Final answer:
The least compressible and largest part of the fetus is typically the head, due to the presence of the fetal skull, which is rigid compared to other tissues of the body. By the third trimester, the fetus reaches a weight of about 2.5-4 kg and a crown-to-rump length of 35.5-40.5 cm, with significant fetal growth and organ development occurring throughout the term.
Step-by-step explanation:
When considering what is the least compressible and largest part of the fetus, we look towards the end of the fetal development stage. By the third trimester, the largest parts of the fetus are its head and body, which contribute most significantly to its size. The fetal head, particularly the skull, is the least compressible due to the presence of bone, which is more rigid than the soft tissues that make up the rest of the body. However, even the fetal skull has fontanelles — soft spots that allow for some compression during childbirth.
By the end of the fetal stage, particularly in the last several weeks before birth, the fetus has reached its largest form in terms of weight and length. At full-term, the fetus typically weighs approximately 2.5-4 kg (5.5–8.8 lbs) and has a crown-to-rump length of 35.5-40.5 cm (14–16 inches). Post-birth, the measurement changes from crown-to-rump to head-to-toe, and the average length is approximately 51 cm (20 inches).
The development of the fetus is essential, with organs like the heart and blood vessels being among the earliest to develop and function, albeit the circulation during the fetal phase differs from postnatal circulation. Fetal hemoglobin, which is distinct from adult hemoglobin, enables the fetus to extract oxygen from the mother's blood through the placenta. This supports fetal growth, which, barring any complications such as intrauterine growth restriction (IUGR), often proceeds linearly until just before birth, when growth rate slows.