Birth weight, placental weight and gestation time in relation to natural selection in Thailand.

Birth weight, placental weight and gestation time of 26258 single births at Ramathibodi Hospital during 1973-7 were studied in relation to natural selection by fitting quadratic function to natural log of survivors-to-nonsurvivors ratios. The survival criterion was 7 days after delivery. The estimated optimal values of birth weight and placental weight were 0.62 and 0.90 standard deviations, respectively, above the corresponding arithmetic means. Selection on birth weight and placental weight was asymmetrical about the optima, being more intense among the lower than the higher values. On the other hand, selection on gestation time seemed most stabilized with the optimum almost coinciding with the mean.