Superfetation

SUPERFETATION, supergeneration, the fertilization of two ova from different ovulatory periods, i.e., conception during an already existing pregnancy. The possibility of superfetation in animals is absent, since pregnant female animals as a rule do not allow the male to approach them. In humans, not a single completely convincing case of superfetation has been described up to the present time. Usually, those binovular twins in which the twins differ sharply from each other in size, i.e., in length and weight, are taken and described as supergeneration, but such a difference by no means indicates a difference in age, since even monozygotic twins can differ sharply from each other in their development. For example, cases have been described where twins of undoubtedly the same age differed from each other by almost 2 kg in weight and by several centimeters in length. The following arguments speak against the possibility of superfetation: 1) the universally recognized absence of ovulation during pregnancy, as a result of the inhibitory effect of the active corpus luteum on the ovulatory activity of the ovary (true, this position was somewhat shaken by the experiments of Zondek and Aschheim, who managed to induce the rupture of follicles in pregnant mice by administering the hormone of the anterior pituitary lobe; but it is still a question whether this rupture was a true ovulation?); 2) the (supposed) impossibility for spermatozoa to enter the uterus during pregnancy due to the proliferation of the mucous glands of the cervical canal and the abundant cervical mucus resulting from this, which tightly seals the canal of the cervix and consequently the access to the uterine cavity; 3) the proliferation of the parietal decidua around the tubal openings, whereby the decidua apparently even fuses at the entrance to the tube and thus access to the tube is hermetically closed to the spermatozoa even if they should penetrate into the uterine cavity; 4) the impossibility of implantation of an ovum into a well-developed decidual membrane; 5) after 3 months of pregnancy, supergeneration is already completely impossible, since by this time the ovum entirely fills the uterine cavity, and the decidua covering the ovum (decidua capsularis) fuses with the parietal decidua (decidua parietalis)—in other words, not even a slit-like space remains of the uterine cavity. If one accepts the listed arguments as fully conclusive, there still remains the question of the possibility of supergeneration in extrauterine pregnancy (a new uterine or a second extrauterine pregnancy in the presence of an already existing extrauterine pregnancy) or in a double uterus. However, even such cases, insofar as they are described insufficiently fully, do not yet make it possible to resolve the question of the possibility of supergeneration in humans.