WEIGHT, SIZE, AND AGE OF THE HUMAN EMBRYO. 155 



per cent in weight. Since the increment in sitting height for the fourteenth week 

 is 16 per cent, the existing variation of 8 per cent in length yields an accuracy of 

 about half a week. In the same manner the increment in weight for the fourteenth 

 week is 42 per cent. The weight variation at this time being 31 per cent, the result- 

 ing accuracy of age determination from weight would be three quarters of a week. 



At the seventeenth week the mean length of the fetus is 130 mm. and the mean 

 weight 150 mm. The normal range of length for this week extends from 125 to 

 135 mm., i. e., 10 mm., thus showing a variation of 7.6 per cent. Since the length 

 increment for a week at this time is about 10.7 per cent, an accuracy of about 5 

 days is yielded. The variation in weight for fetuses 130 mm. long is from 135 to 

 170 grams, a variation of 35 grams, or 23 per cent. The weight increment for a 

 week at this time is 28 per cent, which for purposes of age determination yields an 

 accuracy of 6 days as compared with three-fourths of a week at the fourteenth week. 



At the twentieth week the mean length of the fetus is 164 mm. and the mean 

 weight 316 grams. The normal range of length for this week extends from 155 to 

 172 mm., i. e., 17 mm., a variation of 10.3 per cent; whereas, under the conditions 

 of our examination, fetuses 164 mm. long have a normal range in weight from 275 

 to 367 grams, i. <?., 92 grams or a weight variation of 29 per cent. Since the incre- 

 ment of length for the twentieth week is 7 per cent and the increment in weight 

 20 per cent, the accuracy for age determination at this time is equivalent to If 

 weeks for both length and weight. 



At the twenty-fifth week the fetus has a mean length of 218 mm. and a mean 

 weight of 723 grams. Specimens of this week range in length from 207 to 229 mm., 

 i. c, a variation of 22 mm., or 10 per cent. On the other hand, fetuses 218 mm. 

 long weigh from 615 to 845 grams, a variation of 230 grams, or 31 per cent. The 

 increment of length for the twenty-fifth week being 4.6 per cent and the increment 

 of weight being 12 per cent, we have an accuracy for age determination of 2 weeks, 

 as based on length, and 2| weeks as based on weight. 



The fetus of the thirtieth week has a mean length of 265 mm. and a mean weight 

 of 1,323 grams. Our fetuses at this time range in sitting height from 252 to 277 

 mm., i. e., a variation of 25 mm. or 9.4 per cent. For a sitting height of 265 mm. 

 they range in weight from 1,150 to 1,550 grams, i. e., a variation of 400 grams 

 or 30 per cent. The increase in sitting height for the thirtieth week is 3.4 per cent 

 and the increment in weight is 11 per cent. This gives an accuracy for age deter- 

 mination of 2| weeks as based on sitting height and 3 weeks as based on weight. 



At the thirty-fifth week the fetus has a mean sitting height of 311 mm. and a 

 mean weight of 2,274 grams. The normal range in sitting height at this time is 

 from 295 to 327 mm., i. e., a variation of 32 mm. or 10 per cent. The range in 

 weight is from 1,970 to 2,650 grams, i. e., a variation of 680 grams or 29 per cent. 

 The increment in sitting height for the thirty-fifth week is 2.8 per cent and the 

 increment in weight is 9 per cent. Therefore, where the age of a fetus of this size 

 is estimated from sitting height alone, it can be done with an accuracy of 3f weeks, 

 and when done from weight alone it can be done with an accuracy of 3j weeks. 



In general it may be said that the normal variation in sitting height for any 

 age over 40 mm. is from 8 to 10 per cent, and that the normal variation in weight 



