BIRTH 289 



is able to maintain its body temperature against low-temperature exposures 

 by increasing its rate of metabolism but as yet has no protection against higher 

 temperatures. 



Birth 



The end of the fetal period of development is widely variable if we rely 

 on the ages assigned to the fetus. The determination of fetus age has always 

 been a very difficult problem, since the time of fertilization is rarely known 

 with certainty. It is now certain that fertilization takes place about fourteen 

 days after the beginning of the last menstrual period. If this date is known 

 with certainty, age can be determined within a few days one way or the other. 

 But since a slight bleeding which may occur for one or two months during 

 pregnancy is sometimes mistaken for menstruation, it is clear that the age of 

 the fetus may be underestimated by as much as a month or two. Thus records 

 of very premature births are suspect unless the mother has been under con- 

 stant observation. If we assume the conventional value of 280 days from the 

 last menstrual period to birth, then 280 minus 14, or 266 days, is the actual 

 age from fertilization. Then we may say that the age varies about 50 days on 

 either side, so that a newborn child may be from 216 to 316 days from 

 fertilization. Actually, somewhat younger newborn have been reported, but 

 these cases may not be valid. 



The important problem is what stimulates birth. A number of com- 

 pounds injected singly or in combination are effective to some extent. How- 

 ever, here we are interested in the normal mechanism. Since labor and birth 

 can occur in the absence of the ovary, and also in the absence of the pituitary, 

 it is only reasonable to assume that the initiation of this process is centered 

 about the embryo, placenta, and uterus. One theory, suggested by the observa- 

 tion that the estrone content of the urine increases before birth, claims that 

 estrone sensitizes the uterus for muscular contractions. On the other hand, 

 since a decrease in pregnandiol 1 in the urine also occurs before birth, it may 

 be that the progesterone content of the placenta falls below a critical value 

 necessary to maintain the uterus in pregnant condition. Injection of proges- 

 terone into pregnant rabbits increases the term of pregnancy. These two 

 theories are not mutually exclusive, since estrone and progesterone are an- 

 tagonistic in their effect on uterine muscle contraction and thus a high con- 



1 Progesterone is converted to pregnandiol, which is excreted in the urine. 



