The umbilical vein carries oxygen-rich blood from the placenta to the fetus. According to Cleveland Clinic, a typical umbilical cord contains three blood vessels: one umbilical vein, which brings oxygenated blood to the fetus, and two umbilical arteries, which return oxygen-poor blood from the fetus to the placenta.
This is the reverse of the usual pattern after birth, when veins generally carry oxygen-poor blood. The vessels are cushioned by a jelly-like tissue called Wharton’s jelly, and once the baby is born and breathing, the cord is no longer needed and is cut.
Source: Cleveland Clinic: Umbilical Cord
Image: Cross section of a human umbilical cord, by Stefańska K, Ożegowska K, Hutchings G, et al., CC BY 4.0, via Wikimedia Commons.