During the first two trimesters of pregnancy the amniotic fluid is clear and yellow; during the third trimester the amniotic fluid becomes colourless; then, approximately from the 33rd-34rd week on, cloudiness and flocculation occur, at first very slowly, after the 36th-37th week steadily faster (Tab. I). At term, the amniotic fluid is moderately cloudy and contains a moderate number of flakes of vernix. The appearance of the amniotic fluid depending on the degree of cloudiness and on the number of flakes, has been expressed by means of a score system, the socalled macroscore (Tab. II). Relationships were observed: a) between the disappearance of the yellow colour (bilirubin) and the initial occurrence of cloudiness and flocculation; b) between the duration of pregnancy and the macroscore; from the 32nd-36th week of pregnancy the mean macroscore increases until the second half of the 40th week; then in the 41 st week there is a drop in the mean macroscore, after which a new increase occurs (Fig. 1,2 and 3). c) between the total gestation period at birth and the progression of the macroscore (Fig. 5); when birth takes place earlier (later), the macroscore will increase earlier (later). d) between the total duration of gestation at birth and the macroscore at the end of pregnancy; with an earlier (later) birth, the macroscore is lower (higher) (Fig. 5 and 6). With the macroscore it is possible to determine the duration of pregnancy (b) and the time before birth even more accurately (c). The fairly large standard deviation of the macroscore per pregnancy week (Fig. 1) also in case of a given duration of gestation at birth (Fig. 7) points to a fairly large interindividual variation in the appearance of the amniotic fluid at a certain duration of pregnancy. The macroscore is determined by elements originating from the fetal skin; the cloudiness and flocculation are caused by release of vernix and the flaking off of cells from the stratum corneum. Hence the macroscore reflects changes in the function of the fetal skin and is an indicator of the functional maturation of the fetal skin. The considerable variation of the macroscore at a given duration of pregnancy indicates a great variation of fetal maturation. The fetus that is maturing faster, will be delivered earlier; the fetus that is maturing slower, later (c). This points to a correlation between the degree of fetal maturation and the start of labour. The higher macroscore during the last days before birth in pregnancies of longer duration (d) (Fig. 5 and 6) may be explained by a less sensitive uterus, requiring a greater maturity of the fetus for delivery to start. The drop of the mean macroscore in the 41 st week of pregnancy is due to a sudden increase of lower scores in this week (Fig. 4). A lower score at a given stage of pregnancy means a later birth (Tab. VI and VII). Thus in the 41 st week of pregnancy a considerable group of pregnant women appears, that has a total duration of gestation that is, on the average, two weeks longer than normally...