Gestational changes in the functional structure of the human placenta in relation to fetal growth: a morphometric study.

The study of different morphometric parameters in 17 placentas collected between 22 and 40 weeks of gestation has shown that, from mid-gestation to term, two stages are clearly discernible in the development of the human placenta. The first stage of growth, which terminates at approximately 36 weeks of gestation, is characterized by a progressive increase in parenchymal components. The second stage, which extends from around 36 weeks to term, is called the maturation stage because it is characterized by substantial fetal growth but without any increase in placental functional tissues. Evidence of this growth in function was also suggested by the demonstration of a sevenfold increase in fetal weight in the last half of pregnancy, despite a twofold decrease in the relative villous surface area. Furthermore, the study has shown that, at a given gestational age, fetuses with relatively small placentas tend to outgrow the functional capacity of their placentas.