Overview of Gestational Sac Evaluation
The gestational sac is a structure filled with fluid that envelops an embryo in the early stages of its development. This is usually during the first few weeks after conception. It is the first thing that can be seen during a pregnancy ultrasound, appearing as soon as 4.5 to 5 weeks into pregnancy. Being able to identify the gestational sac is very helpful in confirming a pregnancy that’s developing in the womb (also called an intrauterine pregnancy), with an accuracy rate of about 97.6%.
Anatomy and Physiology of Gestational Sac Evaluation
The uterus is an organ found in women, responsible for reproduction. You can see it on an ultrasound probe placed either on the belly or inside the vagina. It’s positioned right behind the bladder and in front of the colon. The ovaries, two other major reproductive organs, are located on either side of the uterus in the area known as the pelvic cavity.
The uterus has three main parts: the cervix (the lower part that opens into the vagina), the body (the main part of the uterus), and the fundus (the top part). When a woman is not pregnant, the inside of the uterus (the uterine cavity) is flat. At early pregnancy around 4.5 to 5 weeks, a small structure called the gestational sac can be seen in the middle of the uterus. This sac, which initially measures about 2 to 3 mm in diameter, is where the baby will grow and develop.
Why do People Need Gestational Sac Evaluation
If you’re in your first three months of pregnancy and aren’t having any worrisome symptoms, your doctor will likely recommend an ultrasound between week 11 and 13. This is done to confirm how far along you are and to make sure the pregnancy is developing as expected.
If you’re experiencing symptoms like stomach pain, pelvic pain, or vaginal bleeding during your pregnancy, your doctor will need to take a closer look with a pelvic ultrasound. This will help determine whether the pregnancy is in the uterus (as it should be), whether the location of the pregnancy is unclear, or whether the embryo might be growing outside of the uterus in what is known as an ectopic pregnancy.
Equipment used for Gestational Sac Evaluation
In the first three months of pregnancy, a detailed ultrasound is performed through both the abdomen and vagina to check the development of the baby. This is known as a comprehensive first-trimester two-dimensional obstetric ultrasound.
For the exam done via the stomach or transabdominal, a curved device known as a curvilinear probe is used. This probe, which operates at a frequency between 1 and 6 MHz, sends sound waves into the abdomen to create an image of the baby and its surroundings on a monitor.
For the part of the ultrasound done via the vagina or transvaginal, a high-frequency (7.5 to 10 MHz) probe is used. This type of probe provides a clearer picture of your baby and uterus due to it being closer to these structures. It’s the preferred method for examining the pelvic region during the ultrasound.
How is Gestational Sac Evaluation performed
First, a doctor examines the patient’s abdomen, specifically focusing on checking the uterus and the regions around the ovaries. The patient points out exactly where the pain is, so the doctor can focus on that area. When the doctor looks at images from this area, they also check if there is any sign of bleeding within the abdomen. If they find any fluid outside of the uterus, the doctor might also scan the upper abdomen to see if there is more bleeding.
Next, the doctor does a transvaginal examination. This is an examination inside the vagina, focusing further on the uterus, fallopian tubes, and ovaries. This is done using a transvaginal ultrasound. An ultrasound uses sound waves to create images of the inside of the body. The transvaginal ultrasound produces clearer images than the abdominal ultrasound because it uses higher frequencies and is not distorted by the abdominal wall. Additionally, unlike the abdominal ultrasound, the transvaginal ultrasound does not require the bladder to be full during the assessment.
M-mode ultrasound, a type of scan using motion over time to create images, is considered safe at all stages of pregnancy. Another kind, Doppler ultrasound (that shows blood flow), uses more energy, so it is only recommended when necessary.
Finally, the doctor calculates the Mean Gestational Sac Diameter (mGSD). The gestational sac is what nourishes the embryo in early pregnancy. The mGSD is calculated by averaging three measurements: the height, width, and length of the gestational sac. In other words, the doctor will add these measurements together and divide the total by three.
What Else Should I Know About Gestational Sac Evaluation?
If you’re early in your pregnancy and experiencing symptoms like stomach or pelvic pain, or vaginal bleeding, your doctor will likely use an ultrasound scan of your pelvis along with a blood test that checks for a hormone called human chorionic gonadotropin (B-hCG) to understand what’s happening. These two tests help the doctor recogonise if the pregnancy is in the uterus (an intrauterine pregnancy), if the location of the pregnancy is unknown, or if it might be an ectopic pregnancy, where the pregnancy is outside the womb.
Between the 5th and 7th week of your pregnancy, your baby grows in a way that doctors can predict. Doctors use this information to determine where the pregnancy is located and how far along you are. Around 4.5 to 5 weeks, the doctor would see what’s known as a gestational sac on an ultrasound. This is a round structure that first shows up in the middle section of the uterus, looking small and measuring around 2 to 3 mm. As each day goes by, it grows a little bit, roughly 1.13 mm each day. By the end of 5.5 weeks, a thin, circular object, known as the yolk sac, can be spotted within this gestational sac. This yolk sac will then expand to approximately 6 mm by the time it’s visible. By the 6th week, you’d be able to see a structure in the yolk sac that represents the embryo, or the baby, which measures around 1 to 2 mm. By the time you reach 8 weeks, the baby’s head, body and limb buds (early stage of arms and legs) can be seen.
In ultrasound scans, spotting an intrauterine gestational sac can confirm an intrauterine pregnancy with up to 97.6% accuracy. If a yolk sac is seen, it’s 100% certain. It’s important to note that a normal gestational sac is smooth and either round or oval and is found in the central region of the uterus. If it’s found elsewhere, there could be concerns about an ectopic pregnancy. Sometimes, you might see what’s called a ‘pseudogestational sac’ on an ultrasound. This is not round or oval and may contain debris, and it might suggest that the pregnancy is ectopic.
The predictable growth of the embryo implies that a certain size of the gestational sac, without an embryo, is a sign that the pregnancy may not be progressing. If the gestational sac measures 25 mm or more without an embryo, it’s indicative of pregnancy failure. Additionally, if it measures between 16 and 25 mm without an embryo, there’s a possibility of pregnancy failure. Other signs of potential pregnancy failure include not seeing a yolk sac develop two or more weeks after seeing the gestational sac or not spotting a heartbeat in the embryo 11 or more days after visualizing the yolk sac. Under such circumstances, doctors may recommend a repeat ultrasound after around 11 to 14 days.
In the past, the levels of the hormone B-hCG were used to diagnose or monitor early pregnancies. But studies have found that B-hCG levels can change a lot during pregnancy, and using only B-hCG levels to make a diagnosis when an intrauterine pregnancy can’t definitely be confirmed using an ultrasound isn’t recommended. Instead, doctors advise taking multiple B-hCG tests along with doing repeat imaging when the diagnosis isn’t clear. When an ultrasound shows something unclear in the uterus and B-hCG levels are high in a stable patient, the most likely diagnosis is a pregnancy of unknown location. In such cases, an ectopic pregnancy is less probable as it occurs in about 2% of pregnancies. So, in most cases, this situation likely indicates an early intrauterine pregnancy. In these cases, the doctor would likely suggest doing another ultrasound and B-hCG test after 48 to 72 hours.