Background
Methods
Data sources
Study population
Outcome measures
Main exposures
Covariates
-
year of infant’s delivery
-
period of births, categorized as ‘1990-1998’ and ‘1999-2009’
-
maternal age at the time of delivery, in years
-
parity, defined as the number of previous deliveries
-
the size of the labor unit, as proxy for urban/rural residence
-
maternal smoking status before/ during pregnancy
-
age at migration, categorized as quartiles
-
period of arrival, categorized as ‘<1993’ or ‘≥1993’
-
gestational or pregestational diabetes
-
hypertensive disorders, including pregnancy-induced hypertension, preeclampsia and eclampsia
-
placenta previa
-
placental abruption
-
major birth defects
-
small- for-gestational age (SGA) and large-for-gestational age (LGA) infants (5th and 95th percentiles, respectively, of the majority standard) as proxies for abnormal fetal growth [26].
Statistical methods
Non-immigrants | Immigrants | Immigrants by the length of residence (years) | |||||
---|---|---|---|---|---|---|---|
Births (%) | 868 832 (95.5%) | 40 709 (4.5%) | 20 074 (49.3%) | 11 077 (27.2%) | 4926 (12.1%) | 4632 (11.4%) | |
0-4 | 5-9 | 10-14 | ≥15 | ||||
% | % | % | % | % | % | Pa
| |
Country of origin | <0.001 | ||||||
Iraq | 14.4 | 20.0 | 12.9 | 6.9 | 2.1 | ||
Pakistan | 24.8 | 19.0 | 22.1 | 31.2 | 49.5 | ||
Philippines | 12.5 | 14.6 | 11.6 | 10.4 | 7.2 | ||
Somalia | 19.9 | 22.3 | 24.5 | 14.4 | 4.2 | ||
Sri Lanka | 12.9 | 12.7 | 15.7 | 13.7 | 5.9 | ||
Vietnam | 15.6 | 11.4 | 13.2 | 23.3 | 31.0 | ||
Period of birth | <0.001 | ||||||
1990-1998 | 46.1 | 29.5* | 31.0 | 31.8 | 30.0 | 17.4 | |
1999-2009 | 53.9 | 70.5 | 69.0 | 68.2 | 70.0 | 82.6 | |
Age at migration | <0.001 | ||||||
<20 years | 24.5 | 7.7 | 20.9 | 46.6 | 86.5 | ||
≥20 years | 75.1 | 92.3 | 79.1 | 53.4 | 13.5 | ||
Period of arrival | <0.001 | ||||||
Before 1993 | 37.4 | 16.6 | 36.0 | 69.4 | 97.1 | ||
1993 onwards | 62.6 | 83.4 | 64.0 | 30.6 | 2.9 | ||
Maternal age (y) | <0.001 | ||||||
<20 | 2.7 | 2.2* | 2.8 | 1.6 | 1.9 | 1.0 | |
20-24 | 17.3 | 21.1 | 28.9 | 12.9 | 12.9 | 15.9 | |
25-29 | 35.2 | 34.5 | 37.4 | 35.0 | 23.9 | 31.7 | |
30-34 | 30.6 | 27.4 | 21.8 | 33.7 | 34.8 | 28.4 | |
35-39 | 12.2 | 12.1 | 7.6 | 14.2 | 21.6 | 17.0 | |
≥40 | 1.9 | 2.7 | 1.5 | 2.5 | 5.0 | 6.0 | |
Parity | <0.001 | ||||||
0 | 41.2 | 34.1* | 46.6 | 18.7 | 21.9 | 29.9 | |
1 | 36.0 | 29.4 | 30.0 | 30.7 | 24.1 | 28.8 | |
≥2 | 22.8 | 36.5 | 23.3 | 50.6 | 54.0 | 41.3 | |
