GESTATION 



1017 



the question of cervical dilatability becomes 

 more important because it seems to occur 

 in all species examined thus far. It would 

 seem that relaxin can induce cervical di- 

 latability in conjunction with the sex ster- 

 oids and that cervical dilation is a necessary 

 event in parturition, but whether relaxin 

 controls this event under physiologic con- 

 ditions is still unknown and direct evidence 

 is unavailable. It is also apparent in some 

 species that relaxin can inhibit uterine con- 

 tractions w'ithout interfering with the action 

 of oxytocin. Kroc, Steinetz and Beach 

 (1959) reported that relaxin actually re- 

 stored responsiveness to oxytocin in mice 

 treated with progesterone. Again the ques- 

 tion is raised as to whether this is merely 

 a good experiment or a part of the normal 

 physiologic events. 



In a general way the events leading to 

 labor may be summarized as follows. As 

 pregnancy approaches term, the uterus be- 

 comes subject to increasing pressure from 

 within, due to a differential change in the 

 growth rates of the fetus and the uterus 

 (Woodbury, Hamilton and Torpin, 1938). 

 Concurrently, a reversal from progesterone 

 to estrogen domination occurs, which also 

 contributes to an increase in uterine tonus. 

 As intra-uterine tension increases, spon- 

 taneous contractions acquire a greater ef- 

 ficiency and forcefulness. Because the radius 

 of curvature in the human uterus is greater 

 at the fundus than at the cervix, and be- 

 cause the myometrium is thicker at the up- 

 per pole (by a factor of 2) the contractile 

 force is stronger at the fundus than at the 

 cervical end. This contractile gradient i^ro- 

 duces a thrust toward the cervix. 



Utilization of a type of strain gauge, the 

 tokodynamometer, has afforded informa- 

 tion on the rate and strength of contraction 

 of the various parts of the parturient uterus 

 simultaneously (Reynolds, Heard, Bruns 

 and Hellman, 1948). These measurements 

 have indicated that, during the first stage of 

 labor, the fundus exerts strong contractions 

 of rather long duration. The corpus exhibits 

 less intense contractions, usually of shorter 

 duration, which frequently diminish in force 

 as labor advances. The lower uterine seg- 

 ment is almost inactive throughout the first 



stage of parturition. According to Reynolds 

 (1949), both the fundus and the midportion 

 contract at the same time, but the fundus 

 remains contracted for a longer period of 

 time than the corpus beneath, thus building 

 up a force downward. If cervical dilation 

 has not occurred, the three areas of the 

 uterus will continue to contract. As cervical 

 dilation begins, the contractions in the mid- 

 portion of the uterus decrease in intensity 

 and the contractions in the lower segment 

 disappear. Cervical dilation has been ob- 

 served only when there is a preponderance 

 of rhythmic activity of the fundus over the 

 rest of the uterus. 



When amniotic fluid is lost after the rup- 

 ture of the membranes, the absolute tension 

 within the wall of the uterus is reduced so 

 that the ratio of force between fundus and 

 cervix is increased. Thus rupture of the 

 membranes decreases the tension in the cer- 

 vix more than the fundus and the net effect 

 is an increased force from the fundus. This 

 change tends to precipitate the parturition 

 more rapidly. 



As pregnancy nears term, both increased 

 tonus of the myometrium and rapid growth 

 of the fetus cause a rise in intra-uterine 

 pressure. This rise results in a decrease of 

 effective arterial blood pressure in the pla- 

 centa. During this period also, thrombosis 

 is observed in many of the venous sinuses 

 of the placenta and many of the blood ves- 

 sels become more or less obstructed by giant 

 cells. During parturition, the systemic blood 

 pressure of the mother rises with each con- 

 traction, but, due to the increased intra- 

 uterine pressure produced by the contrac- 

 tions, the effective maternal arterial blood 

 pressure in the placenta decreases to zero. 

 Thus maternal circulation is cut off from 

 the fetus. 



Measurements of intra-uterine pressure 

 at term show that the human uterus con- 

 tracts with a pressure wave which varies 

 from 25 to 95 mm. Hg (Woodbury, Hamil- 

 ton and Torpin, 1938). The uterine wall is 

 subjected to an average tension of 500 gm. 

 per cm.- and, during delivery of the head, 

 may, with the aid of abdominal muscula- 

 ture, develop as much as 15 kg. force. 



In animals giving birth to multiple young 

 (rat and mouse) evacuation of the horn pro- 

 ceeds in an orderlv fashion beginning at the 



