Final answer:
Manual assessment of fetal size and position is generally begun during the second trimester when the fetus has grown substantially and organ development is progressing rapidly. By the end of the second trimester, the fetus becomes active with noticeable movements and reaches about 30 cm in length.
Step-by-step explanation:
Manual assessment of fetal size and position typically begins during the second trimester. This period of fetal development is marked by rapid growth, where the fetus reaches about 30 cm (12 inches) by the end of the trimester. All organs and structures continue to develop, including the placenta, which becomes fully functional in taking over tasks such as nutrition support and waste removal, as well as hormone production.
The second trimester is also when fetal movements become noticeable to the mother. It's during weeks 21-30 that the fetus gains significant weight, important for maintaining body temperature after birth. By the end of the 30th week, the fetus has the proportion of a full-term newborn, though leaner, measuring about 28 cm (11 in) from crown to rump.