The fetal pole is the earliest visible structure of a developing embryo within the gestational sac. Its appearance on an ultrasound confirms an intrauterine pregnancy, meaning the pregnancy is located inside the uterus. This early visualization offers reassurance regarding embryonic development.
When and How the Fetal Pole is Seen
The fetal pole becomes visible on an ultrasound between 5.5 and 6.5 weeks of gestational age. However, some variability exists, and it might not be seen until around 9 weeks. The timing of visibility can be influenced by factors such as the accuracy of pregnancy dating, the quality of the ultrasound equipment, and the skill of the sonographer.
On an ultrasound, the fetal pole appears as a small, thickened area adjacent to the yolk sac, which provides early nourishment. Initially, it may look like a linear or oval structure, measuring 1 to 2 millimeters. As it grows, the fetal pole often takes on a curved shape, with one end representing the developing head (crown) and the other a tail-like structure (rump).
Transvaginal ultrasound is preferred for visualizing the fetal pole in early pregnancy. This method provides clearer and more detailed images due to its proximity to the uterus, allowing for earlier detection compared to transabdominal ultrasounds. A fetal pole can be seen as early as 34 days from the last menstrual period with transvaginal sonography, while transabdominal sonography detects it around 42 days.
What the Fetal Pole Means for Your Pregnancy
The presence of a fetal pole on an ultrasound is a positive indication, confirming that a fertilized egg has implanted in the uterus and that embryonic development has begun. The size of the fetal pole, measured as crown-rump length (CRL), helps determine the gestational age, especially if the date of the last menstrual period is uncertain or irregular.
Following the visualization of the fetal pole, the next expected event is the detection of cardiac activity. A fetal heartbeat is observed shortly after the fetal pole becomes visible, around 6 to 7 weeks of gestation. If the fetal pole measures 7 millimeters or more and no cardiac activity is detected, this may indicate a non-viable pregnancy.
If a fetal pole is not seen when expected, it does not automatically indicate a problem. One possibility is that the pregnancy is at an earlier gestational age than calculated, perhaps due to variations in ovulation timing. In such cases, a healthcare provider may recommend a follow-up ultrasound a week or two later.
However, an absent fetal pole when expected can also signify a non-viable pregnancy, such as a blighted ovum, where a gestational sac forms but no embryo develops. An ectopic pregnancy, where the gestational sac is located outside the uterus, is another consideration if a fetal pole is not found within the uterus. Consulting with a healthcare provider for accurate assessment and guidance is always recommended for any concerns regarding ultrasound findings.