Final answer:
During the first trimester of pregnancy, signs such as amenorrhea and nausea are commonly observed. Other indicators include tender nipple area, increased appetite, frequent urination, and potential weight loss due to morning sickness.
Step-by-step explanation:
The first trimester of pregnancy is the initial 12 weeks after fertilization. Some physiological changes that most likely indicate pregnancy during the first trimester include amenorrhea (missed menstrual period) and nausea. Other signs are tender nipple area, increased appetite, more frequent urination, and weight loss due to morning sickness.