Term Low Birth Weight or Macrosomia among Immigrant Mothers in Korea

Low birth weight (LBW) reflects intrauterine growth. It is an important determinant for perinatal morbidity and death, growth and cognitive development in childhood. It also increases the risk of morbidity later in life such as insulin resistance, obesity, dyslipidemia, and hypertension.1, 2, 3, 4, 5

LBW is defined as a birth weight of an infant of 2,499 g or less, regardless of gestational age.6 LBW can be caused by preterm birth or small for gestational age at term with or without intrauterine growth restriction. LBW at term (TLBW) can be constitutional (i.e., without an underlying pathological cause) or secondary to intrauterine growth restriction. Risk factors for TLBW include poor maternal nutrition,7 maternal infections and other morbidities,8 young maternal age,9 and so on. And it has been reported that TLBW is highly prevalent in low and middle-income countries, particularly in South Asia.10 The numbers of international marriages in Korea has increased significantly during the first decade of the 21st century, from 111 834 in 2007 to 281 295 in 2015.11 The majority (83.9%) of international marriages in South Korea involve Korean men and women from East Asian countries, including China (40.2%), Vietnam (26.3%), Japan (8.3%) and the Philippines (7.3%). Because of this rapid growth in international marriage in Korea, 3.4% of all live births in the country involve immigrant women.11

Although term macrosomia is considered for newborns who weigh 4,500 g or more at birth by American College of Obstetricians and Gynecologists,12 definition of macrosomia in Asian countries remains unclear. Newborn weight exceeding 4,000 g is also a frequently used threshold to define macrosomia.13 Newborns with macrosomia are prone to neonatal complications, as well as cardiovascular or metabolic disease in childhood.14, 15 Immigrant women and children are a vulnerable group that faces health challenges different from native ones.16 Whether immigrant status is a risk factor for adverse fetal and infant health is currently unclear, yet.17 Some studies have reported that immigrant mothers are associated with higher rates of prematurity, lower birth weights and higher rates of perinatal mortality compared to native mothers.18 However, other studies have reported that fetal/infant mortality and the risk of LBW are comparable between immigrant women and native-born women after adjusting for influencing factors.19, 20 The aim of this study was to determine risks of TLBW and term macrosomia associated with immigrant mothers compared to native-born mothers living in South Korea according to economic level of origin countries of immigrant mothers.

De-identified Korean Vital Statistics Birth Certificate data were obtained from Statistics Korea, a nationwide database. This study was approved by Institutional Review Boards (IRB) (approval number: KC17ZESI0265). The IRB waived the need for consent because records were accessed anonymously. Raw data on all live fetuses delivered from 2009 to 2015 (444,849 in 2009, 470,171 in 2010, 471,265 in 2011, 484,550 in 2012, 436,455 in 2013, 435,435 in 2014 and 438,420 in 2015) were analyzed.11 Our results were based on 2,369,329 singleton live births after applying exclusion criteria. Data regarding newborns at gestational age (GA) <37 weeks or >42 weeks (n=207,845), twin (n=98,229), triplet (n=1,865), unknown number of fetuses (n=5,142), unknown maternal nationality (n=451,632), unknown paternal nationality (n=467,562), unknown maternal age or paternal age (n=36,570), extreme maternal age (<15 or >45 years old, n=1,572), extreme paternal age (<19 or >50 years old, n=14,440), unknown paternal education status (n=40,754), unknown maternal education status (n=16,300), unknown paternal occupation status (n=110,820) and unknown maternal occupation status (n=28,715) were excluded. Some of these exclusions (n=726,818) were duplicated. Pregnancy date was determined using the best obstetric estimate rather than the last menstrual period. However, if the estimated age based on the last menstrual period was significantly different from that estimated by ultrasound, GA determined by ultrasound was used. GA correction following birth was not allowed. GA was expressed in intervals of completed weeks. For example, a GA of 40 weeks refers to 40 weeks plus 0–6 days. Body weight was measured to the nearest 10 g.

