At what gestational age can a fetus be seen on ultrasound (US)?

Earliest Visualization of Pregnancy Structures on Ultrasound

A gestational sac can be visualized on transvaginal ultrasound (TVUS) at approximately 5 weeks gestational age, with the embryo typically visible at 6 weeks gestational age. 1

Timeline of Early Pregnancy Ultrasound Findings

• The gestational sac is the earliest sonographic finding of pregnancy, typically visualized on TVUS at 5 weeks gestational age as a round or oval fluid collection surrounded by a hyperechoic rim of trophoblastic tissue 1
• The yolk sac, a thin-rimmed circular structure eccentrically located within the gestational sac, is visualized at approximately 5½ weeks gestational age 1
• An embryo with cardiac activity is typically visualized on TVUS at 6 weeks gestational age 1
• The amnion (thin membrane surrounding the embryo/fetus) is typically visualized on TVUS at approximately 7 weeks gestational age 1

Terminology and Measurements

• Before visualization of a yolk sac or embryo, an intrauterine fluid collection should be reported as a "probable gestational sac" or "probable pregnancy" 1
• The presence of a yolk sac within an intrauterine fluid collection is incontrovertible evidence of pregnancy, and should be interpreted as a "definite gestational sac" or "definite pregnancy" 1
• Mean sac diameter (MSD) can be used to calculate gestational age before visualization of an embryo 1
• Crown-rump length (CRL) should be used for dating as soon as an embryo is measurable, as it is more accurate than mean sac diameter 1

Important Distinctions

• The term "embryo" applies to a conceptus of 10 weeks 6 days gestational age or less 1
• Beginning at 11 weeks 0 days gestational age, the conceptus should be referred to as a "fetus" 1
• The word "pole," as in "embryonic pole" or "fetal pole," is considered obsolete and should be avoided 1
• Embryonic or fetal rhythmic pulsations should be reported as "cardiac activity" or "cardiac motion," avoiding terms like "heart motion," "heartbeat," or "heart tones" 1

Transvaginal vs. Transabdominal Ultrasound

• Transvaginal ultrasound allows visualization of pregnancy structures 1-2 weeks earlier than transabdominal ultrasound 2
• With transvaginal ultrasound, intra-uterine structures can be identified from a measured minimum diameter of 2-3 mm 2

Clinical Implications

• Absence of expected structures based on gestational age may indicate poor prognosis, such as absence of yolk sac in a gestational sac >8 mm or absence of embryo in a gestational sac >16 mm 3, 4
• For embryos with CRL <7 mm without cardiac activity, follow-up ultrasound in 7-10 days is recommended 3
• Definitive diagnosis of embryonic demise requires absence of cardiac activity in an embryo ≥7 mm in CRL 3

Pitfalls to Avoid

• Avoid using terms like "pseudosac" or "pseudogestational sac" as they may lead to clinical errors; instead describe fluid in the endometrial cavity based on its characteristics 1, 5, 3
• Fluid in the endometrial cavity with pointed or non-curved margins should be described as "intracavitary fluid" or "fluid in the endometrial cavity" 1