Final answer:
The third trimester is when a pregnant woman most often feels a backache due to the significant expansion of the uterus and rapid weight gain. The large weight and space taken up by the growing fetus can strain the back, leading to pain and discomfort.
Step-by-step explanation:
The trimester in which a pregnant woman most feels a backache is the third trimester. As the pregnancy progresses into the third trimester, which occurs during weeks 29 through birth (around 40 weeks), the uterus expands significantly, occupying a large part of the woman's abdomen. This plus the more rapid weight gain leads to physical changes that can cause discomfort.
In the third trimester, as the fetus grows larger, its weight and the physical space it takes up can lead to several symptoms for the mother. Notably, back pain may occur due to the change in the woman's center of gravity and the additional pressure on her back muscles. The woman's lower extremities may also swell, and she may experience more frequent urination, varicose veins, and heartburn. Towards the end of this trimester, the fetus tends to turn into a downward position in preparation for birth, which can relieve some pressure on the upper abdomen but simultaneously place more pressure on the pelvic floor and rectum.