Fetal Pole and Early Pregnancy Ultrasound

Upon discovering that you're newly pregnant, your doctor may schedule you for an early pregnancy ultrasound. If the baby isn't far enough along to have developed into a recognizable fetus, the ultrasound tech will look to identify a "fetal pole."

The development of a fetal pole is one of the first stages of embryonic growth. If the fetal pole appears to be missing, you may be left wondering whether everything is OK. Understanding a little background about the fetal pole can help you to feel better prepared for your first prenatal visit.

Fetal Pole Function

The fetal pole appears as a thick area alongside the yolk sac, which is a source of fetal nourishment at the beginning of pregnancy. The fetal pole is a preliminary structure that ultimately develops into a fetus.

Curved in appearance, the fetal pole has the head of the embryo at one end and a tail-like structure at the other. The distance between these two points is used to measure the crown-to-rump length (CRL) which helps determine how far along you are in your pregnancy.

Reasons for a Missing Fetal Pole

There are a couple of possible reasons why the fetal pole may not be visible on an early ultrasound despite getting a positive pregnancy test.

It's Too Early

Given that the fetal pole becomes visible somewhere between 5 1/2 and 6 1/2 weeks of gestational age (typically defined as the fetal age which is determined based on the date of the last normal menstrual period), any small error in dating the pregnancy can throw off an ultrasound interpretation.

For example, incorrectly remembering when you last had your period can change what your doctor will expect to see on an ultrasound as healthcare providers typically use the first day of your last period (LMP) to initially date your pregnancy.

Similarly, if you have an irregular cycle or do not always ovulate 14 days after you begin each period (an assumption based on the 28-day average cycle), your pregnancy may not technically be 5 to 6 weeks along—even if it has been 5 or 6 weeks since your last menstrual period. As a normal menstrual cycle can range anywhere from 21 to 35 days, it's not uncommon to ovulate much earlier or later than cycle day 14.

If your doctor suspects that it's just too early into your pregnancy to detect a fetal pole, they may ask you to come back for a follow-up ultrasound a week or two later.

The Pregnancy Is Not Viable

If, however, you go to the follow-up ultrasound and there's still no sign of a fetal pole (or a gestational sac, which looks like white rim around a clear center), a miscarriage has likely occurred.

In some cases, the empty gestational sac can remain intact for a number of weeks before miscarriage symptoms appear. It may even continue to grow as in the case of a blighted ovum.

Other times, a miscarriage may be diagnosed by a single ultrasound that shows no fetal pole, such as if a gestational sac is seen on the ultrasound that is larger than 25 millimeters but with no fetal pole.

A Word From Verywell

Pregnancy is often filled with emotional ups and downs. It's normal to get your hopes up at the first sign of a positive pregnancy test. Discovering complications along the way can be devastating. While family and friends don't always know the best way to show support, there is help available. Speak to a therapist if you are struggling to cope with issues related to pregnancy or fertility.