Which organ forms during pregnancy and attaches to the uterus?

The placenta is the organ that forms during pregnancy and attaches to the uterus. Its primary function is to facilitate the exchange of nutrients, gases, and wastes between the mother and the developing fetus. The placenta allows for the transfer of oxygen and nutrients from the mother's bloodstream to the fetus while simultaneously removing carbon dioxide and waste products from the fetal blood. This organ is crucial for the sustenance of the fetus throughout pregnancy and plays a significant role in hormones that support pregnancy.

The ovary does not attach to the uterus; rather, it is responsible for producing eggs and hormones. The fallopian tubes are the structures through which eggs travel from the ovaries to the uterus, but they do not form or attach during pregnancy themselves. The amniotic sac encloses the developing fetus in amniotic fluid but is not the organ that attaches to the uterus and performs the vital role of nutrient and gas exchange.