Cross-breeding experiments between large and small strains of mammals have shown the powerful influence of the maternal organism on the control of fetal growth. The prepotency of a maternal regulator has also been demonstrated in humans. Our earlier studies indicated that this regulator acts by means of constraint; there is no equivalent accelerating mechanism. Data on 1092 siblings and 5207 paternal and maternal relatives of 986 probands show different patterns of birthweight among families ascertained, respectively, through very large and very small babies. When constraint is relaxed the Mendelian laws of inheritance are clearly followed. At the lower extreme there is evidence for the transmission of constraint through the female line only. This could be due to the maternal genotype, but our data suggest that a non-Mendelian path might also be involved. Such a process would be adaptive, facilitating fairly fast changes in fetal growth rate as the conditions under which a population lives deteriorate or improve.