If there is evidence of severe fetal anemia, because of either elevated MCA peak systolic velocity or development of fetal hydrops, management is strongly inluenced by gestational age. Fetal blood sampling and intrauterine transfusion are generally performed prior to 34 to 35 weeks (Society for Maternal-Fetal Medicine, 2015a). Intravascular transfusion into the umbilical vein under sonographic guidance is the preferred method of fetal transfusion. Transfusion into the fetal peritoneal cavity may be necessary with severe, early-onset hemolytic disease in the early second trimester, a time when the umbilical vein is too narrow to readily permit needle entry. With hydrops, although peritoneal absorption is impaired, some prefer to transfuse into both the fetal peritoneal cavity and the umbilical vein.
Transfusion is generally recommended only if the fetal hematocrit is <30 percent (Society for Maternal-Fetal Medicine, 2015a). Once hydrops has developed, the hematocrit is generally 15 percent or lower. The red cells transfused are type 0, D-negative, cytomegalovirus-negative, packed to a hematocrit of approximately 80 percent to prevent volume overload, irradiated to prevent fetal graft-versus-host reaction, and leukocyte-poor. he fetal-placental volume allows rapid infusion ofa relatively large quantity ofblood. Before transfusion, a paralytic agent such as vecuronium may be given to the fetus to minimize movement. In a nonhydropic fetus, the target hematocrit is generally 40 to 50 percent. he volume transfused may be estimated by multiplying the estimated fetal weight in grams by 0.02 for each 10-percent rise in hematocrit needed (Giannina, 1998). In the severely anemic fetus at 18 to 24 weeks' gestation, less blood is transfused initially, and another transfusion may be planned for approximately 2 days later. Subsequent transfusions usually take place every 2 to 4 weeks, depending on the hematocrit.
The MCA peak systolic velocity threshold for severe anemia is higher following an initial transfusion-1.70 MoM rather than 1.50 MoYI (Society for Maternal-Fetal Medicine, 2015a). It is hypothesized that the change in threshold compensates for the contribution ofdonor cells in the initial transfusion, because donor cells (from adults) have a smaller mean corpuscular volume. Alternately, the timing ofsubsequent transfusions is based on anemia severity and posttransfusion hematocrit. Following transfusion, the fetal hematocrit generally drops by approximately 1 percent per day. A more rapid initial decline may be encountered in the setting of fetal hydrops.
Outcomes. Procedure-related complications have declined signiicantly at experienced centers in recent years, with overall survival rates exceeding 95 percent (Zwiers, 2017). Complications include fetal death in approximately 2 percent, need for emergent cesarean delivery in 1 percent, and infection and preterm rupture of membranes in 0.3 percent each, respectively. The stillbirth rate exceeds 15 percent if transfusion is required before 20 weeks (Lindenberg, 2013; Zwiers, 2017).
Considering that fetal transfusion is potentially lifesaving in severely compromised fetuses, these risks should not dissuade therapy.
Van Kamp (2001) reported that if hydrops had developed, the survival rate approached 75 to 80 percent. However, of the nearly two thirds with resolution of hydrops following transfu sion, more than 95 percent survived. he survival rate was < 40 percent if hydrops persisted.