Gestational sac – what is its function?

The first sign of a correctly located pregnancy is the visualization of the gestational sac in the uterus during an ultrasound examination. However, the mere presence of the gestational sac does not necessarily indicate a normally developing pregnancy. So, what does it mean if the gestational sac is empty?

What is the gestational sac?

The gestational sac (GS) is a small, round, fluid-filled structure and represents the first stage of early pregnancy. Before the embryo and its heartbeat can be detected, the sac helps determine whether the pregnancy is correctly located within the uterus. Following the appearance of the gestational sac, the yolk sac, embryo, and amniotic cavity develop in that order.

Gestational sac – ultrasound

Most women undergo their first ultrasound after receiving a positive pregnancy test in urine or blood – usually between the 5th and 7th week of pregnancy. The gestational sac should be visible when the blood beta-hCG level reaches approximately 1500 IU/ml. The gynecologist will check the position of the gestational sac in the uterus, measure it, and determine how many sacs are present. If the beta-hCG level is high but no gestational sac can be found in the uterus, there is a high risk of an ectopic pregnancy (extrauterine pregnancy).

Size of the gestational sac

The size of the gestational sac on ultrasound can be used to estimate the gestational age – even before the crown-rump length (CRL) of the embryo can be measured. Guideline values are:

•  Gestational sac 1–4 mm = 4th/5th week of pregnancy
• Gestational sac 5–8 mm = 5th/6th week of pregnancy

Normal daily growth of the gestational sac is about 1–1.2 mm. If the sac is larger than 20 mm and no embryo is visible, there is a very high risk of miscarriage (known as a blighted ovum or empty sac).

Empty gestational sac

If an empty gestational sac is visible on ultrasound, it means the pregnancy has developed without an embryo. The yolk sac and amniotic cavity may develop normally, but the embryo either never forms or is reabsorbed at a very early stage. An empty gestational sac equals a miscarriage. The most common cause is genetic abnormalities. However, it can also be due to immunological disorders or infections. Important to note: An empty gestational sac does not increase the risk of miscarriage in future pregnancies.