Relationship of fetal and placental weight in human beings: fetal/placental weight ratios at various gestational ages and birth weight distributions.

Two thousand consecutive deliveries occurring over an 11-month period were studied for fetal and placental (F/P) weight characteristics. Infant and placental weight pairs were analyzed according to their gestational ages and growth categories (AGA, SGA, LGA). Graphs were constructed to depict normal placental weight gain and F/P ratio changes over a wide range of gestational ages (23 to 43 weeks). Mean placental weights and F/P ratios continued to increase through 42 weeks' gestation in the AGA and LGA groups, though placental weight showed no change in the SGA group after 36 weeks. F/P ratio continued to increase in the SGA group and was indistinguishable from that of the other two groups. Fetal and placental weight were linearly related in the total sample as well as in the individual growth categories. However, when F/P ratio was plotted against placental weight, the infants were segregated into three completely distinct curves, depending upon their growth characteristics. A wide range of placental weight and f/p ratios existed within all three infant birth weight groups. When one-minute Apgar scores were considered, infants with F/P ratios of greater than 10.00 had significantly higher risks of an Apgar score of less than 6.