The discrepancy between the number of boys and girls born has been interpreted as a natural selection response to differential survival prospects. There also exists a discrepancy in birth weight, length, head circumference at birth of boys and girls; on the other hand, placental weights were not so strongly biased by the sex of the fetus. Metabolic differences between the sexes are clearly recognized in adults. It is therefore argued that the anthropometric differences at birth, examples of which are presented in this paper, can only be achieved if the products of conception are also expressing a sexual bias in metabolism and physiology. It would then be this bias which would determine the efficiency of the implantation and growth processes and lead to rates of survival to birth. The speculation arising from this and the experimental manipulation of the sex ratio is that the physiological component most likely to be involved would be the lipid compartment with its strong sex difference.