The gestational sac is the first structure formed in pregnancy, surrounding and protecting the baby. It also develops into the placenta and the amniotic sac, remaining present until about the 12th week of pregnancy.
The gestational sac can usually be seen on an ultrasound between 4.5 and 5 weeks of pregnancy, when it measures about 2 to 3 mm in diameter and is located in the center of the uterus. At this stage, the embryo may not yet be visible and often only appears around the 6th week of pregnancy.
Monitoring the gestational sac is an important way to check if the pregnancy is developing normally. Doctors assess factors such as implantation, size, shape, and the contents of the gestational sac.
Gestational sac size
The gestational sac grows steadily as pregnancy progresses:
These reference values for gestational sac size allow doctors to detect possible issues and abnormalities in pregnancy development at an early stage.
Common problems with the gestational sac
A healthy gestational sac has smooth, symmetrical contours and is well implanted. When it shows irregularities or low implantation, the chances of pregnancy not progressing are higher. The most common problems include:
1. Empty gestational sac
After the 6th week of pregnancy, if no fetus is seen on ultrasound, this indicates an empty gestational sac, meaning the embryo did not develop after fertilization. This condition is also called an anembryonic pregnancy or blighted ovum.
The most common causes of an embryo not developing are abnormal cell division and poor egg or sperm quality. Doctors usually recommend repeating the ultrasound around the 8th week to confirm an anembryonic pregnancy. If confirmed, the doctor may choose to wait for a natural miscarriage or perform a dilation and curettage (D&C), which requires hospital admission.
2. Gestational sac detachment
Gestational sac detachment can occur due to a hematoma in the sac, often linked to physical exertion, falls, or hormonal changes such as progesterone irregularities, high blood pressure, or the use of alcohol and drugs.
Signs of detachment include mild to severe cramping and brown or bright red bleeding. When detachment is greater than 50%, the risk of miscarriage is high. There is no proven way to prevent detachment, but when it occurs, doctors usually recommend medication and complete rest for at least 15 days. In more severe cases, hospitalization may be needed.
How to know if the gestational sac was expelled
When the gestational sac is expelled, symptoms often include abdominal pain and vaginal bleeding. This bleeding is usually similar to menstruation but can be heavier and include larger blood clots. In some cases, the gestational sac may not be visible.
If expulsion of the gestational sac is suspected, it is important to see an obstetrician promptly for evaluation. To confirm whether the sac was expelled, tests such as an ultrasound and blood beta-hCG measurement may be ordered.
When to see a doctor
Medical care is essential if symptoms like intense cramping or bleeding occur. In these cases, it is important to seek immediate care at a maternity unit or emergency department and notify the doctor who is monitoring the pregnancy. Only a doctor can diagnose problems with the gestational sac, usually through ultrasound, which is why starting prenatal care as soon as pregnancy is confirmed is very important.