Education level | <0.001 | ||||||
<12 years | 56.3 | 56.1* | 44.5 | 62.8 | 74.5 | 70.9 | |
≥12 years | 43.6 | 12.0 | 8.7 | 12.1 | 14.9 | 23.2 | |
missing | 0.1 | 31.9 | 46.8 | 25.1 | 10.6 | 5.9 | |
Size of labor unit | <0.001 | ||||||
<1500 | 37.6 | 15.6* | 21.4 | 12.7 | 8.6 | 5.0 | |
1500-2999 | 28.1 | 30.8 | 31.2 | 30.2 | 32.0 | 29.5 | |
≥3000 | 34.3 | 53.6 | 47.4 | 57.1 | 59.4 | 65.5 |
Non-immigrants | Immigrants | Immigrants by the length of residence (years) | |||||
---|---|---|---|---|---|---|---|
Births (%) | 868 832 (95.5%) | 40 709 (4.5%) | 20 074 (49.3%) | 11 077 (27.2%) | 4926 (12.1%) | 4632 (11.4%) | |
0-4 | 5-9 | 10-14 | ≥15 | ||||
% | % | % | % | % | % | Pa
| |
Term gestational days, mean (SD)b
| 282 (9.0) | 280 (9.4) | 280 (9.4) | 280 (9.3) | 279 (9.1) | 278 (9.0) | <0.001 |
Gestational diabetes | 0.7 | 2.6* | 1.7 | 3.1 | 3.3 | 4.3 | <0.001 |
Pregest. diabetes | 0.5 | 1.1* | 0.6 | 1.2 | 1.9 | 2.2 | <0.001 |
Hypertensive disorders | 5.2 | 3.6* | 3.3 | 3.5 | 4.5 | 4.2 | <0.001 |
Abruptio placentae | 0.5 | 0.5 | 0.5 | 0.4 | 0.5 | 0.5 | 0.829 |
Placenta previa | 0.2 | 0.3* | 0.3 | 0.4 | 0.3 | 0.5 | 0.235 |
SGAc
| 4.2 | 9.2* | 9.7 | 8.1 | 8.0 | 10.3 | <0.001 |
LGAd
| 6.3 | 3.0* | 2.5 | 3.5 | 4.2 | 2.8 | <0.001 |
Major birth defects | 2.4 | 2.4 | 2.4 | 2.5 | 2.3 | 2.6 | 0.714 |
Smokinge
| <0.001 | ||||||
Yes | 18.9 | 1.6* | 1.2 | 1.2 | 2.4 | 3.1 | |
No | 61.6 | 67.0 | 68.5 | 66.2 | 65.9 | 63.9 | |
Not disclosed | 19.5 | 31.4 | 30.3 | 32.6 | 31.7 | 33.0 |
Sensitivity analyses
Results
Demographic and health related characteristics
Non-immigrants | Immigrants | ||||||
---|---|---|---|---|---|---|---|
n = 868 832 | Pakistan n = 10 096 | Somalia n = 8094 | Vietnam n = 6336 | Iraq n = 5879 | Sri Lanka n = 5235 | Philippines n = 5069 | |
% | % | % | % | % | % | % | |
Period of birth | |||||||
1990-1998 | 46.1 | 40.4* | 16.8* | 38.8* | 8.7* | 37.7* | 32.3* |
1999-2009 | 53.9 | 59.6 | 83.2 | 61.2 | 91.3 | 62.3 | 67.7 |
Age at migration | |||||||
<20 years | 38.5 | 17.0 | 40.4 | 16.3 | 15.4 | 11.1 | |
≥20 years | 61.5 | 83.0 | 59.6 | 83.7 | 84.6 | 88.9 | |
Period of arrival | |||||||
Before 1993 | 56.3 | 17.1 | 62.5 | 6.9 | 37.6 | 36.4 | |
1993 onwards | 43.7 | 82.9 | 37.5 | 93.1 | 62.4 | 63.6 | |
Maternal age (y) | |||||||
<20 | 2.7 | 2.1* | 3.1* | 1.8* | 3.6* | 1.0* | 0.9* |
20-24 | 17.3 | 27.8 | 20.6 | 20.6 | 22.7 | 15.5 | 13.3 |
25-29 | 35.2 | 35.4 | 33.6 | 35.9 | 32.5 | 37.1 | 31.8 |
30-34 | 30.6 | 23.2 | 26.5 | 27.1 | 26.9 | 32.0 | 33.0 |
