What can signal the beginning of the third trimester in fetal development?

The third trimester of fetal development is marked by significant physiological changes in both the fetus and the mother. One of the key events that signal the onset of this stage is the transfer of maternal antibodies. During the third trimester, the fetus begins to receive immunoglobulin G (IgG) antibodies from the mother through the placenta. This immunological transfer is crucial as it helps to bolster the fetus's developing immune system and provides passive immunity against infections that the mother has immunity to.

While the formation of vital organs and the completion of organ systems are important developmental milestones, they generally occur earlier in fetal development, particularly in the first and second trimesters. The initiation of labor typically marks the end of the pregnancy rather than the beginning of the third trimester. Thus, the transfer of maternal antibodies is the defining process that indicates the third trimester is beginning, as it reflects the continued maturation and preparation of the fetus for life outside the womb.