An NP is evaluating a patient who is at 16 weeks of gestation and is unable to auscultate fetal heart tones using the Doppler fetal monitor. Which of the following diagnostic tests is indicated for further assessment of this finding?

When fetal heart tones cannot be auscultated using a Doppler fetal monitor at 16 weeks of gestation, a formal ultrasound is the most appropriate next diagnostic step. By 10 to 12 weeks a handheld Doppler device should reliably detect fetal cardiac activity, so inability to find heart tones at 16 weeks raises concern for fetal demise, malpositioning, or other complications and requires immediate structural and viability assessment. Ultrasound provides real-time visualization of fetal cardiac motion, fetal size, placental location, and amniotic fluid volume, making it the definitive tool for this scenario. Doppler velocimetry evaluates blood flow patterns through fetal and placental vasculature and is used later in pregnancy to assess fetal compromise in conditions such as intrauterine growth restriction; it is not the initial test when fetal viability is uncertain. A nonstress test requires the fetus to be at a gestational age where reactivity is expected, typically after 28 weeks, and presupposes fetal viability — it cannot be used to confirm whether the fetus is alive. Rh(D) incompatibility screening is an important prenatal blood test but addresses isoimmunization risk, not assessment of fetal cardiac activity. Therefore formal ultrasound is both the safest and most informative immediate investigation.