PAPP101 - S03: How demographers think about populations: age and sex

Sex ratios (cont.)

Sex ratios at birth are remarkably consistent across time and populations, usually falling within the range of 104-107 male births for every 100 female births.
As countries develop and health improves, the sex ratio at birth tends to increase slightly. This is because pregnancies in which the foetus is male are more likely to end in spontaneous abortion than pregnancies in which the foetus is female. As the health situation of a country improves, those pregnancies which may have ended in spontaneous abortion are more likely to reach full term, thus increasing the number of male infants born.
However some Asian countries such as China, India and South Korea, have sex ratios far above this normal range, reaching 115 or even 120 male births for every 100 female births.