Daily changes in amniotic and allantoic fluid during the last three months of pregnancy in conscious, unstressed ewes, with catheters in their foetal fluid sacs
Authors: Mellor D. J. , Slater J. S. 
Published: December 2014

1. Catheters were inserted into the maternal and foetal vasculatures of ten ewes, 100–139 days pregnant, and daily samples of uterine and umbilical blood and maternal jugular vein blood were taken for periods of 5‐27 days after operation.

2. Catheters were inserted into the fluid sacs of nineteen foetuses, 60–97 days post‐conception, and daily samples were withdrawn for up to 90 days from amniotic sacs (eleven foetuses) and for up to 70 days from allantoic sacs (eight foetuses). Maternal jugular plasma was obtained 3 times weekly and an approximation from its composition to that of uterine and umbilical plasma was made using results from the ewes and foetuses with vascular catheters.

3. The pH, osmolality, [Na+], [K+], [Cl−], [urea] and [amino acid] of all samples were measured.

4. The nutritional status of all ewes was monitored throughout pregnancy. Most lambs were born naturally at ∼ 147 days post‐conception and their subsequent progress was observed.

5. Results at operation and from acute experiments were compared with those from conscious ewes and foetuses of the same gestational age to assess the nature and extent of the influence of the operative procedures on foetal fluid composition. The composition of amniotic fluid was influenced mainly by the anaesthetic and surgical procedures while that of allantoic fluid was affected largely by starvation of the ewe.

6. Changes during recovery from operation were followed and indicated that maternal and foetal plasma required about 3 days and the foetal fluids up to 7 days before stability of composition was achieved.

7. After recovery from operation, daily changes in the composition of each foetal fluid showed the same general pattern in all foetuses, but the absolute values of constituents sometimes showed large differences.

8. It is suggested that flow of foetal urine into the amniotic sac increased from 80 days gestational age, that urine flow into the allantoic sac decreased until about 100 days but did not cease thereafter, and that relative to foetal urine the influence of foetal pulmonary fluid on amniotic fluid composition was not great.

9. A relative impermeability of the amnion appeared to be a major factor influencing amniotic fluid composition, whereas pumping mechanisms in the chorioallantois seem to have been responsible largely for changes in the composition of allantoic fluid.

10. The quantity of solute relative to that of water within each sac appears to be a major determinant of changes in foetal fluid volumes.

11. Changes in the [Na+] and [K+] of allantoic fluid during the normal course of pregnancy were consistent with an increasing action of mineralo‐corticoids on pumping mechanisms in the chorioallantois. Similar but more rapid changes seemed to be associated with acute and chronic episodes of maternal hypoglycaemia. Under these circumstances foetal hypoglycaemia may effect a relative increase in the secretion of foetal corticosteroids having an action on the chorioallantois.

12. The results from this study demonstrate clearly the value of using chronically catheterized animals, and it is suggested that their use in physiological studies on the conceptus must eventually supersede that of acute, anaesthetized preparations.