Is a yolk sac a good sign

Written by Dr Sarah Merritt

Mainly thanks to the development of transvaginal ultrasound (TVUS), we can now see a pregnancy as soon as during the 4th week of gestation. An early pregnancy is very small but grows quickly and uniformly over the first 10 weeks in particular. What is seen on scan in early pregnancy is rapidly changing as more and more of the developing anatomy is seen.  The changes are quite remarkable and each week brings something different as the embryo forms and develops into a fetus.

A normal developing pregnancy within a normal womb can first be seen on scan at approximately 4 weeks and 3 days gestation on a transvaginal scan. A transabdominal scan cannot identify a pregnancy this early. At this gestation a small pregnancy or gestational sac is visible, sometime only measuring a few millimetres (2-5mm) in diameter. The sac is spherical in shape and regular and is normally seen near the top of the womb, implanted within the lining of the womb (endometrium) to one side of the midline of the endometrium.

During the 5th week of gestation a small yolk sac is seen within the gestational sac.  The yolk sac helps provide the embryo with the nourishment it requires in early pregnancy and can be seen as a small white ‘circle’ within the gestational sac. Towards the end of the 5th week, a tiny embryo can be seen measuring 2-3mm in length represented by a tiny white straight line.  The embryo normally can be seen sitting on the edge of the yolk sac, it may or may not have a heartbeat. If a heartbeat can be seen it is usually slow at about 100bpm. This is completely normal in an early healthy ongoing pregnancy.

At 6 weeks gestation an embryo is visible and measures about 4mm in size, the heart beat is over 120bpm (beats per minute). The embryo is now more kidney bean- shaped and has moved away from the yolk sac. The embryo is measured from head to bottom, or crown to rump, otherwise known as the crown rump length (CRL). The CRL at this gestation is about 4-8mm.

In the 7th week the CRL is 9-14mm. An area of brain can be seen as a small black diamond shaped cavity in the head, which enable head to be distinguished from bottom. The spine is often visible, as 2 brighter white lines running parallel to each other from the head to the bottom. The amniotic sac or bag of waters can usually also be seen surrounding the embryo. The yolk sac is outside the amniotic sac. The umbilical cord can also be seen.

During the 8th week of pregnancy the CRL is now 15-22mm. The shape of the embryo is more recognisable and limb buds appear. These are the hands and feet growing out of the embryo. Sometimes the embryo can be seen doing small movements of the body and the growing limbs. The umbilical cord is longer and the amniotic sac enlarges.

In the 9th week of gestation the CRL is 23-32mm. The limbs are now longer and hands and feet can be seen as tiny brighter white stripes representing the fingers and toes.  The heart beat can reach as high as 170-180bpm and then over the coming weeks will reduce again. Movements in the arms and legs increase.

In the 10th week of gestation the CRL is about 32-41mm and the embryo is now a fetus. The profile of baby is very clear and often can be seen moving all limbs and body. From 10 weeks gestation, scans are more commonly performed through the abdomen.

In the world of electronic medical records, patients sometimes get access to their pregnancy imaging results before they have a chance to visit with their doctor about the findings. Words such as "abnormal" or "hemorrhage" can trigger concerns about increased risks of miscarriage.

This is especially true after the first trimester ultrasound. During this exam, we measure three items that tell us whether a fetus is developing as expected:

• Yolk sac
• Crown-to-rump length
• Heart rate

We also look at other features that may influence the outcome of the pregnancy, such as the presence of subchorionic hemorrhage or bleeding between the sac and the uterus.

Patients often ask, "If everything looks OK at my first-trimester ultrasound, am I out of the woods for miscarriage risk?" Unfortunately, that is never truly the case until delivery. The average pregnancy has an approximate 5% risk of miscarriage. However, if nothing unusual is detected with all four of these markers, patients likely are not at increased risk.

Not every abnormal measurement means something is wrong. For example, a smaller than expected crown-to-rump (head to bottom) number may mean your pregnancy is not as far along as you thought.

However, certain abnormal readings are concerning, especially when seen together. A 2018 study found that in pregnancies with a combination of a low fetal heart rate and a small crown-to-rump length, the risk of miscarriage increases from 5% to 21%. Still, 20% of pregnancies with both abnormal findings resulted in successful live births.

So, what do these markers mean for your pregnancy? There is no general answer – each patient should discuss her results with her doctor. In the meantime, I wanted to share an overview of these findings so you can know what to expect at your follow-up visit.

Related reading: Pregnancy without ultrasound? Pros and cons

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