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.

When an ultrasound takes place around six weeks gestation, one of the things the doctor and technician look for is a yolk sac.

When this structure doesn't appear to be there, it could mean that the pregnancy isn't viable—in other words, 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. Here's what it may be helpful to know about either scenario. 

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 and can be seen on a transvaginal ultrasound—in which the ultrasound wand is inserted into the vagina rather than pressed against the abdomen—between three to five weeks gestation. 

The yolk sac isn't visible until around five and a half to six weeks gestation.

 The yolk sac provides nutrition to the developing embryo until the placenta takes over. That's why it's a good indicator of the health of the pregnancy.

No Yolk Sac at 6 Weeks

Seeing no yolk sac on an ultrasound at this stage of pregnancy could simply mean the fetus's 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 six weeks pregnant but has no yolk sac, she'll 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.

When It's a Sign of Miscarriage

Seeing no yolk sac at six weeks can also be a sign of miscarriage. Most likely you'll have to wait until that follow-up ultrasound to be sure. If it 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 mm 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.

An Empty Sac Pregnancy

When the gestational sac is empty—meaning there's no yolk sac or embryo by the time there should be— it is known as an empty sac pregnancy.

An empty sac pregnancy may also be referred to as an  "anembryonic" pregnancy or a blighted ovum (a term that's 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 to 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.

Research shows that empty sac pregnancies tend to have high levels of 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 you may want to think of it as nature's way of keeping unhealthy pregnancies from continuing.

Sources:

American College of Obstetrics and Gynecologists. "Dilation and Curettage (D&C." Feb 2016.

American Pregnancy Association. "Blighted Ovum." Aug 2015.

American Pregnancy Association. "Concerns Regarding Early Fetal Development." Aug 2015.

Doubilet, P.M. et al. (2013). "Diagnostic Criteria for Nonviable Pregnancy Early in the First Trimester." N Engl J Med, Oct 10, 369, 15, 143-51.

Tulandi T, Al-Fozan HM. "Spontaneous Abortion: Risk Factors, Etiology, Clinical Manifestations, and Diagnostic Evaluation . In: UpToDate, Levin D, Barbieri RJ (Ed)." UpToDate, Waltham, MA. Sept 2016.