Distortions in fetal growth standards.

The study analyzed several errors incorporated in widely used fetal growth standards. Such standards contain measurements from many infants who died as neonates. The present study found many of these nonsurvivors to be growth retarded so new standards were developed which contain only measurements from survivors. The study also found that weights and other body measurements of prematurely born neonates cluster at intervals consistent with maternal vaginal bleeding at monthly intervals in early pregnancy. Such bleeding is often mistaken for the last menstrual period. To avoid this error, the probability plot method was used to separate the clusters of measurements at each gestational age. The cluster with the lowest values at each gestational age was used as the data base for the new growth standards. Standard deviations from most mean values in the new growth standards are much smaller than are such deviations in the older standards.