A new study involving Oxford University researchers suggests that the migrant status of couples in Hong Kong is a key factor in their babies' birth weights.
The study suggests that native couples have significantly lighter babies than parents who come from mainland China.
Mainland Chinese mothers who give birth in Hong Kong are only half as likely to have low-weight children and just one-fifth as likely to have very low-weight children as Hong Kong-born couples.
The researchers from Oxford University and the University of Piraeus in Greece examined the data of more than 825,000 babies in the Special Administrative Region of Hong Kong between 1995 and 2009.
According to the study, published in the online early edition of Journal of Biosocial Science, low birth weight (under 2.5kg) and very low birth weight (under 1.5kg) are important predictors of infant death and illness and can be linked to ill health in childhood and later life.
The paper also cites other studies showing that high birth weight babies (over 5kg) have a greater likelihood of developing type 1 diabetes in childhood and rheumatoid arthritis, while high birth weight females have a higher chance of becoming obese adolescents and developing type 2 diabetes.
Researcher Dr Stuart Basten from the Department of Social Policy and Intervention at Oxford University said: 'Our study appears to support the "healthy migrant theory" of previous studies that suggests recent immigrants have better health than women of the same ethnicity who are not foreign-born.
'We can speculate that in this case, women who have travelled from mainland China to live in Hong Kong are generally more socially and economically advantaged than the couples who were born in Hong Kong, which is why their babies are heavier.'
Overall, the study found that the prevalence of low-weight births in Hong Kong was low compared with other parts of Asia and Europe. However, migrant women from south and south-east Asia were found to be around 1.3 times more likely to give birth to very low-weight babies than women born in Hong Kong.
Meanwhile, mothers from developed countries were over five times more likely to give birth to heavy babies than couples where both partners were born in Hong Kong.
The study found that teenagers and single mothers, and especially mothers over the age of 35, face higher risks of poor pregnancy outcomes and particular risks of low and very low-weight births. Older women are also more likely to give birth to heavy babies.
Researcher Dr Georgia Verropoulou from the University of Piraeus said: 'This large-scale study has identified specific high-risk groups that should help policy makers target groups that need the most support in pregnancy.
'Single mothers represent a fast-growing group worldwide. Meanwhile, immigrants from south and south-east Asia also seem to be an especially susceptible group linked to low birth weight babies.
'Meanwhile, this study suggests that particular vigilance is also needed for women born in developed countries who should be monitored for heavy birth weight babies.'