What happens during the 4th to 6th months of pregnancy?

During the 4th to 6th months of pregnancy, one significant milestone is that all major organ systems in the fetus are typically formed and begin to mature. This period is often when the structures of the body become more defined, and although they may not be fully functional or mature yet, the foundational development has completed.

The fetus also begins significant weight gain, but this occurs more noticeably later in the pregnancy. Fetal movement, often referred to as "quickening," can generally be detected around the 18th to 20th week, which aligns with the 4th to 6th months, but it is not the main development focus of that time. Additionally, preparations for birth don't start until the later stages of pregnancy when the fetus undergoes final growth and positioning in preparation for delivery. Therefore, the correct answer highlights the completion of major developmental systems during this mid-term phase of pregnancy.