Intrauterine volume in pregnancy.

Intrauterine volume, a three-dimensional measure of growth, was evaluated in pregnancy. The ultrasonic method for measurement of volume was investigated for its accuracy and error potentials and then used to collect information on the growth of three volume components, - total intrauterine, intra-amniotic and placental volume, in the latter half of normal pregnancy. Against reference standards from the normal population, two other main groups, a non-selected and a pre-selected high-risk group, were studied, to test the hypothesis that a small intrauterine volume would predict the presence of a small-for-dates and possibly compromised baby. In previous times main interest in uterine volume had centered on its possible role in parturition. More recently ultrasound had been used to measure intrauterine volume in early pregnancy and a simple ultrasonic method of calculating total intrauterine volume had been used for diagnosing the small-for-dates fetus. The study showed that in a phantom model situation, a method based on serial parallel planimetric area measurements was significantly more accurate than a linear measurement method that assumed the uterine shape to be like a prolate ellipsoid. With the former method 93.3% of measurements were within +/- 5% of the true volume with a mean error of -0.43%. When the method was evaluated for its accuracy in women undergoing midtrimester termination or in women delivering near term, there was good correlation between ultrasonically and directly/indirectly measured intra-amniotic volume suggesting that the same level of accuracy could be expected in pregnancy as found in the phantom models. Intra- and interobserver variation was small. The PPA method was used for all measurements in pregnancy because of its superior accuracy. Data from 115 healthy women with no pregnancy complications were evaluated longitudinally to study normal growth and derive reference standards. Median and mean values for total intrauterine volume increased from around 1,000 ml at 20 weeks to 4,500 ml at 40 weeks. The standard deviation was large at any stage of pregnancy, also for intra-amniotic and placental volume. The rate of volume increase was near constant for total intrauterine and intra-amniotic volumes, except during the period 30-35 weeks, when a faster rise was observed. Placental volume growth decreased after 30 weeks. With growth of total volume following a largely linear trend, a tendency for ranking of volume levels was observed, but predictability of one measurement on the basis of earlier ones weakened with increasing separation of time points.(ABSTRACT TRUNCATED AT 400 WORDS)