During which trimester are all major systems of the fetus developed?

The first trimester is the critical period during which all major systems of the fetus begin to develop. This trimester spans from conception through the end of week 12. Key developments include the formation of the heart, neural tube, limbs, and foundational components of the circulatory, digestive, and respiratory systems. By the end of the first trimester, essential organs are in place, setting the stage for further growth and maturation in the subsequent trimesters.

During the second trimester, while development continues and growth becomes more pronounced, many systems that began developing in the first trimester start to refine and enhance functional capabilities rather than undergo initial formation. The third trimester focuses largely on growth, preparation for birth, and further maturation of the already established systems. The preembryonic stage, which occurs during the first couple of weeks after conception, involves fertilization and initial cell division, but it is not the stage where major systems develop. Thus, the first trimester is uniquely critical for the foundational developments of all major fetal systems.