Here the time of fetal viability outside the womb may be considered. Langstein, of the Augusta Victoria Hospital in Berlin, reported[36] a study of the growth and nutrition of 250 prematurely born infants, and he found that a weight of 1000 grammes (2-1/5 pounds) and a full body length of 34 centimetres (13-3/5 inches) are the lowest limits for viability under proper circumstances. A fetus 1000 grammes in weight and 34 centimetres in length has completed the sixth solar month, or the sixth and a half lunar month; that is, it is viable at the _beginning_ of its seventh month, _servatis servandis_. [36] _Berliner klinische Wochenschrift_, June 14, 1915. The child at term, as a rough average, is from 48 to 52 centimetres (19 to 20-1/2 inches) in length, and it weighs from about 6-3/5 to 7-1/2 pounds. It is impossible, however, to obtain the sizes and weights of infants _in utero_ with scientific accuracy, because the date of conception cannot be determined with absolute certainty, and individual fetuses vary as do infants after birth. A full-term infant sometimes may weigh only 3-1/2 pounds when the mother is diseased, and again an eight-month fetus will weigh as much as 8 pounds. Large muscular and fat women have large babies; women of the well-to-do classes have larger babies than do the poor; women who work during gestation bear smaller babies than do those women that rest. Mothers who work in tobacco, lead, or phosphorus have puny babies; white children are larger at birth than negro children; boys at term are 3 to 5 ounces heavier than girls. Langstein says that prematurely born infants weighing from 900 grammes (31-1/2 ounces) to 1500 grammes (3-1/2 pounds)--that is, all born before the seventh solar month--must be kept in hot-water incubators in a room with ordinary ventilation. Babies weighing 2000 grammes (4-1/2 pounds) or more get along in an ordinary crib if they are kept surrounded with hot-water bags. Such children are to be fed with human milk through a catheter passed into the mouth or they die of inanition. Only a few of them are strong enough to suck from a bottle, and these give up the effort after a few days and die. They cannot utilize fat, even from milk; and all artificial food is dangerous. Most of the prematurely born become rachitic, and even human milk is not preventive of this condition. Rachitis is a constitutional disease, characterized by impaired nutrition of the bones and changes in their shape. In the third or fourth month craniotabes is frequent--that is, an atrophy of the skull bones with the formation of small conical pits. These infants show also a morbid tendency to convulsions--spasmophilia. Such diseases are caused by a lack of mineral salts, which normally are carried to the fetus by the placental blood during the last two months of gestation. Because of this lack premature infants require the administration of lime salts in their food; they also need iron because they are anemic.