What is the Fetal Pole in Pregnancy?

The Earliest Visible Sign of an Embryo

Ultrasound during pregnancy
Medic Image/Universal Images Group/Getty Images

A fetal pole is the earliest sign 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.

What Does "Fetal Pole" Mean?

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 this "pole" 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.

According to the American Pregnancy Association, 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 a repeat ultrasound in three to seven days or so. 

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

If the follow-up ultrasound shows no development from the previous ultrasound, the doctor can conclusively diagnose miscarriage. In these circumstances, an empty gestational sac or a blighted ovum 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 .

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

A Word From Verywell

If you have any questions or worries or questions about how your baby is developing, please speak with your doctor. How a baby develops is a complex process, so it is natural to be curious or even have some anxiety.

Sources:

American College of Obstetrics and Gynecology. (May 2015). Practice Bulletin: Early Pregnancy Loss.

American Pregnancy Association. Concerns Regarding Early Fetal Development

Continue Reading