It is perhaps useful to distinguish those cases of premature expulsion which occur before from those which occur after the fourth month, inasmuch as they seldom prove dangerous before that time, from the diminutive size of the ovum and from the slight degree of development which the uterine vessels have undergone; whereas, after this period the hæmorrhage is more severe, and the general disturbance to the system greater. In other respects it will be more simple to divide premature expulsion of the ovum under two heads only; those cases which happen before the twenty-eighth week, or seventh month, being termed _abortions_, and after this period (as before) _premature labours_. This division is highly important in a practical point of view, since it marks the period before which the child has little chance of being born alive; whereas, after this date it may with care be reared.[61] A foetus may be expelled, at a very early stage of pregnancy, not only alive but capable of moving its limbs briskly for a short time afterwards, but it is unable to prolong its existence separate from the mother beyond a few hours. Cases do occur now and then where a child is born in the sixth month, and where it manages to struggle through, but these are rare, and must rather be looked upon as exceptions to the general rule. Abortions usually occur from the eighth to the twelfth week, a period which is decidedly the least dangerous for such accidents. "The liability to abortion is greater in the early than in the later periods of pregnancy; for as the union between the chorion and decidua is not well confirmed, as the attachment of the latter to the internal face of the uterus is proportionably slight, and as the extent of surface which the ovum now presents is very small to that which it offers in the more advanced state of pregnancy, and as it can of course be affected by smaller causes, it will be seen that a separation will be more easily induced, and prove much more injurious to the well-being of the embryo, than a larger one at another stage." (Dewees, _Compendious System of Midwifery_, § 929.) Abortions coming on at a later period, viz. from the sixteenth to the twenty-eighth week, which corresponds to the second division, or _miscarriages_, of the continental authors, are not only more dangerous than abortions at an early stage, for the reasons above-mentioned, but also than premature labours, as in this last division the uterus has attained such a size as to make the process rather resemble that of natural labour at the full term.