The trophoblast (from Greek trephein: to feed; and blastos: germinator) is the outer layer of cells of the blastocyst.

[2][3] They form during the first stage of pregnancy and are the first cells to differentiate from the fertilized egg to become extraembryonic structures that do not directly contribute to the embryo.

Trophoblasts are specialized cells of the placenta that play an important role in embryo implantation and interaction with the decidualized maternal uterus.

Some of these trophoblasts even replace the endothelial cells in the uterine spiral arteries as they remodel these vessels into wide bore conduits that are independent of maternal vasoconstriction.

This ensures that the fetus receives a steady supply of blood, and the placenta is not subjected to fluctuations in oxygen that could cause it damage.