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

The relations of birth weight placental weight and gestation time to neonatal death were analyzed for 26258 single births during 1973-77 at Ramathibodi Hospital in Bangkok Thailand. The survival criterion was surviving beyond the 7th day after delivery. The dead included deaths in utero and sillbirths. The death rate was 9.25/1000. Sex specific rates were 9.96 for males and 8.08 for females. All mothers delivering at the hospital received prenatal care and most or all belonged to upper socioeconomic groups. Gestation time was calculated form expected date of confinement and birth date but only 19647 cases reported complete expected dates. The estimations of the optimal values of birth weight placental weight and gestation time were done by fitting quadratic functions to a natural log of survivors-to-nonsurvivors ratios using the model of Karn and Penrose. The male and female optimal birth weight values were 3389.19 and 3332.02 gm respectively .58 and .67 standard deviations above the corresponding means. From extrapolation the curve below 2000 gm approached zero more rapidly than that beyond 4200 gm. The relative fitness curve for placental weight followed virtually the same pattern as that for birth weight. Optimal values in males and females were 722.00 and 710.52 gm or .91 and .89 standard deviations above the respective means. 5 cases whose placentae weighed more than 1300 gm were essentially a separate category in which the chance of survival was zero. The optimal gestation values were 277.43 days for males and 281.15 for females almost coinciding with the mean values. Females had higher fitness than males at all values and the gap between the 2 curves widened at the 2 ends. Selection on birth weight and placental weight was asymmetrical about the optima being more intense among the lower values. Selection on gestation time was symmetrical about the mean with the optima almost coinciding with the mean.