season of birth, n (%) | ||||
| January–March | 126,971 (52) | 77,154 (32) | 31,084 (13) | 9,189 (4) |
| April–June | 133,726 (52) | 80,246 (31) | 31,909 (13) | 9,382 (4) |
| July–September | 141,982 (52) | 85,831 (32) | 32,989 (12) | 9,859 (4) |
| October–December | 126,493 (53) | 74,904 (31) | 28,620 (12) | 8,850 (4) |
| Maternal age (years), n (%) | ||||
| <25 | 65,613 (48) | 43,750 (32) | 20,418 (15) | 5,541 (4) |
| 25–29 | 192,586 (53) | 111,068 (30) | 47,825 (13) | 15,087 (4) |
| 30–34 | 187,860 (54) | 110,196 (32) | 39,819 (11) | 11,949 (3) |
| ≥35 | 83,113 (53) | 53,121 (34) | 16,540 (11) | 4,703 (3) |
| Maternal smokinge, n (%) | ||||
| No | 426,069 (54) | 249,304 (31) | 92,204 (12) | 28,852 (4) |
| Yes | 103,103 (48) | 68,831 (32) | 32,398 (15) | 8,428 (4) |
| Maternal BMI, n (%) | ||||
| <18.5 | 10,032 (55) | 6,153 (34) | 1,543 (8) | 616 (3) |
| 18.5–24.9 | 157,553 (56) | 93,070 (33) | 23,844 (8) | 9,328 (3) |
| 25–29.9 | 52,754 (56) | 28,471 (30) | 9,861 (10) | 3,327 (4) |
| ≥30 | 32,085 (56) | 16,351 (29) | 6,491 (11) | 2,084 (4) |
| Missingf | 276,748 (50) | 174,090 (31) | 82,863 (15) | 21,925 (4) |
| Maternal educationg, n (%) | ||||
| Primary school | 111,898 (49) | 73,156 (32) | 34,718 (15) | 9,406 (4) |
| High school | 247,303 (53) | 141,880 (30) | 60,735 (13) | 17,958 (4) |
| Higher education | 169,971 (54) | 103,099 (33) | 29,149 (9) | 9,916 (3) |
| Maternal employment statusg, n (%) | ||||
| Employed | 431,525 (53) | 258,286 (32) | 98,825 (12) | 29,387 (4) |
| Unemployed | 28,828 (48) | 18,710 (31) | 9,620 (16) | 2,868 (5) |
| Not seeking work | 68,819 (52) | 41,139 (31) | 16,157 (12) | 5,025 (4) |
| Maternal incomeh, n (%) | ||||
| Q1 | 128,705 (52) | 78,541 (32) | 31,045 (13) | 9,452 (4) |
| Q2 | 132,575 (53) | 71,866 (29) | 36,213 (14) | 11,060 (4) |
| Q3 | 134,520 (53) | 75,804 (30) | 33,217 (13) | 9,761 (4) |
| Q4 | 133,372 (52) | 91,924 (36) | 24,127 (9) | 7,007 (3) |
| Missing | 128,705 (52) | 78,541 (32) | 31,045 (13) | 9,452 (4) |
| Paternal age (years), n (%) | ||||
| <25 | 32,318 (48) | 22,662 (33) | 10,196 (15) | 2,662 (4) |
| 25–29 | 142,498 (52) | 84,008 (31) | 36,884 (13) | 11,422 (4) |
| 30–34 | 198,549 (53) | 114,862 (31) | 44,513 (12) | 13,649 (4) |
| ≥35 | 155,807 (53) | 96,603 (33) | 33,009 (11) | 9,547 (3) |
| Paternal educationg, n (%) | ||||
| Primary school | 107,349 (49) | 69,249 (32) | 32,089 (15) | 8,712 (4) |
| High school | 272,371 (53) | 155,999 (30) | 67,969 (13) | 20,005 (4) |
| Higher education | 144,258 (55) | 88,909 (34) | 23,115 (9) | 8,245 (3) |
| Missing | 5,194 (48) | 3,978 (36) | 1,429 (13) | 318 (3) |
| Paternal employment statusg, n (%) | ||||
| Employed | 477,356 (53) | 282,767 (31) | 111,471 (12) | 33,211 (4) |
| Unemployed | 18,294 (46) | 13,328 (34) | 6,038 (15) | 1,712 (4) |
| Not seeking work | 31,959 (52) | 20,749 (34) | 6,780 (11) | 2,302 (4) |
| Missing | 1,563 (49) | 1,291 (40) | 313 (10) | 55 (2) |
| Paternal incomeh, n (%) | ||||
| Q1 | 126,569 (51) | 81,466 (33) | 30,459 (12) | 9,635 (4) |
| Q2 | 131,468 (52) | 73,180 (29) | 35,687 (14) | 10,738 (4) |
| Q3 | 135,287 (53) | 76,857 (30) | 32,226 (13) | 9,421 (4) |
| Q4 | 135,570 (53) | 86,422 (34) | 26,188 (10) | 7,481 (3) |
| Missing | 278 (52) | 210 (39) | 42 (8) | 5 (1) |
All X2 tests for difference between strata were significant at P < 0.001 except for sex (P = 0.04).
aThe study population: full-term singleton live births in Denmark from 1 January 1991 to 31 December 2015 to Danish-born parents where the mother resided in Denmark for the full pregnancy and who had a nitrate estimate for each day of pregnancy, and with non-missing covariates in the base model.
bMunicipalities having a town with >100,000 inhabitants.
cMunicipalities having a town with between 10,000 and 100,000 inhabitants.
dMunicipalities in Denmark where the largest town has <10,000 inhabitants.
eFor children born in the period before 1997 smoking was recorded at the first visit with the midwife with no specifications as to the timing. For children born from 1997 onward smoking is during pregnancy.
fAvailable from 2003 onward only.
gAs reported 2 years before birth.
hAs reported 2 years before birth and standardized to 2013 values.