Baseline characteristics of the study population
| Demographic variables | No of individuals with missing data | No of individuals | No of congenital Zika syndrome cases | Prevalence × 100 000 (95% CI) |
| Year of birth | 0 | – | – | – |
| 2015 | – | 2 913 812 (25.6%) | 1226 | 42.1 (39.8 to 44.5) |
| 2016 | – | 2 769 476 (24.4%) | 1560 | 56.3 (53.6 to 59.2) |
| 2017 | – | 2 829 285 (24.9%) | 369 | 13.0 (11.8 to 14.4) |
| 2018 | – | 2 854 113 (25.1%) | 198 | 6.9 (6.0 to 8.0) |
| Sex of the newborn | 11 (<0.1%) | – | – | – |
| Male | – | 5 818 371 (51.2%) | 1566 | 26.9 (25.6 to 28.3) |
| Female | – | 5 548 304 (48.8%) | 1779 | 32.1 (30.6 to 33.6) |
| Birth region | 3511 (<0.1%) | – | – | – |
| South | – | 1 567 773 (13.8%) | 74 | 4.7 (3.7 to 5.9) |
| Southeast | – | 4 496 224 (39.6%) | 813 | 18.1 (16.9 to 19.4) |
| North | – | 1 230 977 (10.8%) | 185 | 15.0 (12.9 to 17.4) |
| Northeast | – | 3 172 975 (27.9%) | 2010 | 63.4 (60.6 to 66.2) |
| Midwest | – | 895 226 (7.9%) | 271 | 30.3 (26.8 to 34.1) |
| Maternal ethnicity | 461 094 (4.1%) | – | – | – |
| White | – | 4 004 683 (35.2%) | 593 | 14.8 (13.6 to 16.1) |
| Black | – | 606 749 (5.3%) | 227 | 37.4 (32.7 to 42.6) |
| Mixed race/brown | – | 6 157 534 (54.2%) | 2293 | 37.2 (35.7 to 38.8) |
| Asian | – | 43 912 (0.4%) | 12 | 27.3 (14.1 to 47.7) |
| Indigenous | – | 92 714 (0.8%) | 20 | 21.6 (13.2 to 33.3) |
| Maternal education (Years) | 149 407 (1.3%) | – | – | – |
| 1–3 | – | 314 986 (2.8%) | 132 | 41.9 (35.1 to 49.7) |
| 4–7 | – | 1 904 078 (16.8%) | 762 | 40.0 (37.2 to 43.0) |
| 12–8 | – | 6 768 809 (59.5%) | 2034 | 30.1 (28.8 to 31.4) |
| >12 | – | 2 229 406 (19.6%) | 372 | 16.7 (15.0 to 18.5) |
| Maternal marital status | 111 412 (1.0%) | – | – | – |
| Married/union | – | 6 268 759 (55.2%) | 1555 | 24.8 (23.6 to 26.1) |
| Single | 4 833 219 (42.5%) | 1724 | 35.7 (34.0 to 37.4) | |
| Widow | 19 506 (0.2%) | 6 | 30.8 (11.3 to 66.9) | |
| Divorced | – | 133 790 (1.2%) | 28 | 20.9 (13.9 to 30.3) |
| No of prenatal appointments | 283 429 (2.5%) | – | – | – |
| None | – | 63 847 (0.6%) | 52 | 81.4 (60.8 to 106.8) |
| 1–3 | – | 673 788 (6.1%) | 307 | 45.6 (40.6 to 51.0) |
| 4–5 | – | 1 370 981 (12.4%) | 534 | 38.9 (35.7 to 42.4) |
| 6+ | – | 8 974 641 (80.9%) | 2328 | 25.9 (24.9 to 27.0) |
| Maternal age (years) | 139 (<0.1%) | – | – | – |
| <20 | – | 1 920 351 (16.9%) | 776 | 40.4 (37.6 to 43.4) |
| 20–34 | – | 7 850 607 (69.1%) | 2205 | 28.1 (26.9 to 29.3) |
| 35+ | – | 1 595 589 (14.0%) | 372 | 23.3 (21.0 to 25.8) |