35-39 | 12.2 | 9.2 | 12.8 | 11.8 | 11.8 | 12.7 | 17.2 |
≥40 | 1.9 | 2.2 | 3.5 | 2.8 | 2.6 | 1.8 | 3.7 |
Parity | |||||||
0 | 41.2 | 28.8* | 23.4* | 39.8* | 35.6* | 41.0 | 45.9* |
1 | 36.0 | 26.0 | 21.8 | 33.3 | 29.2 | 37.1 | 35.5 |
≥2 | 22.8 | 45.2 | 54.9 | 26.9 | 35.1 | 21.9 | 18.6 |
Education level | |||||||
<12 years | 56.3 | 58.5 | 60.7 | 67.0 | 42.9 | 64.6 | 37.0 |
≥12 years | 43.6 | 7.3* | 3.8* | 13.8* | 10.9* | 15.1* | 30.2* |
missing | 0.1 | 34.3 | 35.4 | 19.2 | 46.2 | 20.2 | 32.7 |
Size of labor unit | |||||||
<1500 | 37.6 | 1.5* | 18.6* | 11.7* | 17.7* | 23.1* | 33.6* |
1500-2999 | 28.1 | 33.0 | 30.3 | 38.7 | 32.5 | 19.5 | 27.1 |
≥3000 | 34.3 | 65.4 | 51.2 | 49.6 | 49.8 | 57.3 | 39.3 |
Term gestational days, mean (SD)a
| 282 (9.0) | 279* (9.0) | 283* (9.3) | 277* (9.3) | 280* (8.7) | 278* (9.7) | 278* (9.0) |
Gest.diabetes | 0.7 | 3.4* | 1.6* | 1.5* | 1.7* | 5.4* | 2.1* |
Pregest. diabetes | 0.5 | 1.6* | 0.7 | 0.5 | 1.0* | 1.8* | 0.8* |
Hypertensive disorder | 5.2 | 3.9* | 4.2* | 1.8* | 2.8* | 3.7* | 5.3 |
Abruptio placenta | 0.5 | 0.4 | 0.5 | 0.5 | 0.4 | 0.6 | 0.5 |
Placenta previa | 0.2 | 0.2 | 0.2 | 0.3 | 0.3 | 0.3 | 0.9* |
SGAb
| 4.2 | 12.4* | 8.5* | 9.0* | 6.9* | 10.4* | 5.3* |
LGAc
| 6.3 | 2.5* | 3.3* | 2.3* | 3.1* | 3.1* | 4.4* |
Major birth defect | 2.4 | 2.8* | 2.7 | 1.6* | 2.7 | 2.3 | 2.0 |
Smokingd
| |||||||
Yes | 18.9 | 0.6* | 1.6* | 2.0* | 2.1* | 0.3* | 3.4* |
No | 61.6 | 62.1 | 62.9 | 71.0 | 69.6 | 69.2 | 72.5 |
Not disclosed | 19.5 | 37.3 | 35.4 | 27.0 | 28.3 | 30.5 | 24.1 |
Overall PTD rates
PTD | Spontaneous PTDa
| Non-spontaneous PTDb
| ||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Births | Cases | By the length of residence (years) | Cases | By the length of residence (years) | Cases | By the length of residence (years) | ||||||||||||||||
0-4 | 5-9 | 10-14 | ≥15 | Pc
| 0-4 | 5-9 | 10-14 | ≥15 | Pc
| 0-4 | 5-9 | 10-14 | ≥15 | Pc
| ||||||||
Population group
|
n
|
n
|
%
|
%
|
%
|
%
|
%
|
n
|
%
|
%
|
%
|
%
|
%
|
n
|
%
|
%
|
%
|
%
|
%
| |||
Non-immigrants | 868 832 | 45 438 |
5.2
| 31 492 |
3.6
| 13 946 |
1.6
| |||||||||||||||
All immigrants | 40 709 | 2753 |
6.8
| 6.4 | 6.4 | 7.3 | 8.8 | <0.001 | 1935 |
4.8
| 4.8 | 4.4 | 4.6 | 5.7 | 0.089 | 818 |
2.0
| 1.6 | 2.0 | 2.7 | 3.1 | <0.001 |
Immigrants by country of birth: | ||||||||||||||||||||||
Iraq | 5879 | 308 |
5.2
| 5.4 | 4.6 | 6.7 | 3.1 | 0.691 | 221 |
3.8
| 4.0 | 3.0 | 4.4 | 2.1 | 0.259 | 87 |
1.5
