Final answer:
The third trimester is characterized by the most rapid fetal growth, with the fetus reaching up to 4 kg and 50 cm in length. The mother experiences various symptoms due to the fetus's size and position. The trimester concludes with labor and delivery, with hormones driving the process.
Step-by-step explanation:
The third trimester of prenatal development is a period of significant growth and change both for the fetus and the expectant mother. During these weeks, which span from week 29 to birth at about 40 weeks, the fetus undergoes its most rapid growth, increasing in weight to 3 to 4 kg (6.5-8.5 lbs.) and reaching a length of about 50 cm (19-20 inches). The fetus's organ development continues intensively during this time, with some systems, such as the nervous system and liver, continuing to develop even after birth.
As the fetus grows, the mother may experience various symptoms, such as back pain, swelling of the lower extremities, frequent urination, varicose veins, and heartburn. The uterus expands significantly, causing the abdomen to grow and change shape. Eventually, the fetus will turn to a downward position preparing for birth, which can lead to increased pressure on the pelvic floor and rectum but may relieve pressure on the upper abdomen.