Fetal Pole Medically Reviewed

Last Updated: June 10, 2020

Definition - What does Fetal Pole mean?

The fetal pole is the earliest visual evidence of a developing fetus during very early pregnancy. A transvaginal ultrasound can detect a fetal pole at around 6 weeks gestational age as a 1-2 mm oval structure.

In a viable pregnancy, the fetal pole will continue to develop into a fetus and eventually to a fully formed infant. The absence or malformation of a fetal pole may indicate a complication with the pregnancy.

A fetal pole may also be called an embryonic pole or is often referred to synonymously as an embryo. An alternative spelling is foetal pole.

FertilitySmarts explains Fetal Pole

Depending on timing, an early ultrasound in the first trimester of pregnancy can normally show three structures. These include:

  • Fetal pole. This is the developing embryo.
  • Yolk sac. The structure that provides nutrients to the developing embryo.
  • Gestational sac. Surrounds and contains the fetal pole and yolk sac.

The development of these structures can provide information about early pregnancy including:

  1. Help to identify the location of the pregnancy. The gestational sac and yolk sac should be located in the uterus.
  2. Determine the number of fetuses. This can be done by counting the number of fetal poles, or fetal heart bears after around 6 weeks.
  3. Estimate gestational age. This is most accurately done by measuring the greatest length of a fetal pole, called crown-to-rump length.

Measuring a fetal pole with crown-to-rump length (CRL)

The fetal pole, or embryo, is typically curved like the letter “C” with the beginning of a head (crown) at one end and what resembles a tail (rump) at the other.

The distance between these two points is called the crown-to-rump length (CRL), and this measurement can be used to estimate gestational age between weeks 6 and 13 of pregnancy.

Calculating Gestational Age

From6 to 9.5 weeks, the growth of the fetal pole grows around 1mm per day. The approximate age is determined by using the length of the fetal pole in mm and adding 42 days weeks.

So a 6mm fetal pole would be 6 weeks and 6 days gestation.

What does no fetal pole mean on an ultrasound?

If no fetal pole is seen during imaging, a provider may request another follow-up scan in 3-7 days. It is possible that:

  • Gestational age is wrong. The pregnancy has not progressed enough to be visualized.
  • Anembryonic pregnancy (blighted ovum). This occurs when a fertilized egg implants and the body produces pregnancy hormones and a gestational sac, but does not continue to grow and is a form of early miscarriage. If there is no visible fetal pole but the gestational sac is measured (with mean sac diameter) to be 25mm or greater, it is an anembryonic pregnancy that is not viable.
  • Ectopic pregnancy. The pregnancy is ectopic and not located in the uterus. This is a medical emergency.
  • Pregnancy of unknown location (PUL). This is defined as a positive pregnancy test but no signs of intrauterine or extrauterine pregnancy. This is typically not a final diagnosis.

What does it mean if a fetal pole has no heartbeat?

A fetal pole that has a CRL of 7mm or greater (typically around 7 weeks) but has no fetal heart activity is considered to be a non-viable pregnancy. This means that the pregnancy will not progress any further and will end in miscarriage.

Fetal Pole Development by week

What should you expect a fetal pole in typically developing pregnancy to look like week-by-week?

Fetal pole at 5 weeks

  • The earliest a fetal pole may be visualized is typically around 5.5 weeks.
  • Fetal heart activity is typically not expected until around 6 weeks but can be detected as early as 5 weeks + 2 days.
  • Yolk sac can appear as early as 5 weeks but is more likely around 5.5 weeks.
  • The appearance of a only a gestational sac is most likely at this stage.

Fetal pole at 6 weeks

  • On average, the fetal pole can be visualized by transvaginal ultrasound around 6 weeks. Fetal heart activity may also be detected. A typical heart rate is 100-120 beats per minute (BPM).
  • It is not necessarily problematic if the fetal pole is not detected if the CRL is less than 5 mm.

Fetal pole at 7 weeks

  • A fetal pole with a CRL of 7mm or greater should have fetal cardiac activity. This can be estimated to occur around week 7.
  • Fetal heart rate is approximately between 120-159 bpm late week 7 to early week 8.

Fetal pole at 8 weeks

  • The fetal pole should be visualized when the mean sac diameter (MSD) of the gestational sac is at least 25 mm, which typically occurs at the end of 7 weeks or early week 8.
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