Final answer:
A nurse should report blood-streaked vaginal discharge as a potential complication following an amniocentesis at 18 weeks of gestation, as it may indicate bleeding, infection, or damage to the uterus.
Step-by-step explanation:
The findings a nurse should report to the provider as a potential complication following an amniocentesis at 18 weeks of gestation is d) Blood-streaked vaginal discharge. While mild cramping and a clear amniotic fluid are normal post-procedure observations, and a fetal heart rate of 150 beats per minute is within the normal range for a fetus at this stage of gestation, the presence of blood-streaked vaginal discharge may signal a potential complication such as bleeding, infection, or damage to the uterus or surrounding structures and needs to be evaluated by the healthcare provider immediately.