Final answer:
Hemopoiesis during the second trimester in a fetus primarily takes place in the bone marrow. The bone marrow begins to function in this role by the end of the first trimester, continuing throughout fetal development.
Step-by-step explanation:
Hemopoiesis, or the formation of blood cellular components, occurs in various sites within the body during fetal development. During the second trimester, hemopoiesis primarily occurs in the bone marrow. Early in fetal life, this process is carried out by the yolk sac, and then by the liver and spleen. As development progresses, the bone marrow gradually takes over this crucial function.
The transition to the bone marrow beginning this process typically starts around the end of the first trimester and is well established by the second trimester. By the end of week 12 of fetal development, the bone marrow is actively involved in the production of erythrocytes, a task previously managed by the liver.
All organs and structures, including the bone marrow, continue to develop during the second trimester, and the fetus experiences a significant amount of growth, eventually reaching about 30 cm (12 inches) in length by the end of this trimester. The placenta also plays a critical role in supporting the developing fetus by taking over the functions of nutrition and waste management as well as producing hormones such as estrogen and progesterone.