Can a fetal sac be seen on ultrasound at 9 weeks and 2 days of gestation?

From the Guidelines

A fetal sac should definitely be visible on ultrasound at 9 weeks and 2 days of gestation. By this stage of pregnancy, not only should the gestational sac be clearly visible, but you should also be able to see the embryo (now nearly a fetus), a yolk sac, and cardiac activity 1. The embryo typically measures about 23-25mm in length at this stage. The gestational sac appears as a round or oval fluid collection surrounded by a hyperechoic rim of trophoblastic tissue, and the embryo appears as a small structure within this sac 1. Cardiac activity, reported as rhythmic pulsations in the embryo/fetus, should be detectable, usually at a rate that can be measured using M-mode or cine clip 1. If these structures are not visible at 9 weeks and 2 days, it could indicate potential concerns such as incorrect dating of the pregnancy, early pregnancy loss, or other complications.

Some key points to consider when evaluating an ultrasound at this stage of pregnancy include:

• The gestational sac is typically visualized at around 5 weeks gestational age (GA) as a round or oval fluid collection surrounded by a hyperechoic rim of trophoblastic tissue 1.
• The yolk sac is visualized at approximately 5½ weeks GA and confirms a definite gestational sac/pregnancy 1.
• The embryo is typically visible by 6 weeks GA, and cardiac activity should be clearly detectable by 6 weeks GA 1.
• The mean sac diameter can be used to calculate the gestational age before visualization of an embryo, but once the embryo is measurable, the crown-rump length should be used for dating as it is more accurate 1.

It's essential to follow the Society of Radiologists in Ultrasound consensus conference recommendations for a lexicon for first-trimester ultrasound to ensure accurate interpretation of ultrasound findings 1.

From the Research

Fetal Sac Visibility on Ultrasound

• The visibility of the fetal sac on ultrasound at 9 weeks and 2 days of gestation can be determined by considering the gestational sac growth and its correlation with gestational age 2.
• A study conducted in 1991 found that a gestational sac could be identified at 5 weeks' gestation using transvaginal ultrasound, and its diameter increased linearly with gestational age 2.
• Another study published in 1985 correlated maternal serum human chorionic gonadotropin (HCG) levels with sonographic uterine findings and found that a gestational sac was always seen when the HCG level was greater than 1800 mIU/ml 3.
• However, the studies do not provide specific information on the visibility of the fetal sac at 9 weeks and 2 days of gestation.

Gestational Sac Growth and Ultrasound Detection

• The growth of the gestational sac and its detection by ultrasound can be influenced by various factors, including the imaging protocol used and the sonographer's suspicion 4.
• A systematic review and meta-analysis published in 2022 found that first-trimester ultrasound examination of the fetal heart allowed identification of over half of fetuses affected by major cardiac pathology, but the detection rate varied depending on the imaging protocol used 4.
• The accuracy and reliability of fetal heart rate monitoring using maternal abdominal surface electrodes were compared with Doppler ultrasound and found to be more reliable and accurate than ultrasound 5.

Ultrasound Screening and Fetal Development

• Ultrasound screening plays a crucial role in prenatal care, and its accuracy and reliability are essential for detecting fetal abnormalities and monitoring fetal development 6, 4.
• The introduction of non-invasive screening tests, such as non-invasive prenatal testing (NIPT), has expanded the options for prenatal screening, but ultrasound remains a vital tool for evaluating fetal growth and development 6.