We examined two birth outcomes: low birth weight and preterm births between immigrant and native Taiwanese mothers by socioeconomic status using nationally representative data of a 2005 birth cohort. By assessing the interaction between foreign-born status and family income, we were able to examine the role of foreign-born status at different family income levels and whether an income gradient exists in low birth weight and preterm births among native Taiwanese and foreign-born mothers. Our results show that foreign-born status plays a protective role against low birth weight and preterm birth among mothers with lower family incomes, whereas this protective role was not observed among mothers with high family income (Table
3). The influence of family income on low birth weight varies among foreign-born mothers and native Taiwanese mothers (Table
4). The gradient between income and low birth weight exists among native Taiwanese mothers; that is, the probability of experiencing low birth weight is higher for mothers with low incomes, whereas no association exists between income and low birth weight among foreign-born mothers. In addition, no association exists between family income and preterm birth among either native Taiwanese or foreign-born mothers. Our findings support the literature on “epidemiological paradox” showing that the association between socioeconomic status and birth outcomes was weaker among foreign-born mothers
All foreign spouses are eligible to apply for the National Health Insurance (NHI) 4 months after entering Taiwan. Taiwan's NHI covers at least 10 free prenatal visits (more if necessary), regardless of nationality. As part of Taiwan’s maternal and child care systems, a schedule of prenatal visits and examinations is recommended in multi-language maternal-child handbooks/passports provided to all pregnant women during their first prenatal visit. Although the available prenatal care resources are the same as those offered to native Taiwanese mothers, Chinese and Southeast Asian mothers were less likely to initiate prenatal care during the first 3 months of pregnancy, and also had fewer prenatal visits. In the context of the negative relationship between the use of prenatal care and birth outcomes, adverse birth outcomes were not observed among native Chinese or Southeast Asian mothers of different family income levels (Table
3). The question of why this is so and the significance of prenatal care remains unknown.
A second possible explanation is that foreign-born mothers had healthier lifestyles during pregnancy. Cigarette smoking is known to have adverse effects on pregnancy outcomes
, and our study showed that foreign-born mothers were less likely to smoke during pregnancy; therefore, they were less likely to have pregnancy complications (Table
1). This suggests that differences in culture and behavior during pregnancy may exist between the two maternal groups.
A third possible explanation may involve the psychosocial characteristics of women born in other countries. Evidence has indicated that social support may be associated with favorable birth outcomes
. As the numbers of cross-border marriages and foreign laborers increase, it may not be too difficult for a foreign-born mother in Taiwan to form friendships with other immigrants from the same country to gain the same social support that native Taiwanese mothers receive. Conversely, perceived discrimination and other social barriers may cause stress, which has been found to be a risk factor for adverse birth outcomes
. Health status may deteriorate over time as a response to cumulative discrimination. This effect of growing social inequality on women’s health may later affect fetal health
. Foreign-born mothers may experience discrimination and stress prior to and during pregnancy, but exposure may be insufficient to result in adverse birth outcomes because of their short stay in Taiwan (the mean length of stay was 3.3 years and 3.5 years for China-born mothers and South Asia-born mothers, respectively). However, additional studies are required to clarify this assumption.
Finally, as consistent with previous studies in the United States and Europe, our results indicate the existence of a healthy migrant effect, indicating that healthier people are more likely to migrate
[23, 28]. Foreign-born status was significantly associated with lower odds of low birth weight and preterm health in income groups of less than NT$70,000 (Table
3). The results for favorable birth outcomes suggest that foreign-born mothers may have undergone selection processes before they entered Taiwan. According to the immigration regulations of the Ministry of Foreign Affairs in Taiwan, marriage immigrants from China and Southeast Asia must pass a physical examination prior to entering Taiwan. Therefore, the health examination may act as a favorable health selection for foreign-born mothers in Taiwan. This selection may also cancel out the income gradient of low birth weight among foreign-born mothers, leading to the observation of no association between family income and low birth weight among foreign-born mothers.
The strength of this study is described as follows: (1) This study was based on data from the Taiwan Birth Cohort Study, the first and the largest longitudinal birth cohort in Taiwan. (2) This study addressed comprehensive explanatory factors of birth outcomes that may not have been available to previous studies, including birth order, maternal age, family income, maternal education, place of residence, smoking during pregnancy, pregnancy complications, and prenatal care use among infants of foreign-born mothers. (3) Future studies could benefit from following up the birth cohort to provide a clearer understanding of the epidemiological paradox of the salutary birth outcomes observed among infants of foreign-born mothers in Taiwan.
A chi-square goodness-of-fit test used to determine TBCS sample representativeness showed that the TBCS sample was consistent with the population. The distribution of infant sex (x
2 = 0.74, p = 0.3896), multiple births (x
2 = 0.37, p = 0.8311), birth weight (x
2 = 1.79, p = 0.4086), and gestational age (x
2 = 1.20, p = 0.5488) were not significantly different between the TBSC sample and the population, except for birthing setting (x
2 = 14.29, p = 0.0007). This may be due to the large sample size and additional hospital births in the TBCS sample (68.43% in sample, and 67.30% in population). Therefore, we conclude that the results in the present study may be generalized to the population.
This study was limited by the data being collected by using a self-report procedure. The self-report results may be subject to mistakes, exaggeration, or underreporting resulting from recall or social desirability bias. Using LMP to determine gestational age may also lead to miscalculation resulting from recall bias; however, adjusting gestational age by using obstetric ultrasonography should minimize the bias. The rates of maternal complications in the TBCS are higher than the rates of the same complications in the Taiwan Birth Reporting Database. This difference may be due to recall bias in the TBCS, or to underreporting in the Taiwan Birth Reporting Database. Higher rates of maternal complications in clinical reports
[29, 30] suggest the underreporting in the Taiwan Birth Database may be the reason. Data on syphilis, a predisposing maternal factor for adverse birth outcomes, were not collected in the TBCS. In previous studies
[19, 31], foreign-born mothers in Taiwan (including China-born and Southeast Asia-born) were more likely to have had syphilis than native Taiwanese mothers. Although syphilis had an effect on birth weight
, it was statistically insignificant on preterm birth in Taiwan