Which of the following describes some of the changes that occur during the third trimester of pregnancy? a) the fetus doubles in mass, and the body begins to store nutrients and minerals within its tissues. b) cells divide into three cell layers during gastrulation, and the heart can be heard with a stethoscope. c) the ectoderm, mesoderm, and endoderm are made, and most of the major organs become fully formed. d) bone starts to replace cartilage for form the skeleton, and the fetus begins to form and discharge urine.