CHANGES IN UTERINE CONTRACTILITY FOLLOWING INTRA‐AMNIOTIC INJECTION OF HYPERTONIC SALINE TO INDUCE THERAPEUTIC ABORTION

DESPITE much research the mechanism which controls the onset of labour remains unsolved. As assessed by uterine contractility, labour may be preceded by a period of increased activity sometimes lasting several weeks or may commence without warning so that the recording of the sequential changes is time consuming and difficult. By comparison, strong uterine contractions begin quickly after hypertonic saline has been injected into the amniotic sac of the quiescent uterus at mid-pregnancy to induce therapeutic abortion (Aburel, 1938) and the sequence of events can be studied according to a pre-arranged plan. It may be argued, of course, that the uterine contractions induced by hypertonic saline in early pregnancy have little in common with the uterine contractions of normal labour at term, but Bengtsson and Csapo (1962) state that in both cases the uterus contracts vigorously because it has been freed from the restraining influence of the progesterone produced by the placenta, which they claim “blocks” the conduction of excitation waves through the myometrium over the placenta. Bengtsson and Stormby (1962) believe that the hypertonic saline exerts its effect by causing a “necrotizing placentitis” and report a fall in the urinary excretion of pregnanediol in these cases, although Short et al. (1965) found no change in the concentration of progesterone in uterine venous blood after intra-amniotic injection of saline. Wood, Booth and Pinkerton (1962) showed that intra-amniotic injection of hypertonic glucose effectively induced labour in patients with an anencephalic foetus or a dead foetus. It is impossible to study the endocrine mechanisms of normal labour in these patients since steroid hormone production is known to be abnormal in such cases. We have had the opportunity to investigate the effect of hypertonic saline used to induce abortion in normal pregnancies which had reached at least 19 weeks. Continuous records of uterine activity have been made before and after the saline injection, up to the time of abortion. Most previous studies report only intermittent measurements of contractility. Wiqvist and Eriksson (1964) attempted a quantitative analysis but onIy recorded activity for the first eight hours after the saline injection or in the 20th hour after saline or in the hour before abortion. In our view, only continuous recordings, analysed in detail, can give an adequate picture of the sequence of events and the variation in the strength and frequency of uterine contractions.