Fetal Pole: What It Is and What It Means for a Pregnancy

Ultrasound during pregnancy
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A fetal pole is the term used for the earliest signs of an embryo in an early pregnancy ultrasound -- before the baby has formed a recognizable human shape. Appearing as a thickened area near the yolk sac, within the gestational sac, a fetal pole can usually be detected with a transvaginal ultrasound by around 6 weeks of gestation.

If you are curious about the origin of the term, it comes from the use of "pole" to describe one of two morphologically different areas at the ends of an axis in an organism.

In a newly developing baby, the fetal pole appears as a thick area on the side of the yolk sac, so it is a "pole" that is different from the rest of the yolk sac and which will ultimately become a fetus (baby).

Around the same time that a fetal pole is identified on ultrasound, a fetal heartbeat may also be found.

While a fetal pole in no way resembles the human being a baby will become, the structure has a curved appearance, with the head of the embryo at one end and what appears to be a tail-like structure at the other. Thanks to this, a fetal pole is now used to measure crown-to-rump length (CRL), which helps date a pregnancy more accurately.

When the CRL reaches 2 to 4 millimeters, the fetal pole is generally visible with a transvaginal ultrasound, and when the CRL reaches 5 millimeters, a heartbeat can usually be detected. 

When No Fetal Pole Is Found

When a vaginal ultrasound fails to find a fetal pole or heartbeat at the expected gestational time, it can be for a few different reasons.

First, the estimation of pregnancy dates can be incorrect and the structures may not yet have developed. For this reason, when no fetal pole or heartbeat is found, generally a physician will recommend another ultrasound within three to seven days.

If the problem was dating, the followup ultrasound should show continued development with a larger gestational sac and most likely the appearance of a fetal pole.

If the followup ultrasound shows no development from the previous ultrasound, the doctor can conclusively diagnose miscarriage.

In a few cases, a miscarriage can be diagnosed from a single ultrasound that shows no fetal pole. These situations would include:

  • A gestational sac larger than 16mm with no fetal pole
  • Having falling hCG levels and no fetal pole
  • Other evidence that rules out the possibility of incorrect dates, such as if the pregnancy was officially confirmed at 4 weeks gestational age and an ultrasound one month later showed development of only 5 weeks gestational age (especially alongside miscarriage symptoms)

Absent Fetal Pole and Miscarriage

When a follow-up ultrasound has confirmed the absence of a fetal pole and heartbeat, a diagnosis of miscarriage can be confirmed. In these circumstances, an empty gestational sac, blighted ovum, or anembryonic sac may be used to describe the failed pregnancy. In some cases, the empty gestational sac can remain intact for a number of weeks before miscarriage symptoms appear, and it may even continue to grow .

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