Twin to twin transfusion syndrome (TTTS) is a disease affecting maternal or identical twin pregnancies who share a common monochorionic placenta.

There are two kinds of twin:

1. Those from a single fertilized egg and sperm: identical (monozygotic) twins 
2. Those from two different fertilized eggs and sperms: non-identical (dizygotic) twins.

All dizygotic twins have two separate placentas and membranes, all are dichorionic and diamniotic.

Monozygotic twins can be:

• Dichorionic-Diamniotic if they split before days 2-3 after fertilization
• Monochorionic-Diamniotic if they split between day 3-8 after fertilization
• Monochorionic-Monoamniotic if they split after 8 days after fertilization

All monochorionic twins, those that split after 3 days after fertilization share a placenta. The shared placenta may contain abnormal blood vessels, which connect the umbilical cords and circulations of the twin and which may exchange blood from one twin to the other twins leading to TTTS a condition that is potentially life-threatening to one or both twins. 

Depending on the number, type and direction of the connecting vessels, blood can be transfused disproportionately from one twin (the donor) to the other twin (the recipient). The recipient could produce more urine than usual and develop polyhydramnios (too much amniotic fluid).

TTTS can occur at any time during pregnancy, and the earlier it is diagnosed the more problematic it could be. Chronic TTTS describes those cases that appear early in pregnancy (12-26 weeks’ gestation).  Without treatment, most twins with early onset chronic TTTS  would not survive and of the survivors, most would have handicaps or birth defects.

Acute TTTS describes those cases that occur suddenly, whenever there is a major difference in the blood pressures between the twins. This may occur in labor at term, or during the last third of pregnancy whenever one twin becomes gravely ill or even passes away as a result of the abnormalities in their shared placenta. Survival is usually better in acute late-pregnancy TTTS if diagnosed early.

The Quintero staging of twin-twin transfusion syndrome

 

Stage I

MVP of amniotic fluid MVP <2 cm in donor sac; MVP >8 cm in recipient sac

Stage II

Fetal bladder Symptoms of Stage I except Donor has no measurable fluid, Nonvisualization of fetal bladder in donor twin over 60 min of observation

Stage III

Symptoms of Stage II with Doppler anomalies in the Umbilical artery, ductus venosus, and umbilical vein Absent or reversed umbilical artery diastolic flow, reversed ductus venosus a-wave flow, pulsatile umbilical vein flow

Stage IV

Symptoms of Stage III with Fetal hydrops Hydrops Fetalis in one or both twins

Stage V

Single or Double Loss Fetal demise of one or both twins
MVP = maximal vertical pocket

Management of a monochrionic pregnancy usually includes close observations, staging, frequent ultrasound examinations, early delivery or treatment with laser to occlude blood vessels shared between the twins.