Abstract: The birth records of Chinese child-bearing age women ( aged 15- 49) available at the census in 1989 from January 1st to December 31st are used to analyse the relationship between the birth rates of total, DZ and MZ twins and the maternal ages and the twins sex ratios in China by means of Winberg 's difference method to compute the zygosity of the twins. The results show that the birth rates of total, DZ and MZ twins in China are 0. 787 ± 0. 002% , 0. 573± 0. 002% and 0. 214± 0. 001% respectively. The birth rate of DZ twins increase monotonously with the increasing maternal age before 33, and then decrease, but increase again after 46. The birth rate of MZ twins keeps relatively stable before 34, and then increase with maternal age. The twins sex ratio is lower than that of the same population significantly in China, and the twins sex ratios at the age groups of 15- 19 and 45- 49 which are possibly caused by the lower MZ twins sex ratios are lower than that at any other age groups.

Key words: Birth rate of twin, Maternal age, MZ, DZ