Fetal Pole and Early Pregnancy Ultrasound

When you're newly pregnant, your caregiver may have you go in for an ultrasound before the baby is far enough along to have taken on a recognizable human shape. At that peek into your womb, one structure the technician and doctor will be looking for is the "fetal pole."

The development of a fetal pole is one of the first stages of growth for an embryo, so it certainly can be unnerving if it appears to be missing during an early pregnancy ultrasound. But even though it sometimes can mean that the pregnancy isn't viable, it's just as everything is fine. Here's why.

Fetal Pole Basics

The fetal pole appears as a thick area alongside the yolk sac, which will provide nourishment to a new fetus in the very early stages. It's actually the structure that becomes that fetus, but at this point of development in no way resembles a human being.

The fetal pole 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. The distance between these two points of the fetal pole is now used to measure the crown-to-rump length (CRL), which helps date a pregnancy more accurately.

According to the American Pregnancy Association (APA), when the CRL reaches 2 to 4 millimeters, it's usually visible with a transvaginal ultrasound. When the CRL reaches 5 millimeters, it's usually possible to hear a heartbeat.

When the fetal pole isn't visible on an ultrasound, there are a couple of possibilities why.

It's Earlier in the Pregnancy Than Estimated

Given that the fetal pole becomes visible somewhere between 5.5 and 6.5 weeks of gestational age (defined by the APA as "the age of the pregnancy from 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 the expected findings on an ultrasound. If you have an irregular cycle and do not always ovulate two weeks after you begin each period, your pregnancy may not technically be 5–6 weeks along—even if it has been 5 or 6 weeks since your last menstrual period.

If it's possible this is the reason the fetal pole isn't yet visible, most doctors will have a woman come back for a follow-up ultrasound a week or two after the first one. This will give the pregnancy time to develop.

The Pregnancy Isn't Viable

When in a follow-up ultrasound there's still no sign of a fetal pole (or of a gestational sac, which appears as a white rim around a clear center and will eventually contain amniotic fluid and enclose the developing baby) it means a miscarriage has occurred.

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. Other times, a miscarriage may be diagnosed by a single ultrasound that shows no fetal pole. One example is if a gestational sac is seen on the ultrasound that is larger than 25 mm (with no fetal pole).

All in all, how a baby develops is a complex process from the very beginning, so if you have any worries, don't hesitate to ask your doctor.

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