Webster's New World Medical Dictionary

placebo response
A positive medical response to
taking a placebo, as if it were an active medication. 
placenta
A temporary organ that joins the mother
and fetus, transferring oxygen and nutrients from
the mother to the fetus and permitting the release of
carbon dioxide and waste products from the fetus.
The placenta is roughly disk-shaped, and at full
term it measures about 7 inches in diameter and
slightly less than 2 inches thick. The upper surface
of the placenta is smooth, and the under surface is
rough. The placenta is rich in blood vessels. The
placenta is expelled with the fetal membranes dur-
ing the birth process; together, these structures
form the afterbirth.
placenta, accessory
A condition in which there
is an extra placenta that is separate from the main
placenta. Also known as a succenturiate or super-
numerary placenta.
placenta, low
See 
placenta previa.
placenta, succenturiate
See 
placenta, accessory.
placenta, supernumerary
See 
placenta,
accessory.
placenta accreta
The abnormal adherence of the
chorion of the placenta to the myometrium of the
uterus. Normally there is tissue intervening between
the chorionic villi and the myometrium, but in
placenta accreta, the vascular processes of the
chorion grow directly in the myometrium. Placenta
accreta can progress into placenta percreta.
placenta percreta
A condition in which the pla-
centa invades the uterine wall. In placenta percreta,
the vascular processes of the chorion (chorionic
villi), a fetal membrane that enters into the forma-
tion of the placenta, can invade the full thickness of
the myometrium. This can cause an incomplete rup-
ture of the uterus. The chorionic villi can go right on
through both the myometrium and the outside cov-
ering of the uterus (serosa), causing complete and
catastrophic rupture of the uterus.
placenta previa
A condition in which the pla-
centa is implanted near the outlet of the uterus, so
that at the time of delivery the placenta precedes the
baby. Placenta previa can cause painless bleeding in
the last trimester of pregnancy, and it may be a rea-
son to perform a C-section. Also known as low
placenta.