Superfætation, or the circumstance of two distinct conceptions occurring in the same woman at a considerable interval, so that two fetuses of different ages—the offspring possibly of different fathers—may co-exist in the uterus, is a subject of great interest both in a scientific and in a medico-legal point of view. Two centuries ago there was a universal belief in not only the possibility but the comparative frequency of this occurrence. Early in the 19th century it was as universally disbelieved; and now again (owing to the investigations of various inquirers) we are returning to the belief of our ancestors. The cases described as instances of superfætation may be arranged in three classes; but it is only to the cases of the third class that the term superfetation is truly applicable. The first class includes the numerous undisputed cases in which two mature children, bearing evidence, from their different colours, that they are the offspring of different parents, are born at the same time. (In the slave states of America it was by no means uncommon for a black woman to bear at the same time a black and a mulatto child—the former being the offspring of her black husband, and the latter of her white lover; and the converse has occasionally occurred—a white woman at the same time bearing a white and a mulatto child.) There is no difficulty in accounting for these cases, which may be examples of nearly contemporaneous conception rather than true superfetation. The second class includes those cases in which a twin has aborted, leaving its fellow undisturbed in the uterus, to be matured and born in due time, or in which twins have been produced at the same time, one of which was fully formed, while the other was small and apparently premature, from being 'blighted' or arrested in its development at an early period, or in which the birth of two children, both mature or nearly so, is separated by a short interval. Cases of these kinds are by no means rare; but there is no reason for believing that the infants were conceived at different periods.
The third class only presents serious difficulty. 'In a case of genuine superfetation,' says Dr Bonnar of Cupar, 'a woman must bear two (or more) mature children, with an interval of weeks or months between the birth of each; or, if she part with the whole contents of the uterus at the first delivery, the difference of the ages of the foetuses, or the mature child and the foetus, as the case may be, must be unmistakable, and there must be the absence of all marks of blight of the latter, so as to leave no doubt that, had it remained in utero, it would have gone on to perfect maturity.' Several apparently well-authenticated cases are on record where a second living child has been born three, four, or five months after the first; and these seem inexplicable on any hypothesis except that a second conception has taken place during the progress of the first pregnancy. Theoretical objections to the possibility of such an occurrence cannot outweigh the recorded facts, and by some of the highest authorities are not considered valid for the early months of pregnancy. Cases, moreover, of double uterus occasionally occur; and in the absence of clear evidence to the contrary in any particular case it is possible that the second conception may have taken place in the unoccupied division of such an organ. See Taylor's Medical Jurisprudence for further details.