Three random events occur in identical (monozygous) twins that determine whether the twins develop TTTS: the timing of the twinning process which determines placental type, the number, type and direction of the connecting vessels, and the degree to which the twins share their common placenta equally or asymmetrically. Most identical twins have a monochorionic placenta and approximately 15% develop TTTS. The placenta findings determine when in pregnancy TTTS occurs, the degree and severity of the transfusion, and the outcomes (survival with and without abnormalities in the babies) with various treatments. In addition to TTTS, the placental abnormalities can cause the early loss of one twin, as well as developmental birth defects in one twin. Couples with monochorionic twins are at high risk and should be examined often by their physician, have frequent ultrasound scans (The TTTS Foundation advocates weekly ultrasounds from 16 weeks through delivery), and plan to have careful placental examination after delivery.