86 CARNEGIE INSTITUTION OF WASHINGTON. 



weight is slightly more than twice that of the percentage increment 

 in height and, like the latter, steadily decreases as the fetus becomes 

 larger. 



From our plotted curves it is possible to read off the weight increment 

 for each millimeter increase in length. It is found that in fetuses under 

 60 mm. long it is less than 1 gram. In fetuses between 70 and 80 mm. 

 long there is an average increase of 1 gram per millimeter. This be- 

 comes 2 grams per millimeter in fetuses between 90 and 100 mm. and 4 

 grams per millimeter in fetuses between 130 and 140 mm. long. In 

 fetuses of about 200 mm. the weight increase is 10 grams per millimeter; 

 for 300 mm. it is 20 grams per millimeter; and at term it reaches 25 

 grams per millimeter. Although there is this progressive increase in the 

 actual-weight increment throughout the whole fetal period, the con- 

 trary is true for the percentage- weight increment, which progressively 

 decreases. 



It has been generally known that the length of the fetus exhibits 

 less variation than its weight, and on this account has been regarded 

 as a more accurate criterion of age. From the data contained in our 

 tables it is found that the normal variation in sitting height for any 

 given age over 40 mm. is between 8 and 10 per cent and that the normal 

 variation in weight for any given sitting height is about 30 per cent. 

 However, since the weekly weight increment is about three times as 

 great in its percentage as the sitting-height increment, the difference 

 in accuracy, when used for the determination of age, is slight. The 

 accuracy is greater, both for weight and sitting height, in the earlier 

 weeks of pregnancy and becomes progressively less accurate towards 

 the later weeks, ranging from about 4 days at the fourteenth week to 

 over 3 weeks at the thirty-fifth week. The joint use of the two deter- 

 minations, however, correspondingly increases the accuracy of the 

 age estimation. 



For purposes of age determination the foot length and head size, 

 when used alone, do not have as much value as do weight and sitting 

 height. The head size shows considerable variation, due in part to the 

 mechanical molding which occurs in the great majority of specimens 

 and in part to the varying influence of formalin on the soft tissues of 

 the scalp, which, when distended, affect the size-reading considerably 

 more than the corresponding increase in sitting height. The foot 

 length, as compared with the sitting height, possesses the disadvan- 

 tage of being smaller and having a smaller weekly increment. Never- 

 theless, the head size and foot length serve as additional controls and, 

 in case of dismembered specimens, frequently constitute the only 

 reliable criteria for the determination of age. From an uninjured 

 foot one can arrive fairly closely at the normal sitting height, 

 weight, and menstrual age of the specimen by means of our curve of 

 correlation. 



