Over the last 20 years, the following factors have come to be thought important in predicting outcomes in TTTS:

  • Gestation Age at Diagnosis: Prior to 25 weeks is more serious because the babies cannot be delivered at this time, and they will be exposed to the syndrome longer. Most calls to the Foundation are from couples at eighteen weeks’ gestation.
  • Gestational Age at Delivery: At 28 weeks and beyond, or with an estimated birth weight of 1500gm (3lbs. 5oz.) or more, doctors become more optimistic regarding the outcome for the twins with delivery. In TTTS, the doctors are often faced with the question, ‘Are the babies better off out than in?’ as they trade off the risks of early delivery versus continuing a TTTS pregnancy.
  • Degree of Growth Discordance: This implies that the babies are found to be different sizes on the ultrasound scan. A difference of over 20% is though significant, but this depends on gestational age that the difference appears (sometimes the difference is given in weeks rather than a percentage). The twin’s size difference may be due to either the transfusion of nutrients or unequal sharing of the common placenta or both.
  • Degree of Discordance in Amniotic Fluid: The recipient may have quarts of excess amniotic fluid (polyhydramnios) and its bladder always appears full on ultrasound scan. The donor may produce so little urine that its amniotic sac may be empty (oligohydramnios) and the baby’s bladder impossible to see with ultrasound.
  • Presence of Hydrops in One Twin: Hydrops implies fluid buildup in the baby’s skin and body cavities, and is usually due to heart failure. It can be seen on ultrasound, and it usually involves the recipient twin who is overwhelmed with too much blood.
  • Status of The Uterine Cervix: The cervix can be damaged by the excess amniotic fluid (polyhydramnious), typical of TTTS. A "short" or "funneled" cervix can be seen by ultrasound. This can lead to pregnancy loss.
  • Maternal Nutritional Abnormalities: Most TTTS moms show signs of anemia and low blood proteins. These can directly and indirectly affect the twins and the mother's health as the pregnancy and TTTS treatments continue.
  • TTTS outcomes are ultimately determined by the number and type of connecting blood vessels, and the way the twins share the placenta (which both occur randomly). Since no two placentas are the same, the outcome is always hard to predict.
  • Regardless of the therapy chosen, the majority of TTTS babies survive and the majority of survivors will be normal. However, the various treatments available do differ in their outcomes: the number of survivors, the number of healthy survivors, and the ability to prolong pregnancy.
  • The majority of TTTS twins, with and without treatment, will be born prematurely and need to spend some time in the newborn intensive care unit.
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