Trend of sex ratio at birth in a public hospital in Hong Kong from 2001 to 2010.

OBJECTIVES To identify factors affecting the sex ratio at birth. DESIGN Cross-sectional study. SETTING Obstetric department of a public hospital in Hong Kong. PARTICIPANTS All pregnant women delivered between 2001 and 2010. MAIN OUTCOME MEASURES Sex ratio at birth versus women's eligibility status, age, parity, number of miscarriages or terminations of pregnancy, and number of fetuses were analysed using the Chi squared test. Multivariate regression was used to determine the effects of multiple factors on the sex of the newborn. RESULTS A total of 54 039 cases were reviewed. The sex ratio at birth changed since 2003, and became unbalanced (>107 males per 100 females) since 2006 revealed by a significant increase in males per 100 females, from 106.6 in 2001-2005 to 111.4 in 2006-2010. From 2001 to 2010, the sex ratio at birth increased from being balanced to becoming unbalanced in eligible persons, and became more unbalanced in non-eligible persons. The ratio increased in eligible persons after having two children, but in non-eligible persons after having one child. The sex ratio at birth was unbalanced (1.095) in singleton pregnancies, but balanced (1.019) in multiple pregnancies. Based on logistic regression, the chance of a male baby being born increased with parity of 2 or above (odds ratio=1.1; P<0.001), non-eligible person status (odds ratio=1.05; P=0.034), and delivery in the period 2006-2010 (odds ratio=1.04; P=0.019). The ratio was not increased with advanced maternal age, the number of miscarriages/terminations of pregnancy, and number of fetuses. CONCLUSION Compared with 2001-2005, the sex ratio at birth became unbalanced in 2006-2010. An unbalanced ratio ensued in the latter period in both eligible and non-eligible persons, but to a greater extent and even after having one child in the latter group.