What is the Gestational Sac?

Nicole Madison
Nicole Madison

The gestational sac is a structure that surrounds an embryo, which is a baby in the very early stages of development. It encloses not only the embryo, but also the amniotic fluid, which helps to nourish and protect the developing baby. The gestational sac is the structure ultrasound technicians look for when they need to confirm the presence and viability of an early pregnancy. In fact, it may be the first pregnancy structure ultrasound technicians can see in early pregnancies.

The gestational sac encloses the embryo and amniotic fluid.
The gestational sac encloses the embryo and amniotic fluid.

In the early weeks of pregnancy, a woman may take a pregnancy test and miss a period. These things often serve as confirmation that a woman is pregnant. Unfortunately, they are not a 100% reliable confirmation. A woman may miss a period for a number of reasons, and pregnancy tests can be wrong. For example, a woman may have a chemical pregnancy, which means an egg is fertilized but does not implant in the woman's uterus. To confirm a healthy pregnancy that is progressing as it should, some obstetricians order early ultrasounds.

Ultrasound technicians look for the gestational sac when they look to confirm early pregnancy.
Ultrasound technicians look for the gestational sac when they look to confirm early pregnancy.

Generally, the gestational sac is visible around the fifth week of pregnancy. Obstetricians often use gestational dates for pregnancy, which means a woman is five weeks pregnant when five weeks have past since her last menstrual period. In reality, her baby may be only three weeks into development by this time. The detection of a gestational sac at this stage is a positive sign. It is not a guarantee the pregnancy is healthy and will proceed, however.

A yolk sac may be visible as early as the middle of the fifth week following a pregnant woman's last menstrual period.
A yolk sac may be visible as early as the middle of the fifth week following a pregnant woman's last menstrual period.

Shortly after the sac becomes visible, an ultrasound technician may be able to detect a yolk sac within it. The yolk sac may be detectable as early as the middle of the fifth week following a pregnant woman’s last menstrual period. The detection of this structure is an important milestone, as the yolk sac is the developing embryo’s first nutritional source. As with the gestational sac, its visibility does not guarantee a healthy pregnancy. Its absence after the sixth week of pregnancy, however, may be a cause for serious concern.

Vaginal ultrasound probes may be inserted into the vagina during pregnancy to obtain an image of the uterus.
Vaginal ultrasound probes may be inserted into the vagina during pregnancy to obtain an image of the uterus.

In some cases, ultrasound images may show characteristics of the sac that indicate problems with a developing pregnancy. It may not grow as expected, for example. After the fifth week, it should grow about 0.0394 inches (1 millimeter) per day until until around the ninth week of pregnancy. A gestational sac may be shaped abnormally or even appear to be empty long after the yolk sac should be visible.

The gestational sac is typically visible around the fifth week of pregnancy.
The gestational sac is typically visible around the fifth week of pregnancy.
Nicole Madison
Nicole Madison

Nicole’s thirst for knowledge inspired her to become a wiseGEEK writer, and she focuses primarily on topics such as homeschooling, parenting, health, science, and business. When not writing or spending time with her four children, Nicole enjoys reading, camping, and going to the beach.

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Discussion Comments


As per my LMP (30.10.14) it's eight weeks, two days. Yesterday I had my first TVS USG showing six weeks sic days. The doctor told me that there is one week off and will see for one more month. I miscarried at eight weeks last time and am pregnant again, just after one successful period after my miscarriage. It will be a problem in coming weeks?

My TVS report as follows:

The study shows an anteverted, bulky uterus with a single live intra-uterine gestation. The CRL of the fetal component is 9.6mm which corresponds to 6 weeks 6 days. There is a well defined yolk sac measuring 2.2mm.

The fetal heart rate is 133beats/min, regular. There is no peri-chorionic haematoma. The cervix measures 3.4cms in length. The internal os is closed.

A corpus luteum is identified in the right ovary. There is no adnexal mass. There is no free fluid in the P.O.D.

