The component of the trophoblast bathed by maternal blood in the placenta (and thus covering the chorionic villi). Derived by differentiation and fusion of cytotrophoblast cells, so the syncytiotrophoblast only ever comprises just one cell (albeit with very many cell nuclei), through which almost everything reaching or leaving the embryo or fetus must pass. By the end of normal pregnancy, its area is about 12 square meters, or over 100 square feet. Most hormones and proteins produced by the placenta, such as progesterone, hCG and PAPP-A, come from it.

Cells in the developing placenta that are derived from the early embryo's outer layer (the trophectoderm of the blastocyst) and then persist through pregnancy, producing either syncytiotrophoblast or extravillous trophoblast.

The space in the placenta in which the maternal blood flows. Washes around the chorionic villi, which contain blood capillaries from the fetus, so that oxygen and nutrition pass to the fetus, while carbon dioxide and waste pass to the mother. Formed from lacunae and lined by the syncytiotrophoblast.

Latin for "lakes": one lacuna, two lacunae. Refers particularly to spaces that form within the syncytiotrophoblast (which therefore continues to line the spaces), separating the SCT into strips of land and little peninsulas that will develop into chorionic villi, as the lakes coalesce into one large intervillous space, into which maternal arteries will open, delivering blood to the placenta as it is being formed.

'Pregnancy-associated plasma protein A': a protein made by the syncytiotrophoblast of the placenta to increase its sensitivity to grow. Found in increasing levels in the pregnant woman's serum as pregnancy develops. Decreased if the pregnancy is affected by Down syndrome, therefore measured in the triple test used for fetal trisomy 21 screening.