When Does a Fetus Develop a Detectable Heartbeat?

Typically, fetal cardiac activity becomes detectable via transvaginal ultrasound between 5.5 and 6 weeks of gestation—measured from the first day of the woman’s last menstrual period (LMP). At this stage, the embryo is approximately 2 to 4 millimeters in crown-rump length, and the primitive heart tube has begun rhythmic contractions. By 7 weeks, a clear, sustained heartbeat is usually visible and audible on Doppler ultrasound in most pregnancies.

Transabdominal ultrasound may detect the heartbeat slightly later—often around 7 to 8 weeks—due to greater distance from the probe and variable maternal body habitus or uterine position. Absence of cardiac activity beyond 7 weeks’ gestation with a visible embryo measuring ≥7 mm warrants careful clinical evaluation, as it may indicate non-viable pregnancy, including early embryonic demise.

It’s important to note that “hearing” the heartbeat with handheld Doppler devices generally occurs later—typically between 10 and 12 weeks—and should not be used to confirm viability earlier in pregnancy. Confirmation of fetal viability relies on real-time visualization of cardiac motion on high-resolution ultrasound, not auditory detection alone.

Clinicians emphasize that gestational age must be accurately determined—preferably using early ultrasound biometry—since miscalculation based solely on LMP can lead to premature concern. Serial assessment may be indicated if initial findings are inconclusive, particularly in cases of uncertain dates or suboptimal imaging.