Table 2.
Frequency of babies born preterm or small‐for‐gestational age and association with influenza vaccination during pregnancy, by method to estimate gestational age and growth reference, among pregnant women in Lao PDR—April 2014‐February 2015
| Method for estimating gestational age | Live birthsb | Frequency of outcome, n (%) | Adjusted RR (95% CI)a | ||||
|---|---|---|---|---|---|---|---|
| All | Unvaccinated | Vaccinated | Overall | High influenza activity | Low influenza activity | ||
| Preterm birth | |||||||
| Last menstrual period | 3691 | 382 (10) | 247 (13) | 135 (8) | 0.71 (0.58‐0.88) | 0.69 (0.55‐0.86) | 0.89 (0.54‐1.47) |
| Ultrasound anytime during pregnancy | 3306 | 487 (15) | 279 (18) | 208 (12) | 0.69 (0.58‐0.83) | 0.67 (0.56‐0.81) | 1.03 (0.66‐1.61) |
| Ultrasound before 14 wk’ gestation | 1363 | 195 (14) | 81 (16) | 114 (13) | 0.86 (0.66‐1.12) | 0.79 (0.59‐1.05) | —c |
| Ballard Score at delivery | 4355 | 237 (5) | 147 (6) | 90 (4) | 0.89 (0.67‐1.17) | 0.87 (0.65‐1.18) | 0.99 (0.48‐2.03) |
| Algorithmd | 4164 | 585 (14) | 354 (16) | 231 (12) | 0.78 (0.67‐0.92) | 0.73 (0.61‐0.87) | 1.16 (0.77‐1.74) |
| Small‐for‐gestational age | |||||||
| Canadian growth reference | |||||||
| Last menstrual period | 3682 | 885 (24) | 400 (21) | 485 (27) | 1.25 (1.11‐1.40) | 1.22 (1.07‐1.39) | 1.35 (1.03‐1.78) |
| Ultrasound anytime during pregnancy | 3304 | 607 (18) | 257 (16) | 350 (20) | 1.22 (1.05‐1.41) | 1.18 (1.01‐1.39) | 1.42 (0.95‐2.12) |
| Ultrasound before 14 wk’ gestation | 1363 | 247 (18) | 81 (16) | 166 (19) | 1.14 (0.89‐1.45) | 1.15 (0.88‐1.49) | 1.22 (0.58‐2.59) |
| Ballard Score at delivery | 4353 | 894 (21) | 453 (19) | 441 (22) | 1.15 (1.01‐1.30) | 1.13 (0.99‐1.29) | 1.40 (0.93‐2.13) |
| Algorithmd | 4162 | 805 (19) | 387 (18) | 418 (21) | 1.17 (1.03‐1.33) | 1.17 (1.02‐1.35) | 1.12 (0.83‐1.52) |
| Intergrowth‐21st growth standard | |||||||
| Last menstrual period | 3363 | 488 (15) | 249 (14) | 239 (15) | 1.03 (0.87‐1.22) | 0.99 (0.83‐1.20) | 1.28 (0.84‐1.96) |
| Ultrasound anytime during pregnancy | 3272 | 382 (12) | 178 (11) | 204 (12) | 1.03 (0.84‐1.25) | 1.02 (0.82‐1.26) | 1.04 (0.64‐1.70) |
| Ultrasound before 14 wk’ gestation | 1346 | 148 (11) | 47 (10) | 101 (12) | 1.22 (0.88‐1.71) | 1.24 (0.87‐1.79) | 1.51 (0.59‐3.90) |
| Ballard Score at delivery | 4356 | 416 (10) | 223 (9) | 193 (10) | 1.01 (0.83‐1.23) | 1.02 (0.83‐1.26) | 1.04 (0.55‐1.97) |
| Algorithmd | 4139 | 516 (12) | 289 (13) | 227 (11) | 0.90 (0.76‐1.06) | 0.90 (0.75‐1.09) | 0.87 (0.57‐1.31) |
| Equation‐based growth reference | |||||||
| Last menstrual period | 3576 | 275 (8) | 131 (7) | 144 (8) | 1.14 (0.90‐1.44) | 1.04 (0.80‐1.36) | 1.71 (1.00‐2.95) |
| Ultrasound anytime during pregnancy | 3212 | 167 (5) | 73 (5) | 94 (6) | 1.14 (0.84‐1.55) | 1.13 (0.80‐1.58) | —c |
| Ultrasound before 14 wk’ gestation | 1326 | 64 (5) | 19 (4) | 45 (5) | 1.21 (0.72‐2.04) | 1.24 (0.69‐2.22) | —c |
| Ballard Score at delivery | 4324 | 226 (5) | 124 (5) | 102 (5) | 0.98 (0.74‐1.29) | 0.96 (0.71‐1.28) | 1.22 (0.51‐2.94) |
| Algorithmd | 4060 | 237 (6) | 120 (6) | 117 (6) | 1.03 (0.80‐1.34) | 1.04 (0.78‐1.38) | 1.13 (0.64‐2.03) |
Adjusted relative risk (RR) and 95% confidence intervals (CI) estimated from a log‐binomial model, with robust standard errors. Model was adjusted for the mother's education, parity, age, province, and ethnicity, an indicator for ≥4 ANC visits, household income, number of household members, and distance of the mother's home to hospital. The overall model was additionally adjusted for influenza activity in the country at the time of delivery.
Live births among women who were vaccinated at least 2 wk prior to term and women who were unvaccinated.
The log‐binomial model failed to converge.
The algorithm favored a first ultrasound during first trimester, followed by the last menstrual period recalled during 1st trimester, a first ultrasound during 2nd trimester, a first ultrasound during 3rd trimester, and last menstrual period recalled during the 2nd or 3rd trimester.