Heredity and multiparity seem to be the only recognized predisposing factors. The more pregnancies a woman has, the more liable she is to have twins. Twins may occur through a division of the primitive cell through the fertilization of two ova from the same or different ovaries, or by fertilization of a single ovum having two nuclei. (See Fig. 13). The former are called binovular twins, and may or may not be of the same sex. The latter are called uniovular twins and are always of the same sex. Twins are usually somewhat smaller than a single child, and frequently associated with hydramnios. Binovular twins have separate placentæ and uniovular twins have one placenta, with separate cords. Twin pregnancies usually go into labor earlier than the single child, possibly on account of the over-distention of the uterus. _The diagnosis_ is occasionally difficult and at other times easy. Two sets of heart tones must be distinguished and differentiated by their variation in frequency, heard at the same time by different observers. The presence of twins may be strongly suspected also when the external measurements of child and uterus greatly exceed the average. In such cases a systematic and persistent search must be made for the two fœtal heart tones. _The delivery_ is generally uncomplicated, unless the chins become locked. =Displacements of the Uterus.=—In most cases displacements of the uterus are a consequence of conception in organs that are previously retroflected or retroverted. They rarely produce symptoms until the end of the third month, when the attention is directed to the bladder. There may be absolute retention or a constant dribbling from a full bladder (ischuria paradoxa), possibly associated with pain. If recognized early, an attempt should be made to replace the uterus by posture (knee chest) and when replaced, to hold it by pessary or tampon. The prone position in bed will aid. After retention has occurred, the patient should be put to bed and the bladder catheterized regularly every eight or ten hours for three or four days. As a rule, the organ will rise spontaneously into the abdomen. If it does not, it is probably incarcerated under the promontory, and the physician must try to replace the uterus by manipulation or by continuous pressure, but in bad cases, he will empty the uterus before the condition of the patient becomes too serious. In multiparas with weak abdominal walls, or women with spinal curvature or contracted pelves, the uterus may fall forward and, passing between the recti muscles, continue to drop until the fundus lies lower than the symphysis pubis. _Management_, until labor occurs, may be made more effective by using a strong, well-fitting abdominal bandage.