Relative growth of brain in the human fetuses: first gestational trimester.

The relative growth of brain weight was studied in 23 Brazilian human fixed fetuses of known age ranging from 9th to 12th weeks post-conception using the allometric method: Y = aXb. The brain weight (grams) was correlated to the fetal parameters of growth: gestational age (weeks post-conception), cephalic perimeter (millimeters) and fetal weight (grams). The brain weight growth presented negative allometry relative to fetal age (b = 0.87) and cephalic perimeter (b = 2.46) and positive allometry relative to gestational age (b = 6.65). These data are important to compare the fetal growth after ultrasonic investigation.