Twin-to-twin transfusion syndrome

# Overview
Twin-to-twin transfusion syndrome (TTTS, also known as Feto-Fetal Transfusion Syndrome (FFTS) and Twin Oligohydramnios-Polyhydramnios Sequence (TOPS)) is a complication with high morbidity and mortality that can affect identical twin or higher multiple pregnancies where two or more fetuses share a common (monochorionic) placenta. Severe TTTS has a 60-100% mortality rate.

# Prevalence
TTTS is believed to affect roughly 1 in 1000 pregnancies.

# Etiology
As a result of sharing a single placenta, the blood supplies of monochorionic twin fetuses can become connected, so that they share blood circulation: although each fetus uses its side of the placenta, the blood vessels connecting the twins allow blood to pass from one twin to the other. Depending on the number, type and direction of the interconnecting blood vessels (anastomoses), blood can be transferred disproportionately from one twin (the "donor") to the other (the "recipient").  The transfusion causes the donor twin to have decreased blood volume, retarding the donor's development and growth, and also decreased urinary output, leading to a lower than normal level of amniotic fluid (becoming oligohydramnios).  The blood volume of the recipient is increased, which can strain the fetus's heart and eventually lead to heart failure, and also higher than normal urinary output, which can lead to excess amniotic fluid (becoming polyhydramnios).
In early pregnancy (before 26 weeks), TTTS can cause both fetuses to die, or lead to severe disabilities.  If TTTS develops after 26 weeks, the babies can usually be delivered and have a greater chance of survival without disability.
Other than requiring a monochorionic-diamniotic twin (or higher multiple) pregnancy, the causes of TTTS are not known.  It is not known to be hereditary or genetic.

## Imbalance in development
Some doctors recommend complete bed-rest for the mother coupled with massive intakes of protein (generally in the form of "protein shakes" such as Boost or Ensure) as a therapy to try to counteract the syndrome.  Theories for why this would be effective vary, but some doctors claim to have seen it help.  There are, however, no formal clinical trials indicating that the bed rest / high-protein diet is effective.
Research into TTTS is ongoing and best medical practices change quickly with respect to this condition.  For the most up-to-date information, consult with a maternal-fetal medicine specialist.

# Treatment
There are a number of different therapies used to treat TTTS, with varying rates of success.  The oldest, most traditional treatment is through serial amniocentesis, which involves periodically draining amniotic fluid from around the recipient twin in an effort to reduce the pressure of the amniotic fluid.  Because serial amniocentesis increases the risk of premature delivery, it has limited success when performed early in pregnancy, especially before fetal viability.  TTTS can also be treated by surgery during pregnancy, using fetoscopy to find the interconnecting blood vessels, and a laser beam to coagulate the blood in these vessels, blocking them.  This is called fetoscopic laser ablation, and is only performed in a few hospitals worldwide.  Outcomes vary widely from case to case, but as of this writing overall statistics of fetoscopic laser ablation indicate a 75% chance that at least one twin will survive. The overall survival rate is 50 - 60%.

# History
TTTS was first described by a German obstetrician, Friedrich Schatz, in 1875.

# TTTS in art
A painting known as the De Wikkellkinderen (The Swaddled Children), from 1617, is thought to represent a depiction of TTTS. The drawing shows twins that appear to be identical, but one is pale (possibly anemic), while the other is ruddy (possibly polycythemic).  Analysis of the family histories of the owners of the painting suggests that the twins did not survive to adulthood, although whether that is due to TTTS is uncertain.