Growth of the fetal head has been reexamined in a longitudinal study of 32 normally growing fetuses using the head profile area (HA). The Rossavik growth model (characterized by the coefficients c, k, and s) fit these data very well (R2 [+/- SD] 99.6 [+/- 0.3]%), and regression analysis indicated that optimum results are obtained when the coefficient k is kept constant. Head area growth before 28 weeks was found to be related to the coefficient c, which was strongly correlated (R2 69.2%) with the time of HA growth initiation. These results suggest that c reflects the genetic factors controlling growth in the early part of pregnancy. The coefficient s was negatively correlated with c (R2 82.1%) and had two components, one associated with c (pred. s) and one not associated with c (s-residual). This linkage between c and s, together with the parallel head growth seen after 30 weeks, suggests the possibility of a maternal system in which genetically determined head size (c) can be sensed and modified by other factors (s) to maximize compatibility between the fetal head and maternal pelvis.