| 1.4 | 1.6 | 2.3 | 1.0 | 0.296 |
Pakistan | 10 096 | 774 |
7.7
| 7.0 | 7.2 | 7.7 | 9.3 | 0.001 | 493 |
4.9
| 4.9 | 4.5 | 4.5 | 5.6 | 0.256 | 281 |
2.8
| 2.1 | 2.7 | 3.2 | 3.7 | <0.001 |
Phillippines | 5069 | 404 |
8.0
| 6.9 | 8.6 | 9.7 | 11.9 | <0.001 | 297 |
5.9
| 5.3 | 6.4 | 7.2 | 6.6 | 0.066 | 107 |
2.1
| 1.6 | 2.2 | 2.5 | 5.4 | <0.001 |
Somalia | 8094 | 407 |
5.0
| 5.0 | 4.4 | 5.3 | 12.8 | 0.026 | 252 |
3.1
| 3.4 | 2.4 | 2.7 | 7.7 | 0.857 | 155 |
1.9
| 1.6 | 2.0 | 2.7 | 5.1 | <0.001 |
Sri Lanka | 5235 | 386 |
7.4
| 6.1 | 7.9 | 9.6 | 10.5 | <0.001 | 273 |
5.2
| 4.5 | 5.5 | 5.8 | 8.4 | 0.006 | 113 |
2.2
| 1.5 | 2.4 | 3.8 | 2.2 | 0.003 |
Vietnam | 6336 | 474 |
7.5
| 9.3 | 6.8 | 5.7 | 6.8 | 0.001 | 399 |
6.3
| 8.2 | 6.1 | 4.3 | 5.0 | <0.001 | 75 |
1.2
| 1.0 | 0.7 | 1.4 | 1.7 | <0.001 |
Spontaneous PTD
PTD | Spontaneous PTDa
| Non-spontaneous PTDb
| ||||
---|---|---|---|---|---|---|
Adjusted demographic factorsc
| Adjusted demographiccand health factorsd
| Adjusted demographic factorsc
| Adjusted demographiccand health factorsd
| Adjusted demographic factorsc
| Adjusted demographiccand health factorsd
| |
aRR [95% CI] | aRR [95% CI] | aRR [95% CI] | aRR [95% CI] | aRR [95% CI] | aRR [95% CI] | |
Length of residence (years) | ||||||
0-4 (Reference) | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 |
5-9 | 0.97 [0.88-1.06] | 0.93 [0.85-1.02] | 0.90 [0.80-1.01] | 0.89 [0.80-1.00] | 1.15 [0.96-1.38] | 1.05 [0.88-1.25] |
10-14 | 1.00 [0.88-1.14] | 0.93 [0.83-1.05] | 0.86 [0.74-1.00] | 0.83 [0.72-0.97] |
1.43 [1.15-1.78] | 1.17 [0.94-1.67] |
≥15 |
1.22 [1.08-1.38] | 1.10 [0.98-1.24] | 1.07 [0.92-1.24] | 1.03 [0.90-1.20] |
1.68 [1.34-2.10] |
1.28 [1.02-1.61] |
P trend | 0.012 | 0.391 | 0.883 | 0.516 | <0.001 | 0.018 |
Country of birth | ||||||
Iraq (Reference) | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 |
Pakistan |
1.29 [1.12-1.48] |
1.26 [1.10-1.45] | 1.16 [0.99-1.37] |
1.20 [1.02-1.42] |
1.57 [1.21-2.04] |
1.37 [1.06-1.76] |
Phillippines |
1.41 [1.22-1.64] |
1.32 [1.14-1.53] |
1.48 [1.24-1.76] |
1.43 [1.19-1.70] | 1.25 [0.94-1.68] | 1.08 [0.81-1.44] |
Somalia | 0.95 [0.82-1.10] | 0.89 [0.77-1.03] |
0.82 [0.69-0.98] |
0.82 [0.68-0.98] | 1.26 [0.97-1.64] | 1.07 [0.83-1.38] |
Sri Lanka |
1.31 [1.13-1.52] |
1.29 [1.11-1.50] |
1.32 [1.10-1.58] |
1.34 [1.11-1.60] | 1.27 [0.95-1.69] | 1.20 [0.90-1.59] |
Vietnam |
1.26 [1.09-1.46] |
1.38 [1.19-1.59] |
1.53 [1.28-1.81] |
1.60 [1.35-1.89] |
0.64 [0.47-0.89] | 0.81 [0.59-1.12] |