Table 1.
Descriptive characteristics of participants
| Characteristic | No. (%) | |||
|---|---|---|---|---|
| Total | Normal birth weight | Low birth weight | Term low birth weight | |
| Subjects | 4,021,741 | 3,758,013 (93.4) | 263,728 (6.6) | 121,852 (3.0) |
| Birth weight (grams, mean ± SD) | 3176.2 ± 503.2 | 3257.0 ± 394.0 | 2024.8 ± 479.2 | 2196.7 ± 407.9 |
| Gestational age (weeks, mean ± SD) | 38.5 ± 1.7 | 38.7 ± 1.2 | 35.4 ± 3.5 | 38.3 ± 1.0 |
| Infant sex | ||||
| Male | 2,074,664 (51.6) | 1,951,484 (51.9) | 123,180 (46.7) | 51,039 (41.9) |
| Female | 1,947,077 (48.4) | 1,806,529 (48.1) | 140,548 (53.3) | 70,813 (58.1) |
| Season of birth | ||||
| Spring | 976,828 (24.3) | 909,836 (24.2) | 66,992 (25.4) | 30,936 (25.4) |
| Summer | 1,140,987 (28.4) | 1,066,848 (28.4) | 74,139 (28.1) | 34,309 (28.2) |
| Autumn | 980,745 (24.4) | 917,468 (24.4) | 63,277 (24.0) | 28,891 (23.7) |
| Winter | 923,181 (23.0) | 863,861 (23.0) | 59,320 (22.5) | 27,716 (22.7) |
| Number of fetuses | ||||
| Singleton | 3,976,387 (98.9) | 3,738,825 (99.5) | 237,562 (90.1) | 115,228 (94.6) |
| Multiple gestations | 45,354 (1.1) | 19,188 (0.5) | 26,166 (9.9) | 6624 (5.4) |
| Maternal age, yrs | ||||
| ≤25 | 1,453,336 (36.1) | 1,358,339 (36.1) | 94,997 (36.0) | 46,289 (38.0) |
| (25, 35] | 2,133,831 (53.1) | 1,999,070 (53.2) | 134,761 (51.1) | 61,375 (50.4) |
| >35 | 434,574 (10.8) | 400,604 (10.7) | 33,970 (12.9) | 14,188 (11.6) |
| Parity | ||||
| 0 | 1,670,615 (41.5) | 1,543,838 (41.1) | 126,777 (48.1) | 59,097 (48.5) |
| ≥1 | 2,351,126 (58.5) | 2,214,175 (58.9) | 136,951 (51.9) | 62,755 (51.5) |
| Residence | ||||
| City | 3,097,265 (77.0) | 2,897,059 (77.1) | 200,206 (75.9) | 90,849 (74.6) |
| Village | 924,476 (23.0) | 860,954 (22.9) | 63,522 (24.1) | 31,003 (25.4) |
| Education attainment | ||||
| Below high school | 1,927,541 (47.9) | 1,791,513 (47.7) | 136,028 (51.6) | 63,459 (52.1) |
| High school | 1,206,248 (30.0) | 1,133,219 (30.2) | 73,029 (27.7) | 34,050 (27.9) |
| College or above | 887,952 (22.1) | 833,281 (22.2) | 54,671 (20.7) | 24,343 (20.0) |
| Nationality | ||||
| Iranian | 3,873,207 (96.3) | 3,619,299 (96.3) | 253,908 (96.3) | 117,106 (96.1) |
| Non-Iranian | 148,534 (3.7) | 138,714 (3.7) | 9820 (3.7) | 4746 (3.9) |
| Delivery type | ||||
| Cesarean section | 2,059,655 (51.2) | 1,903,604 (50.7) | 156,051 (59.2) | 66,019 (54.2) |
| Vaginal delivery | 1,962,086 (48.8) | 1,854,409 (49.3) | 107,677 (40.8) | 55,833 (45.8) |
| Delivery complications | ||||
| Absent | 3,914,965 (97.3) | 3,657,444 (97.3) | 257,521 (97.6) | 119,018 (97.7) |
| Present | 106,776 (2.7) | 100,569 (2.7) | 6207 (2.4) | 2834 (2.3) |
| Diabetes | ||||
| Absent | 3,919,203 (97.5) | 3,663,529 (97.5) | 255,674 (96.9) | 119,238 (97.9) |
| Present | 102,538 (2.5) | 94,484 (2.5) | 8054 (3.1) | 2614 (2.1) |
| Chronic hypertension | ||||
| Absent | 3,959,776 (98.5) | 3,704,473 (98.6) | 255,303 (96.8) | 119,196 (97.8) |
| Present | 61,965 (1.5) | 53,540 (1.4) | 8425 (3.2) | 2656 (2.2) |
| Environmental exposures | ||||
| NDVI−500m | 0.313 ± 0.21 | 0.315 ± 0.21 | 0.287 ± 0.21 | 0.276 ± 0.20 |
| NDVI−1000m | 0.314 ± 0.20 | 0.316 ± 0.20 | 0.288 ± 0.20 | 0.276 ± 0.20 |
| NDVI−2000m | 0.315 ± 0.20 | 0.317 ± 0.20 | 0.290 ± 0.19 | 0.278 ± 0.19 |
| NDVI−3000m | 0.317 ± 0.19 | 0.319 ± 0.19 | 0.292 ± 0.19 | 0.280 ± 0.19 |
| EVI−500m | 0.225 ± 0.16 | 0.227 ± 0.16 | 0.207 ± 0.15 | 0.200 ± 0.15 |
| EVI−1000m | 0.226 ± 0.15 | 0.227 ± 0.15 | 0.207 ± 0.15 | 0.199 ± 0.15 |
| EVI−2000m | 0.227 ± 0.15 | 0.228 ± 0.15 | 0.209 ± 0.14 | 0.201 ± 0.14 |
| EVI−3000m | 0.229 ± 0.14 | 0.230 ± 0.14 | 0.210 ± 0.14 | 0.202 ± 0.14 |
| PM2.5, μg/m3 | 41.1 ± 12.6 | 41.1 ± 12.6 | 41.9 ± 12.9 | 42.2 ± 12.4 |
| Temperature, °C | 16.7 ± 5.8 | 16.7 ± 5.7 | 17.0 ± 6.3 | 17.3 ± 6.0 |
| Relative humidity, % | 43.5 ± 13.3 | 43.5 ± 13.2 | 42.4 ± 13.7 | 41.6 ± 13.1 |
Delivery complications refer to a range of medical issues and problems that can occur during the process of delivery, including one or more injuries of the parturient canal, placenta abruption, premature rupture of membranes, shoulder dystocia, uterine rupture at delivery, etc. Diabetes is defined as type 1 or type 2 diabetes diagnosed prior to conception (without gestational diabetes). Chronic hypertension is defined as pre-pregnancy hypertension (≥140/90 mmHg) or hypertension before 20 weeks of pregnancy. The sum of percentages from multiple subgroups may not equal 100% exactly due to rounding-off numbers.
SD standard deviation, NDVI normalized difference vegetation index, EVI enhanced vegetation index, PM2.5 fine particulate matter.