SUMMARYIn the present paper the secular trend of twin concordance for late fetal death rate has been analysed. The results show that, while the correlation remains unchanged in MZ twins, it is greatly reduced in DZ twins. This renders untenable the assumption of comparable secular trends in the intrauterine environment of the two types of twins. An appropriate model is given for the estimation of the environmental (intrauterine) factors in late fetal death concordance, under the assumption of a wide range of frequencies for unconditional lethals.