Final answer:
To detect fetal heart tones at the end of the first trimester, a Doppler ultrasound stethoscope should be placed just above the symphysis pubis.
Step-by-step explanation:
During the first trimester of pregnancy, to assess fetal heart tones using a Doppler ultrasound stethoscope, the nurse should begin by placing it just above the symphysis pubis. At the end of the first trimester, the uterus is still a pelvic organ and has not risen much above the pubic bone. Listening to fetal heart tones is a crucial aspect of prenatal care, and proper stethoscope placement is important for obtaining an accurate reading.