At what gestational age should cardiac activity be visible on fetal ultrasound (US)?

From the Guidelines

Cardiac activity on fetal ultrasound should be visible by 6 weeks of gestation. Before this timeframe, you may only see a gestational sac and yolk sac without cardiac activity, which is normal in very early pregnancy. The fetal heart begins to beat around 22-23 days after conception, but conventional transvaginal ultrasound typically cannot detect this activity until a few weeks later when the embryonic structures are large enough to visualize.

Key Points to Consider

• If no cardiac activity is seen by 6 weeks gestation on a transvaginal ultrasound performed by an experienced sonographer with appropriate equipment, this raises concern for a non-viable pregnancy 1.
• Accurate dating is crucial, as miscalculating gestational age can lead to unnecessary worry.
• The development of cardiac activity represents a significant milestone in early pregnancy, as the presence of fetal heart motion significantly increases the likelihood of ongoing pregnancy, with miscarriage rates dropping substantially once cardiac activity is documented.
• According to the most recent study by the Society of Radiologists in Ultrasound consensus conference recommendations 1, cardiac activity should be reported as cardiac activity, or alternatively as cardiac motion, and should be visible on transvaginal ultrasound at 6 weeks GA.

Important Considerations for Clinical Practice

• It is essential to use the crown-rump length (CRL) to date the pregnancy, as it is more accurate than the mean sac diameter.
• The absence of cardiac activity in an embryo measuring 7 mm in CRL confirms embryonic demise, but absence of cardiac activity in embryos < 7 mm is still worrisome for embryonic demise, and the patient should generally be re-evaluated with a follow-up US in 7 to 10 days 1.

From the Research

Cardiac Activity on Fetal US

• Cardiac activity can be detected on fetal ultrasound as early as 6 weeks of gestation, but the ability to image fetal heart structures between 6 and 8 weeks is currently nondiagnostic 2.
• A four-chambered heart can be identified in 52% of patients in the eighth week, improving to 80% in the 10th week and 98% by the 11th week 2.
• The use of color Doppler (CD) significantly increases the detection of cardiac structures on early fetal echocardiography, and can assist with definition of cardiac anatomy at all gestational ages 2.
• The ideal timing of complete early fetal echocardiography, excluding pulmonary vein assessment, appears to be after 11 weeks' gestational age 2.

Detection of Cardiac Activity

• Handheld abdominal ultrasound can detect fetal heartbeats in 73% of healthy pregnancies, with a positive predictive value of 100% and a negative predictive value of 38% 3.
• From gestational week 7, handheld abdominal ultrasound can confirm vitality in 51/54 patients, with a positive predictive value of 100% and a negative predictive value of 79% 3.
• Transvaginal ultrasound is the preferred route for fetal cardiac examination prior to 12 weeks of gestation, whereas after 12 weeks, the fetal heart can be reliably evaluated by transabdominal ultrasound 4.

Factors Affecting Detection

• The accuracy of early detection of congenital heart defects is highly related to the experience of the operator 4.
• The type and frequency of ultrasound equipment, maternal anatomical characteristics, and the visual acuity of observers can affect the detection of cardiac activity 5.
• Obesity, coexisting fibroids, and uterine version can limit the use of transabdominal ultrasound, making transvaginal ultrasound a better option in these cases 6.