No Yolk Sac in Early Pregnancy

Each stage of pregnancy has characteristics that are a fairly reliable indicator of whether the pregnancy is healthy and developing as it should. This is one reason why ultrasounds during pregnancy can be so valuable, even in the first weeks.

Ultrasounds are routinely done around 6 weeks gestation. One of the things the doctor and technician look for is a yolk sac. When this vital structure doesn't appear to be there, the most likely reason is that the pregnancy isn't viable—in other words, that a miscarriage has occurred.

But this isn't always the case. If you're newly pregnant and the yolk sac isn't visible on your six-week ultrasound, it may simply mean you aren't as far along as you thought.

What Is the Yolk Sac?

In early pregnancy, the yolk sac functions as a source of nourishment for the developing fetus. It's the first structure to be visible within the gestational sac, which envelopes the developing fetus and the amniotic fluid.

The gestational sac looks like a white rim around a clear center. It can be seen on a transvaginal ultrasound—in which the ultrasound wand is inserted into the vagina rather than pressed against the abdomen—between 3 and 5 weeks gestation. The yolk sac isn't visible until around 5.5 to 6 weeks gestation when using an abdominal ultrasound. 

Miscalculated Due Date

Occasionally, not seeing a yolk sac on an ultrasound at this stage of pregnancy could simply mean the gestational age may have been miscalculated. This can happen if you made an error in remembering when your last period was or if you have irregular menstrual cycles.

When a doctor suspects incorrect gestational age in a woman who was believed to be around 6 weeks pregnant but has no yolk sac, they usually recommend doing another ultrasound in a week or two. By then, if all is well and the pregnancy is viable, the yolk sac and possibly the fetal pole (a curved structure that will eventually develop into the baby) will be visible.

Sign of Miscarriage

Often, seeing no yolk sac (or a yolk sac that is smaller than normal or otherwise misshapen) at 6 weeks can be a sign of miscarriage. Unfortunately, you'll most likely have to wait until a follow-up ultrasound to be sure. At that time, if the ultrasound does not show continued development of the pregnancy and there's still no visible yolk sac, your doctor will diagnose a miscarriage.

You won't always have to wait to know for sure, however. Sometimes, if the gestational sac is a certain size (25 millimeters or more) on the first ultrasound and there is no yolk sac or embryo, your doctor will be able to diagnose a miscarriage right away.

Empty Sac Pregnancy

When the gestational sac is empty (meaning there's no yolk sac or embryo by the time that there should be), this is known as an empty sac pregnancy. Other terms for an empty sac pregnancy are an "anembryonic" pregnancy or a blighted ovum (a term that's now considered to be outdated).

An empty sac pregnancy is a type of miscarriage, even though the products of conception are still contained in the uterus. If this happens to you, you may be given the choice of letting nature take its course or having a procedure called dilation and curettage (D&C). A D&C involves dilating the cervix to create an opening for a thin surgical instrument to remove tissue from the uterus.

Causes

Research shows that empty sac pregnancies frequently have chromosome abnormalities. It's believed that the woman's body recognizes the problem early on and stops further progress of the pregnancy. An empty sac diagnosis may feel cruel, but it may help to think of it as nature's way of keeping unhealthy pregnancies from continuing.

A Word From Verywell

Not seeing the yolk sac at the pregnancy confirmation appointment can be scary and very disappointing. Remember, the timing might be off, so you could still be pregnant. Other times, the yolk sac is absent due to a miscarriage.

No matter how early you are in your pregnancy, learning that the fetus is not viable can be very upsetting. But rest assured that early miscarriages are very common, often happening before women even know they're pregnant—and having had one in no way indicates that you won't carry to term next time.