Final answer:
A decrease in amniotic fluid during the last trimester of pregnancy is called oligohydramnios, which can lead to complications such as hypoplastic lungs. This is different from placenta previa, which involves the placenta covering the cervix.
Step-by-step explanation:
A condition occurring most during the last trimester in pregnancy characterized by a decrease in the amount of amniotic fluid is called oligohydramnios. This condition can have serious consequences for the fetus, including the development of contractures of the limbs, clubbing of the feet and hands, and a life-threatening condition known as hypoplastic lungs. Oligohydramnios is in contrast to polyhydramnios or hydramnios, where there is too much amniotic fluid. Both conditions can indicate potential complications for the mother and baby, although most pregnancies proceed normally and result in the birth of a healthy baby.
Placenta previa is a different condition where the placenta partially or completely covers the cervix, leading to potential complications such as antepartum hemorrhage. This latter condition is not related to the quantity of amniotic fluid but to the placement of the placenta.