Sonar estimates of "gestation sac" volumes in the first trimester of pregnancy were made from a series of 319 measurements; the volumes estimated included the amniotic fluid, the extraembryonic coelom and the fetus. The technique employed involved the use of parallel section scans taken in series from one end of the gestation sac to the other, followed by planimetric measurements of the sac areas so produced. The potential errors which may be incurred in these measurements are discussed, and it is considered that the technique carries an overall possible error in the order of plus or minus 10 per cent. Growth curves produced from the measurements in this series show that the sac increases in volume from a mean of 1 ml. at 6 weeks to a mean of 100 ml. at 13 weeks, initially in an exponential fashion but latterly in a more linear manner. The mean values of the sonar gestation sac fluid volumes (after subtraction of the estimated fetal volume) in the 10 to 13 week range, show good correlation with those amniotic fluid values reported in the literature where direct measurements were made at the time of hysterotomy. As a method of assessing the maturity of a pregnancy this technique is of lesser value than the sonar measurement of fetal crown-rump length because of the relatively wider scatter of results. It has, however, found a useful place in clinical practice in the early sonar diagnosis of blighted ova or anembryonic pregnancies.