What weeks of a pregnancy fall within the third trimester?

The third trimester of pregnancy begins after about 26 to 28 weeks of pregnancy and lasts until birth. According to the Mayo Clinic, it is the final development stage before delivery.

WebMD lists several discomforts women may feel during the final weeks of pregnancy, including backaches, vaginal discharge or bleeding, breast enlargement and contractions. For the baby, eyes will open, bones and the nervous system will develop, fingernails grow, and he or she will begin to practice breathing. This is also a critical time to keep up with healthcare appointments, as a doctor may screen a mother-to-be for gestational diabetes, anemia and Group B strep.