Final answer:
By the 12th week of pregnancy, generally the intestines complete their retraction from the umbilical cord into the abdomen as the fetus grows and more space becomes available.
Step-by-step explanation:
By 16 weeks of pregnancy, the organ that becomes part of the abdomen is the intestines. During the sixth week of gestation, the gastrointestinal system develops too rapidly for the embryonic abdomen to accommodate it, leading to a temporary protrusion of the intestines into the umbilical cord. However, as the fetus grows and there is more room in the abdomen, the intestines retract from the umbilical cord and reside within the abdominal cavity, a process that is generally complete by the end of the 12th week.
It is critical to avoid exposure to toxins during this delicate stage of development since the organs, including the liver and lungs, are still maturing and are susceptible to external influences that can affect the fetus' survival. During weeks 9-12, the liver becomes capable of secreting bile and develops alongside other major organ systems.