At what gestational age can viability be seen on ultrasound?

From the Guidelines

Viability on ultrasound can typically be confirmed between 5.5 to 6.5 weeks of gestational age, as indicated by the presence of cardiac activity, which is the definitive sign of a viable pregnancy 1. This gestational age is based on the latest consensus recommendations from the Society of Radiologists in Ultrasound, which provide a standardized lexicon for first-trimester ultrasound terminology 1. At this stage, the embryo's cardiac activity becomes detectable, and this finding is considered the most reliable indicator of viability. Prior to this timeframe, an ultrasound may only show a gestational sac or a yolk sac, but these findings alone do not confirm viability. The timing can vary slightly based on factors such as the quality of the ultrasound equipment, the skill of the sonographer, and individual variations in early embryonic development. Some key points to consider when assessing viability on ultrasound include:

• Transvaginal ultrasound provides better visualization in early pregnancy compared to transabdominal ultrasound 1.
• If an ultrasound is performed too early and viability cannot be confirmed, a follow-up scan in 1-2 weeks is typically recommended rather than making immediate conclusions about pregnancy viability.
• The presence of a gestational sac, yolk sac, or embryo does not necessarily confirm viability, and cardiac activity must be detected to confirm a viable pregnancy 1. It's also important to note that the term "viability" should be avoided in the first trimester, as it implies the ability of the fetus to survive in the extrauterine environment, which is not applicable at this stage of development 1. Instead, the presence of cardiac activity should be reported as the indicator of a viable pregnancy.

From the Research

Gestational Age and Viability on Ultrasound

• The gestational age at which viability can be seen on ultrasound is a crucial aspect of prenatal care.
• According to 2, in the United States, viability presently occurs at approximately 24 weeks of gestational age.
• However, the study also notes that viability is not an intrinsic property of the fetus and should be understood in terms of both biological and technological factors.
• Another study 3 found that emergency physicians can quickly and accurately determine gestational age using ultrasound, with an overall accuracy of 96% in determining fetal viability.

Factors Affecting Viability

• The ability to confirm viability is significantly related to gestational age, as noted in 4.
• The chance of confirming viability increases rapidly per day of gestation until 49 days and thereafter plateaus.
• The study 4 suggests that in asymptomatic women with no previous ectopic pregnancy, ultrasound should be delayed until 49 days to reduce the number of inconclusive scans.

Ultrasound Estimation of Gestational Age

• Ultrasound estimation of gestational age can be done using various fetal measurements, including biparietal diameter, head circumference, abdominal circumference, and femur length, as discussed in 5 and 6.
• The accuracy of these measurements varies, with head circumference and corrected biparietal diameter being more accurate predictors of gestational age in the second trimester, and head circumference, corrected biparietal diameter, and femoral length being the best predictors in the third trimester, as found in 6.