Embryo Project Encyclopedia Articles

Fetus in fetu is a rare variety of parasitic twins , where the developmentally abnormal parasitic twin is completely encapsulated within the torso of the otherwise normally developed host twin. In the late eighteenth century, German anatomist Johann Friedrich Meckel was the first to described fetus in fetu, which translates to “fetus within fetus.” Fetus in fetu is thought to result from the unequal division of the totipotent inner cell mass , the mass of cells that is the ancestral precursor to all cells in the body. The unequal division is thought to occur during the formation of the blastocyst, which can also result in parasitic and conjoined twins . Fetus in fetu represents a developmental anomaly that has prompted developmental biologists to further examine the mechanisms for how twins arise.

Parasitic twins, a specific type of conjoined twins, occurs when one twin ceases development during gestation and becomes vestigial to the fully formed dominant twin, called the autositic twin. The underdeveloped twin is called parasitic because it is only partially formed, is not functional, or is wholly dependent on the autositic twin. In most cases, the phenotype of parasitic twins is one normal functioning individual with extra appendages or organs, leading to questions about whether or not the additional limbs and organs are in fact another person or just a mutation of the individual's body. Researchers think that parasitic twins result from mechanisms similar to those that produce Vanishing Twin Syndrome. On a developmental continuum with vanishing twin syndrome on one end and developmentally normal twins on the other, researchers propose that the patterns of conjoined twins fall in the middle.

Twin-to-Twin Transfusion Syndrome (TTTS) is a rare placental disease that can occur at any time during pregnancy involving identical twins. TTTS occurs when there is an unequal distribution of placental blood vessels between fetuses, which leads to a disproportionate supply of blood delivered. This unequal allocation of blood leads to developmental problems in both fetuses that can range in severity depending on the type, direction, and number of interconnected blood vessels.

In humans, multi-fetal pregnancy occurs when a mother carries more than one fetus during the pregnancy. The most common multi-fetal pregnancy is twins, but mothers have given birth to up to eight children (octuplets) from a single pregnancy. Multiple fetusus can result from the release of multiple eggs or multiple ovulations, the splitting of a single fertilized egg, and fertility treatments such as in vitro fertilization (IVF) which involves the insertion of many fertilized eggs into the mother's uterus. The specific ways that multiples are conceived determines the degree of relatedness between individuals within the set. Once conceived, there are many possibilities for arrangement of placentas, where the egg implants, and amniotic sacs. The detection of multiple fetuses can be made by using ultrasound technology, hormone testing, and through the discovery of multiple heart beats. Some multiple births may be deemed high-risk due to the number of fetuses, their arrangement, or due to complications during development.