The evaluation of early pregnancy and its complications by diagnostic ultrasound

In the two decades since the use of ultrasound was first applied to the problems of obstetrics, its role has expanded dramatically, and, in good hands it may now be considered to be one of the most important diagnostic aids available to the obstetrician. In 1972 however, when the work described in this thesis was initiated, the ultrasonic techniques employed in early pregnancy were either subjective, or their limits of accuracy, in such areas as fetal heart movement detection and the estimation of gestational age, were less than optimal. Thus, the objectives of this study were: to develop a reliable method of detecting fetal life earlier in the first trimester of pregnancy; to devise objective criteria for the early diagnosis of unsuccessful pregnancies; and to develop a more accurate means of estimating the gestational age of a pregnancy in the first trimester. In pursuit of these objectives a "second generation" ultrasound equipment was used, the Diasonograph NE4102, an equipment which is more sophisticated electronically than earlier models, and which has many novel and useful facilities. Using this equipment three new ultrasonic techniques were developed: a combined B-, A-scan method for detecting small movements of the fetal heart; a system for estimating the volume of the gestation sac; and a technique for the measurement of the fetal crown-rump length (CRL). The technique devised to detect early fetal heart movements was based on the principle of first locating the fetus on a two-dimensional B-scan image, then examining the fetal echoes closely on an expanded uni-dimensional A-scan display. In this way it proved possible to detect movements of the fetal heart structures with reliability from seven weeks of amenorrhoea, and on occasion from as early as six weeks. Conversely, absence of such movements proved reliable as a means of diagnosing early fetal death, and with increasing confidence in the technique, patients were offered an elective termination of pregnancy, at times, on the sole evidence of the ultrasound examination. Two techniques were developed with a view to obtaining objective, and more accurate estimates of gestational age in the first trimester. These were gestation sac volume, and CRL measurements, of which the latter has proven to be relatively simple in its application, to be reproducable, and to give an accuracy of ± 4.7 days in 95 per cent of cases. This figure was confirmed in a large clinical "blind" trial. After correction for the systematic errors of the technique (1 mm plus 3.7%), the mean CRL growth curve, derived from 424 measurements, correlated well with the data published by some embryologists, but not with those by others. The possible reasons for these discrepancies are discussed. Using CRL measurements as a baseline, the accuracy of clinical bimanual estimation of uterine size in the first trimester, and of biparietal diameter measurements at approximately 20 and 36 weeks, was assessed in a further "blind" trial. The 2SD limits of each of these estimations were found to be ± 25.5 days, ± 11 days, and ± 30.4 days respectively. In view of this, and the desirability of having a reliable estimate of the gestational age in the modem management of pregnancy, it is recommended that at least all patients with "at risk" pregnancies, together with those whose menstrual histories are in any way unreliable, should be referred for an ultrasound examination at the time of their first antenatal clinic visits. It is concluded that the application of these three techniques allows a more objective and more reliable evaluation of early pregnancy and of its complications than was hitherto possible