Mathematical modeling of fetal growth: II. Head cube (A), abdominal cube (B) and their ratio (A/B).

Growth of the fetal head and abdomen have been monitored by measurement of the head cube (A), the abdominal cube (B) and A/B, from 12 to 41 weeks menstrual age. Growth curves for these parameters have been determined using a specially developed growth curve model [P = c(t)k+s(t)]. R2 values of 98.0%, 96.6%, and 56.2% were obtained for A, B, and A/B, respectively. Similar results were obtained when this model was used with HC and AC data from a previous study. Variability analysis indicated a progressive increase in variability with menstrual age for both A and B while the variability of A/B decreased. The variability in the 12- to 17-week interval was significantly greater for all three parameters and the A deviation distribution was asymmetric (shifted toward negative values) after 36 weeks. Variability data were used with the growth curve models to determine standard curves for A, B and A/B.