Vanishing twin
A vanishing twin is a fetus in a multi-gestation pregnancy which dies in utero and is then partially or completely reabsorbed by the mother Template:Harv.
The occurrence of this phenomenon is sometimes referred to as twin embolisation syndrome or vanishing twin syndrome (VTS), since the 1980s when twin pregnancies were made visible early on by means of ultrasound.  The term wombtwin has been used since 2003 to describe the deceased embryo or fetus.
Occasionally, rather than being completely resorbed, the dead fetus will undergo mechanical compression by its wombmate(s), resulting in a flattened, parchment-like state known as fetus papyraceus Template:Harv.
If the fetus is absorbed completely, there are usually no further complications to the pregnancy, other than first trimester vaginal bleeding Template:Harv. However, if the event occurs in the second or third trimester, serious complications may include premature labor, infection due to the demise of the fetus, and hemorrhage. Even at the end of the pregnancy, a low-lying fetus papyraceus may block the cervix and require a cesarean to deliver the living twin.
The wombtwin can die owing to a poorly implanted placenta, a developmental anomaly that may cause major organs to fail or be missing completely, or there may be a chromosome abnormality incompatible with life. Frequently the twin is a blighted ovum, one that never developed beyond the very earliest stages of embryogenesis.

# Frequency
According to Charles Template:Harvcoltxt, who is a professor in the Department of Pediatrics in the Brody School of Medicine and adjunct professor of biology at East Carolina University, vanishing twins occur in up to one out of every eight pregnancies and may not even be known in most cases.
However, Patricia J. Sulak, M.D. has reported otherwise. "In reporting 300 early pregnancies evaluated by means of ultrasound noted 21 twins, one triplet, and one quadruplet. Only three pairs of twins survived to term (85.6% loss of the second twin), whereas the triplet pregnancy evolved into a normal twin gestation, and the quadruplet pregnancy reported here also evolved into a singleton gestation. Additonally, the triplet pregnancy reported here also evolved into a pregnancy diagnosed by means of ultrasound that evolved into a twin gestation. These very high resorption rates, which cannot be explained on the basis of the expected abortion rate, again suggest intense fetal competition for space, nutrition, or other factors during early gestation, with frequent loss or resorption of the other twin(s)." Multiple gestation occurs at 7.6% in this particular study, and vanishing syndrome effect occurs within 85.6%. Therefore 6.88% of the 300 pregnancies experienced vanishing twin syndrome. Thus it seems more likely that VTS occurs 19.68 pregnancies out of 300, or about six out of every hundred pregnancies. 
 Article 7:  The Vanishing Twin: pathologic Confirmation of an Ultrasonographic Phenomenon
Sulak, M.D., L., Dodson, M.D., M.
Since it is hypothesized that in some instances vanishing twins leave no detectable trace at birth or before, it is impossible to say for certain how frequent the phenomenon is. It was hypothesized for a long time that non-righthanded and left-handed individuals may be the survivors of "mirror image" identical twinning Template:Harv but recent research does not seem to support that view Template:Harv.
"Vanishing" twins are frequently encountered in pregnancies created as a result of IVF. Ultrasound scans are taken very early in these pregnancies (5-8 weeks), so that, where a multiple conception has occurred, it frequently happens that more than one amniotic sac can be seen in early pregnancy, whereas a few weeks later there is only one to be seen and the other has "vanished" Template:Harv.

# Miscellaneous
It has been speculated that the children born of such a pregnancy may have some memories of their vanished twins, and may feel lonely because of this Template:Harv. There is no scientific evidence to support this claim.  Talk shows, such as Coast to Coast AM, have discussed the alleged phenomenon.
The vanishing twin syndrome has been cited by biotech company Acu-Gen as an ad hoc hypothesis to explain false results of the company's Baby Gender Mentor test. According to the company, on occasions where their pregnancy gender test has apparently given the incorrect gender of the fetus, the apparent mistake can be explained by a fetus having been present at the time of testing, but later being reabsorbed as a vanished twin. According to the company's critics, this excuse does not seem plausible.