Placental growth and fetal/placental weight (F/P) ratios throughout gestation--their relationship to patterns of fetal growth.

Available data would indicate that patterns of growth and velocity of growth in the placenta and fetus may be determined early in gestation, at or shortly after implantation. Such early growth characterization may be mediated through regulation of DNA synthesis, thereby limiting potential cell number in the developing placenta. If this assumption were true, many casualties would be possible: (1) chromosomal or genetic, (2) implantation abnormalities, (3) early acquisition of congenital viral disease, (4) radiation or toxin exposure, or (5) maternal autoimmune attack on the placental/fetal unit. Once the placenta's maximal cell number has been established, well defined placental and fetal growth velocities are predictable in the majority of gestations. However, within each growth category (AGA, SGA, LGA) a wide range of "normal" placental weights (and F/P ratios) are available and compatible with normal growth. When each growth category is considered separately, eventual placenta size (and therefore fetal size) may rely more directly on total protein accretion reflected in cell size. At this point in growth, other factors such as inadequate maternal nutrition, high altitude, maternal smoking, intrinsic maternal vascular disease, or yet later factors such as preeclampsia may be responsible for placental growth limitations. Such factors, many as yet undefined, would account for the wide variability of F/P ratios within the AGA, SGA, and LGA groups. Since mean F/P ratios vary little between such groups, the placenta appears to select an ideal fetal growth rate independent of predetermined growth category. Both AGA and LGA infants demonstrate a consistent increase in placental size until 42 wk of gestation.(ABSTRACT TRUNCATED AT 250 WORDS)