Final answer:
Most babies measure similarly until 20-22 weeks, when ultrasound becomes less accurate due to variable growth rates. Fetal measurements and menstrual history are then used to estimate the due date. The fetus is nearly ready for birth by week 38 and continues to grow until around week 40.
Step-by-step explanation:
Most babies measure the same until 20-22 weeks. At this point, ultrasound is not as accurate because growth may differ. After this point, fetal measurements and the mother's menstrual history are used to determine Estimated Due Date (EDD).
By the end of week 38, most organ systems are mature and the fetus is almost ready for birth, typically weighing about 6 pounds and measuring around 18.5 inches long. The fetus continues to gain weight and grow in length until approximately 40 weeks, reaching an average length of approximately 51 cm (20 in.) and a weight range of approximately 2.5-4 kg (5.5-8.8 lbs).
Fetal growth and development are rapid and critical from the end of the eighth week until birth. By the end of week 37, all of the fetus's organ systems are developed enough that it could survive outside the uterus without many of the risks associated with premature birth.