The Quintero classification uses ultrasound findings to describe the severity of TTTS:

In stage I, there is severe polyhydramnios (increased amniotic fluid) around the recipient and severe oligohydramnios (decreased amniotic fluid) around the donor, but the donor is still able to produce enough urine to fill its bladder which is therefore visible on ultrasound.

In stage II, the donor is no longer able to fill its bladder. The bladder is not visible on ultrasound.

In stage III, there are 'critically abnormal Dopplers'. This means that the blood flow pattern becomes severely abnormal in one or more of: the umbilical arteries, umbilical veins, or the ductus venosus (an important blood vessel near the liver in an unborn baby).

In stage IV, there are now obvious signs of heart failure in one or both twins seen as fluid accumulating around the heart or lungs, in the abdomen, or under the skin.

Clearly, stages III and IV are more severe than stages I and II. The prognosis is better for stage I than for higher stages.

Unfortunately it is very difficult to predict how a particular case will progress. Some twins have been seen to remain in stage I for many weeks or even improve on their own, while others have progressed very rapidly to a higher stage sometimes even skipping some stages. This is the reason you will be asked to return for frequent review by ultrasound once the diagnosis of TTTS has been made. The treatment option that is best for you will depend on the stage, and how early in your pregnancy TTTS is diagnosed. Laser treatment, for example, would mostly be offered between 16 and 26 weeks' gestation to stages III and IV, and sometimes stage II.

The risk of the disease progressing (getting worse), remaining stable or regressing (getting better) is summarised for each of the stages below: