Final answer:
During the third trimester, a fetus experiences its most rapid period of growth, gaining approximately 3 to 4 kg and developing organ systems that continue to mature post-birth. The mother's increased weight is partly due to the growing fetus, the enlarged uterus, and changes in her own body, with only a moderate increase in caloric intake recommended.
Step-by-step explanation:
The fetus typically gains significant weight during the third trimester of pregnancy, approximately 3 to 4 kg (6.5 to 8.5 lbs.), and reaches about 50 cm (19-20 inches) in length. This period is marked by the most rapid growth phase during pregnancy, with extensive development and maturation of various organ systems, such as the nervous system and liver, which continue to develop even after birth. The mother may experience discomfort as the fetus and associated structures like the amniotic fluid and placenta contribute to maternal weight gain.
Although the demand for additional calories is moderate, with only an extra 300 calories per day suggested, the mother's weight gain is primarily driven by the growing needs of the fetus, along with the enlarged uterus, additional breast tissue, and increased blood volume. During the entirety of the third trimester, the fetus prepares for birth, with organ systems like the lungs maturing sufficiently to function outside the uterus. Ultimately, around the 40-week mark, the fetus's growth culminates as birth becomes imminent.