This study used anonymous registry data. The mother's origin was defined based on her nationality at birth. Immigrants were defined as women born in a country other than Korea. An immigrant's country of origin was classified according to income group defined with the 2015 World Bank Atlas Method.21 Based on gross national income per capita, a country's economy was categorized as low income (US $1025 or less), lower middle income (US$2016–$4035), upper middle income (US $4035–$12475), or high income (US $12475 or more). This method considers the average income of a country's citizens that reflects social, economic and environmental well-being of the country and its people. Then we combined ‘low income’ and ‘lower middle income’ to ‘lower middle income countries’. And we combined ‘upper middle income’ and ‘high income’ to ‘high and upper middle income countries’.

The present study analyzed associations between economic level of maternal origin of country and risks of TLBW and macrosomia using a nationwide registry data. In this study, we found significant differences in risks of TLBW and macrosomia of immigrant mothers compared to Korean mothers. Overall, the risk of TLBW was higher while the risk of macrosomia was lower in immigrant mothers. However, results were variable according to economic level of maternal origin of country based on the classification of country income by word bank. The risk of TLBW was significantly higher in immigrant women from low and lower middle income countries compared to that it in Korean mothers, but significantly higher in immigrant women from high and upper middle income countries. In addition, the risk of term macrosomia was significantly lower in immigrant mothers from low and lower middle income countries but significantly higher in immigrant mothers from middle or high-income countries compared to that in Korean mothers after adjusting for neonatal sex, paternal age, maternal age, gestational week at delivery, paternal occupation, maternal occupation, paternal nationality, maternal ethnicity, paternal ethnicity, and parity. Economic level of South Korea is included in the high income countries, but also included in Asia. Therefore, the results from multivariate analysis adjusted for variables can be meaningful.

A previous study has reported that Korean father-foreign mother pregnancies have higher risk of TLBW compared to Korean father-Korean mother pregnancies.22 A recent study with similar population has shown that Chinese mothers have higher risk of term macrosomia but lower risk of TLBW compared to Korean mothers while mothers from Vietnam or Philippines have inverse risks.23 Nowadays, the origin of mothers is getting more and more diverse. Therefore, it is important to evaluate available pregnancy outcomes in immigrant mothers according to their socioeconomic background including the country of origin.

There have been similar studies worldwide focusing on neonatal outcome of immigrant mothers. Women from Asia, the Pacific islands and sub-Saharan Africa have higher risks of low birth weight and preterm birth than women from Finland in a Swedish study performed between 1978 and 1990.24 Newborns of Spanish mothers have higher likelihood of low birth weight but the lowest risk for macrosomia compared to immigrants from Morocco, Romania, Ecuador, Bolivia, or Colombia.25 This Vik et al.26 (2019) also reported that migrant women from certain countries, mostly low and lower middle income countries, are at increased risk of stillbirth.

The current study has several limitations. First, we were unable to exclude maternal complications that could affect neonatal birth weight. In addition, database did not include duration of stay in Korea for immigrant women or individual family income level. Lastly, this study did not include maternal body mass index, individual economic level, due to the limitation of data. However, in the present study, the risk of TLBW was increased in immigrant women from low and lower middle income countries and the risk of term macrosomia was increased in immigrant women from high and upper middle income countries after adjusting for neonatal sex, paternal age, maternal age, gestational week at delivery, paternal occupation, maternal occupation, paternal nationality, maternal ethnicity, paternal ethnicity, and parity. Therefore, we can consider that maternal origin of country is an independent risk factor for TLBW and term macrosomia. For example, adequate nutritional support and education for women from low and lower middle income countries can be more emphasized, but balanced diet with regular exercise plan can be more emphasized in women from high and upper middle income countries. In this study, higher level of maternal and paternal education independently decreased the risk of TLBW and term macrosomia in this study. This suggests that more education of expecting parents might lead to better pregnancy outcomes.

Cultural globalization is accelerating due to technical development, resulting in more international marriages from diverse countries, in accordance with increasing ‘marriage-based immigration’ in rural areas, in Korea. However, conservative culture or unified policy does not help diverse couples. Results of this study suggest that risks of abnormal neonatal birth weights could be different for immigrant mothers depending on economic level of maternal country of origin. Antenatal care and education including diet, exercise, weight gain and possible adverse pregnancy outcomes according to maternal background and socioeconomic characteristics might be able to improve pregnancy outcomes that could affect future health of neonates in childhood and adulthood, especially for immigrant women.