During which trimester do a baby's organs and body structures begin to develop?

The first trimester is the crucial period for the development of a baby's organs and body structures. During this time, which spans from conception to approximately 12 weeks, all major organ systems begin to form. This includes the heart, brain, spinal cord, and other vital structures. By the end of the first trimester, the fetus has developed a more recognizable human shape and most of the foundational systems are in place, even though they will continue to mature throughout the pregnancy.

The second trimester focuses more on growth and refinement of the structures already formed, while the third trimester is primarily about further growth and development, including fat accumulation and maturation of organ functions. Saying that development occurs throughout the pregnancy is correct in a general sense, but the foundational organ development specifically initiates in the first trimester, making this period particularly significant in fetal development.