Impression: Single live intra-uterine gestation corresponding to 6 weeks and 6 days.


I had a abdominal ultrasound done to determine I am six weeks pregnant. The ultrasound showed two sacs present. One of the sacs is black and the other sac has a baby, but the doctor is saying there is no fetal heartbeat. No blood work was done to check my HCG levels and the doctor says I may have miscarried one twin and I will more than likely miscarry the other. I have had no signs/symptoms of miscarrying. I have been feeling fine and steadily gaining weight.


I'm confirmed 5 weeks, 4 days pregnant and my first scan said that the yolk sac is visible, but not the fetal node, so they me advised to come back a week later. Is something wrong?


My wife and I had unprotected sex in late September 2012. Is it possible for the doctor's report to show that she is six weeks pregnant?


I had a scan a couple of days ago. My last period was in late October, so I should be about 12 weeks. They couldn't see my baby, but there was an egg sac. It was only 20mm and they said it was the size of a 6 weeks pregnancy. I have had no bleeding at all. Have I lost my baby? I have to go back for another scan next week.


Can anyone tell me why, a day after my 10 week ultrasound, my doctor would call and ask me to schedule another ultrasound in four weeks because he could not get a good measurement of the egg sac yolk? I saw the baby on the ultrasound and also heard the heart beat. I'm just confused. Please help.


My pregnancy test is positive but the doctor said the sac is not visible. It has been 35 days since I missed my period. Could I be pregnant or could there be a problem?


My LMP was a couple of days ago. I was determined pregnant the third week in April, went to first doctor visit and she said I was about seven weeks pregnant. I went for an ultrasound and they said they saw the gestational sac and yolk but could not see the baby.

They asked me to come back in a week. I went back today and they said they saw everything from last week but a little fetal pole with no heartbeat. My doctor is saying I am going to miscarry, saying she believes it is blighted ovum. She also said the sac did not grow from last week but a fetal pole was not there last week either. Are there any answers, please?


I found out I was pregnant in early January. About ten days later, I started bleeding like a period for about four days. They sent me for an early scan and it showed no signs of pregnancy. They did bloods and the hcg came back at 53, so two days later, they did more bloods that came back at 75 and again two days later, they came back at 165. Then yesterday, they did another scan transvaginally and they found a 2.2mm sac in my womb and a cyst on my right ovary, but said that was nothing to worry about and they also did my bloods yesterday. That came back at 389, which seems to be low.

Is this likely to be an ectopic pregnancy, or is something wrong and why is my hcg low? I think I am four weeks along. Any advice would be great and I have to go back for another scan in six days.


I have a question. I have a very irregular period. My last cycles were 49, 51 and 63 days, my LMP was early November and I have had all these symptoms since mid November: headache daily, nausea without vomiting, dizziness, feeling of fainting, lower back pain, tiredness, cramping on and off daily, and not any spotting or bleeding since then.

Please note before I have taken norcolut to postpone my period for my wedding day for two weeks (20th Oct-3rd Nov). Once I stopped I had my period an my LMP. Due to all the symptoms, I have taken seven pregnancy tests and all were negative, and one abdominal ultrasound in five weeks. It was negative, too. Can anybody advise me? I'm so sick of feeling like this.


what does a large gestational sac at 7.3 weeks indicate? they found a gestational sac measuring 4.8 by 4.2 but the doctor says the fetus is small -- about six weeks. They found the heartbeat too, transvaginally. Is there any reason to worry if the gestational sac is this big? The doctor seemed to be repeating how it's big, which has me all tensed up!


It should be noted that another reason for a positive pregnancy test and no gestational sac is an ectopic pregnancy. This is where sac and egg form in the fallopian tubes, rather than the womb. When an ultrasound is performed and there is no sac visible, and it is at least the fifth week after the first day of the last period, this is one of the first things they look for. An ectopic pregnancy can be very dangerous as it can cause the fallopian tube to burst and cause internal bleeding.


what does it mean if the sac is too low during an ultrasound?

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