Various interpretations have been attempted, but no definitive theory has yet been established as to the etiology of the low-birth-weight infant. We carried out the present study in an attempt to explore the relationship between platelet functions and fetal growth during normal pregnancy. For this purpose, we made a retrospective study of 130 pregnant women aged 23-35 with no clinical abnormalities throughout pregnancy or at delivery. Blood samples were taken at 29-30 weeks of gestation (referred to as the 2nd trimester) and 37-38 weeks of gestation (referred to as the 3rd trimester). The cases studied were divided into two groups: i) those with a birth weight of under 2,500g (n = 32), and ii) those with a birth weight of over 2,500g (n = 98). In these two groups, we studied the platelet functions in maternal blood in the 2nd and 3rd trimesters. The following results were obtained: 1) In the 2nd to 3rd trimester, the platelet count showed no significant variation with the birth weight. 2) In the 2nd to 3rd trimester, the platelet aggregation was found to be moderately depressed in cases with a birth weight of under 2,500g, while it was found to be moderately activated in cases with a birth weight of over 2,500g. 3) In the 2nd to 3rd trimester, platelet factor 4 was significantly lower (p < 0.005) in cases with a birth weight of under 2,500g, while it was moderately higher in cases with a birth weight of over 2,500g.(ABSTRACT TRUNCATED AT 250 WORDS)