At what gestational age should a fetal pole be visible on ultrasound?

From the Guidelines

A fetal pole, now more accurately referred to as an embryo, should be visible on ultrasound by 6 weeks of gestational age. According to the most recent and highest quality study 1, a transvaginal ultrasound can typically detect an embryo when it reaches about 5-6 mm in length, which generally corresponds to approximately 6 weeks gestation. The term "fetal pole" is considered obsolete and should be avoided, with "crown-rump length" being the preferred term for measurement of an embryo or fetus based on its greatest dimension 1.

Key Considerations

• The visualization of an embryo on ultrasound is an important developmental milestone in early pregnancy assessment.
• Factors that can affect visualization include the quality of the ultrasound equipment, the skill of the sonographer, maternal body habitus, and the actual gestational age, which may differ from the estimated age based on last menstrual period.
• If an embryo is not visible by 7 weeks on transvaginal ultrasound when dates are certain, this may indicate an abnormal pregnancy.

Clinical Implications

• The detection of cardiac activity should be visible shortly after the embryo appears, usually by 6.5-7 weeks 1.
• The use of crown-rump length for dating is more accurate than the mean sac diameter once the embryo is measurable 1.
• It is essential to follow the consensus that an embryo is 10 weeks 6 days gestational age or less, and beginning at 11 weeks 0 days gestational age, the conceptus should be referred to as a fetus 1.

From the Research

Visibility of Fetal Pole on Ultrasound

• The fetal pole can be visible on ultrasound as early as 34 days from the last menstrual period with transvaginal sonography 2.
• Transabdominal sonography can detect the fetal pole at around 42 days from the last menstrual period 2.
• The visibility of the fetal pole is also dependent on the crown-rump length, with fetal heart motion detectable in fetal poles with a crown-rump length of 3 mm or greater using transvaginal sonography, and 6 mm or greater using transabdominal sonography 2.

Gestational Age and Fetal Pole Visibility

• A study found that the chance of confirming viability increases rapidly per day of gestation until 49 days, and thereafter plateaus 3.
• The same study suggests that transvaginal sonography should be delayed until 49 days in asymptomatic women with no previous ectopic pregnancy to reduce the number of inconclusive scans 3.
• Another study found that a fetal pole can be seen at 40.3 +/- 3.4 days from the last menstrual period, with a corresponding hCG level of 5113 +/- 298 mIU/ml of the IRP or 3783 +/- 683 mIU/ml of the second IS 4.

Comparison of Transvaginal and Transabdominal Sonography

• Transvaginal sonography is more sensitive than transabdominal sonography in detecting the fetal pole and fetal heart motion 2, 5, 6.
• Transabdominal sonography may underestimate the gestational age and miss the detection of a continuing pregnancy 5.
• The sensitivity of transabdominal sonography for detecting a gestational sac is higher than for detecting a fetal pole, with a sensitivity of 100% for gestational sac detection and 68% for fetal pole detection 5.