These two case reports confirm that sirolimus administered orally to the pregnant woman transfers from the maternal to the fetal circulation and may be used in the treatment of extensive, complicated fetal CLVMs in utero.

Prenatal sirolimus treatment may improve the prognosis for fetuses with complicated CLVMs causing fetal distress, such as bleeding and anemia, and seems to be safe both for the pregnant woman and the fetus.

On the basis of our findings, we consider that maternal trough sirolimus levels from 7 to 12 ng/mL may be sufficient to achieve therapeutic sirolimus levels in the fetal circulation.