After a pregnancy test comes back positive, an ultrasound done around six weeks after your last period can provide the next clue as to how everything’s progressing in there. While you won’t be able to see much yet, there’s a good chance that you’ll spot a 3- to 5-millimeter circle about the size of a pencil eraser on the screen. 

This is known as the yolk sac, and it's key to learning if your pregnancy is developing the way it should in its early stages.

What is the yolk sac?

The yolk sac is part of the gestational sac, the protective covering that surrounds a developing baby and contains the amniotic fluid. It appears about a week or two after the embryo has implanted in the uterus (during week 4), and it disappears near the end of the first trimester.

During that time, the yolk sac provides all the nutrients a little embryo needs. It also produces red blood cells until the placenta fully forms and takes over. 

When should you see the yolk sac on an ultrasound?

You should see the yolk sac when you go for your first ultrasound, typically between weeks 6 and 9 of pregnancy. 

The gestational sac is technically visible before that, around the fourth or fifth week. It looks like a round structure in your uterus that’s 2 to 3 millimeters in diameter, and it increases by about a millimeter each day early on in the pregnancy. 

About a week later, the yolk sac has grown enough to appear on an ultrasound too. It'll look like a round, dark mass with a bright rim measuring only a few millimeters around. Like the gestational sac, it will get bigger over the next few weeks. By the 10-week mark, a yolk sac will typically measure a (still tiny!) 6 millimeters. 

What if there’s no yolk sac during a 6-week ultrasound?

If the yolk sac isn’t visible at your six-week ultrasound, you may feel scared and confused. But it’s also possible that you got your dates wrong and your pregnancy isn’t as far along as you thought. 

This is especially true if you have less-than-regular menstrual cycles. That’s why your practitioner will likely send you home and have you come back in a week or two to see what’s going on. Most of the time, you’ll be greeted with the sign of a gestational sac.

Sometimes, however, the technician can see a yolk sac, but it doesn’t look right. This can indicate a problem with the pregnancy and portend a miscarriage — but future healthy pregnancies are still very much possible. Signs include:

  • An irregular shape (wrinkled with indented walls)
  • Calcifications in the yolk sac that make it harder to see through
  • A yolk sac that’s smaller than 2 millimeters or greater than 5 millimeters
  • A yolk sac that appears to be moving within the gestational sac

Does an empty gestational sac mean miscarriage?

If the gestational sac is still empty at a follow-up appointment, then it’s what’s known as a blighted ovum, or anembryonic pregnancy. This type of miscarriage occurs when an embryo never develops or stops developing. The most common reason this happens is because of a chromosomal abnormality, and the odds of a future healthy pregnancy remain very good. 

Most of the time, an anembryonic pregnancy occurs so early on that you wouldn’t even know about it unless you were keeping an eye on the calendar, which many hopeful moms-to-be are. 

You can have a blighted ovum and still experience pregnancy symptoms like sore breasts, nausea and vomiting. This is because the early embryo still secretes a pregnancy hormone — human chorionic gonadotropin (hCG) — until it stops developing.

With a blighted ovum, a doctor will most likely advise watching and waiting for a couple weeks. If your body recognizes that an embryo isn’t developing, it will expel the contents with bleeding similar to a heavy period. But if it doesn’t, a medical provider may prescribe a medication that induces bleeding. He or she may also recommend a brief surgical procedure (a D&C) to empty the uterus.

The yolk sac is a positive sign and an important indicator of pregnancy health. If you see it, it’s reassurance that all is well and your little nugget is starting to develop as expected. But even if one’s absent or irregular right now, the odds are still good that you’ll successfully conceive again and give birth to a healthy baby.