During pregnancy, the first thing to appear, even before the baby, is the gestational sac. Many women find themselves with an empty sac visible on an ultrasound, especially when it is done at the very beginning of pregnancy. This image can bring discomfort, a scare, and also concern due to the uncertainty of whether the pregnancy is healthy. But in fact, what is the gestational sac and what is the ideal size in relation to gestational weeks?
When does the gestational sac appear?
The gestational sac appears when the beta HCG reaches approximately 3,000 mIU in the blood, or around 4 to 5 weeks. It begins what will become the amniotic sac and will grow as the pregnancy progresses. Inside it, at first, the baby will form, which is still microscopic, along with the yolk sac and also a build-up of amniotic fluid.
It originates from part of the blastocyst, which forms from fertilization and divides into two parts. One part forms the baby, and the other part will create all the structures for the pregnancy to grow healthy, such as the placenta and the sac.
Besides providing support to the baby initially up to the 12th week, the gestational sac is also a parameter to know if the pregnancy is progressing as it should1. In a healthy pregnancy, it will appear around the end of the fourth week at a small size, usually viewed as a mark on the uterus.
What should be evaluated in the gestational sac?
From the end of the fifth week onward, it becomes more visible and its shape is more centered and regular. The main characteristics to be assessed to evaluate the initial health of the pregnancy are studied by the gestational sac and through an ultrasound. The doctor will take into account: the shape, implantation, content, and size.
Normally, the baby can be seen within the gestational sac around the 6th week. Before that, only the yolk sac might be visualized. It is advised for the pregnant person to repeat the ultrasound in a few weeks to track development.
Most of the time, a smaller-than-expected gestational sac for the gestational age refers to a pregnancy resulting from late ovulation. In other words, the woman will be less far along in pregnancy than would be calculated by her period.
Gestational sac measurement table
Table of measurements to establish a parameter for gestational age:
| GESTATIONAL AGE | DIAMETER (MM) | VARIATION (MM) |
| 4 | 5 | 2 to 8 |
| 5 | 10 | 6 to 16 |
| 6 | 16 | 9 to 23 |
| 7 | 23 | 15 to 31 |
| 8 | 30 | 22 to 38 |
| 9 | 37 | 28 to 16 |
| 10 | 43 | 35 to 51 |
| 11 | 51 | 42 to 60 |
| 12 | 60 | 51 to 69 |
Problems with the gestational sac
When all goes well, a healthy gestational sac is characterized by regular contours. This means it has formed correctly and is a suitable environment for the baby to grow. But there are cases where the gestational sac forms with problems and shows an unviable pregnancy even before the baby appears. When the gestational sac presents irregular and asymmetric contours, a decidual sac or low implantation, the chances of the pregnancy not progressing are high.
Is it possible to fix a problem with the gestational sac?
It is possible to have problems with a healthy gestational sac that develops a hematoma, better known as sac detachment. In most cases, if there is a hematoma from detachment, the hematoma will disappear spontaneously, meaning the body will absorb it and the pregnancy will proceed as normal. In other cases, the pregnancy will unfortunately end in miscarriage.
The doctor will assess the size of the gestational sac in relation to the gestational period2. If the mean diameter of the gestational sac is between 20mm and 25mm and there is no embryo (empty gestational sac), the doctor will likely diagnose a case of anembryonic pregnancy. Along with the gestational sac measurements, the age of the pregnancy will be determined by the CRL (crown-rump length).
It is extremely important that the woman maintains proper nutrition3. If this is not possible due to nausea, it is recommended that even before the 12th week of pregnancy, she start using a prenatal multivitamin. FamiGesta is a complete vitamin that contains Methylfolate, the active form of folic acid, which is much more efficient in preventing problems in the baby.
Readers’ questions:
What does the gestational sac look like?
The sac is a thin membrane that holds a clear liquid inside. Its appearance is similar to that thin “skin” that surrounds sausages, just as sensitive and easy to tear.
How does the gestational sac form?
The sac forms as the cells destined to become the baby divide. The blastocyst divides in two, with one part forming the sac and the other, the baby.
How to avoid gestational sac detachment?
Placental detachment is a feared complication among pregnant women, but unfortunately, it is not something possible to predict or prevent. Performing prenatal exams and receiving proper care can help control the problems that could result in detachment. Some factors to check include whether progesterone production is correct or even excessive weight gain during pregnancy.
What is an anembryonic gestational sac?
An anembryonic gestational sac, or anembryonic pregnancy as it is known, is when the gestational sac is empty without the presence of an embryo as expected. It is diagnosed via ultrasound in the first trimester. This happens when the fertilized egg implants in the uterus but the embryo does not develop.
See also: 5-Week Ultrasound – I Didn’t See the Baby, Now What?
