Overview of Sonography Assessment of Gestational Age
Sonography, also known as an ultrasound, can be used at various stages of pregnancy to determine the age of a developing baby. Knowing the correct age of the fetus is crucial because it affects the care plan for both the mother and baby, the expected delivery date, and measures of the baby’s growth. The most precise way to figure out the baby’s age is between the 8th and 12th weeks of pregnancy; this is when we can measure the crown-rump length, which is essentially the size of the baby from its head to its bottom. This period is also when the baby grows the fastest and most consistently.
Other stages of development can also be used to confirm these dates when necessary. Portable ultrasound machines, even those used right on the spot where care is provided, can do this just as accurately as more advanced machines.
Anatomy and Physiology of Sonography Assessment of Gestational Age
The first trimester of pregnancy is when the baby starts developing and includes the first 12 weeks. During this time, an ultrasound can identify three key structures: the gestational sac, the yolk sac, and the embryonic pole. These are the first visible signs of pregnancy on an ultrasound.
The gestational sac can usually be seen around 30 days after the start of the woman’s last menstrual cycle. It looks like a small, dim structure near the top of the uterus. This sac is the early stage of what eventually becomes the placenta and the fluid-filled sac where the baby grows. It increases in size by about 1mm every day after it becomes visible.
Inside the gestational sac, is the yolk sac. It is a small, bright ring that usually appears about 35 days, or 5 to 6 weeks, after the woman’s last menstrual cycle. The yolk sac also grows about 1mm per day until it reaches its maximum size at about 9 weeks, and then it starts to get smaller. The yolk sac provides the embryo with nutrients before the placenta is fully formed. But it’s not generally used to figure out the age of the pregnancy.
The embryonic pole is the early form of the developing baby, or embryo, and it first appears as a small line that aligns with the yolk sac, giving it a “ring” appearance. This is the first part of the developing baby that can be measured to find out the exact age of the pregnancy. The embryonic pole will develop a head and tail part that will eventually unfold to form the baby’s body and limbs. At about 9 weeks, the baby’s organ formation is complete and this marks the transition from embryo to fetus as the baby focuses on growth and further development.
Why do People Need Sonography Assessment of Gestational Age
If a woman is pregnant and doesn’t know the date of her last menstrual cycle, it’s important to figure out her “gestational age”. Gestational age is basically how long the pregnancy has been going on. This helps the doctor confirm estimated dates for the pregnancy’s progress, and it’s especially important for patients who don’t know how long they’ve been pregnant.
When a Person Should Avoid Sonography Assessment of Gestational Age
There are no known reasons why ultrasound can’t be used to figure out how old an unborn baby is. However, doctors need to make sure they use as little ultrasound energy as possible and only for a very short time to get accurate information.
Equipment used for Sonography Assessment of Gestational Age
How far along you are in your pregnancy can help your doctor choose the best ultrasound tool for your examination. Ultrasound is a safe and painless test that uses sound waves to make images of your baby. If you’re more than 8 weeks pregnant, a low-frequency tool (called a curvilinear probe) is often chosen, especially when used for an ultrasound method called transabdominal, where the ultrasound wand slides on your belly. This is almost as accurate as an endovaginal ultrasound, which is done by inserting a wand into the vaginal canal.
If you’re less than 12 weeks pregnant, a tool called an endocavitary probe is often best. This tool is used directly inside the vagina. Between 8 to 12 weeks of pregnancy, either tool may work to see your baby properly, so it might be needed to use both tools to see your baby and the pregnancy sac (called the gestational sac), its nutritional hub (called the yolk sac), and its preliminary backbone (referred to as embryonic pole).
In these cases, it’s often recommended to start with the curvilinear probe used on your belly. If your doctor can’t see everything they need to, they might switch to the tool that’s used inside the vagina called the endocavitary probe.
Preparing for Sonography Assessment of Gestational Age
When using medical probes for exams, it’s important to clean and take care of them according to the instructions given by the manufacturer. When a patient lies down on their back (a position known as supine), the curvilinear probe gives the best images. If an endovaginal approach is needed, it’s easiest for the doctor to perform the exam when the patient is in the lithotomy position, which is lying on their back with their legs raised and bent.
During a transabdominal exam, which is a scan done from outside the abdomen, it’s recommended that the patient should have a full bladder. This is because the urine-filled bladder creates a clear path for the sound waves, making the images clearer. A little gel is then put on the curvilinear probe which is used to scan the area around the abdomen and pelvis. The scan is done from top to bottom (longitudinal axis) and from side to side (short axis).
For a transvaginal exam, which is a scan done through the vagina, the patient should empty her bladder just before the exam. This is because the cervix, the lower part of the womb, provides a clear path for the scan and a full bladder can move the uterus in a way that makes it more difficult to detect and measure a pregnancy. A small amount of ultrasound gel is put on the tip of the endocavitary probe, and a cover (often with pre-placed gel) is placed over it. Sometimes, a Vaseline gel is used to cover the probe before it’s placed in the vagina.
How is Sonography Assessment of Gestational Age performed
Doctors use ultrasound imaging to get a good look at a baby growing inside a mother’s womb. This can be done from two angles: across the abdomen or from top to bottom. These images are taken from two different kinds of ultrasound probes; one that moves across the belly and another that goes inside the birth canal. After the images are taken, the age of the baby can be determined. This can be done by considering the mother’s last period or from physical exam.
In the First Three Months
The best time to determine the baby’s age is between 8 to 12 weeks of pregnancy. This is when the baby grows the fastest and most steadily. Using this method, a doctor can tell the age of the baby with accuracy up to 5 days. There are two methods used in the first trimester to determine the baby’s age: one by looking at the size of the gestational sac (the structure that forms to house the baby) and the other by measuring the length of the baby from its head to its buttock, also known as the crown-rump length.
The gestational sac can be seen as early as 5 weeks into pregnancy through an internal examination and by 6 weeks through an external examination. It grows about 1 mm per day and around the time it reaches about a size of 14 mm, the measurements become less accurate. The age of the baby is determined by measuring the diameter of the sac from an inside wall to another. If the sac is not perfectly round then three measurements taken from the inside of the sac using different views may be needed. These diameters are then compared to a table of values or entered into a calculation to get the baby’s estimated age.
The crown-rump length measurement is the most accurate way to determine the baby’s age in the first trimester. This measurement can be done as early as 6 weeks into pregnancy when the baby first appears on ultrasound as a small structure on the edge of the yolk sac. It is very precise between 6-10 weeks. The measurement is made from the topmost part of the baby to its rump, excluding the limbs and yolk sac. The age of the baby is usually calculated by the ultrasound machine based on these measurements.
In the Second Trimester
In the second and third trimesters, finding out the baby’s age is less accurate due to differences in growth rates, genetic variations and attributes of the growing fetus. Multiple methods can be used during these stages to get a more accurate age. Five common methods are used in the second trimester including the crown-rump length, the biparietal diameter (measurement from one side of the head to the other), the head circumference, the abdominal circumference and the femur length (measurement of the thigh bone).
Crown-rump length can be used to estimate the baby’s age up to 14 weeks of gestation but it becomes less accurate. Another method is the Biparietal diameter which measures the baby’s head from one side to the other and is accurate up to about a week. This measurement requires a clear view of the head. The head circumference taken in the same view as biparietal diameter is also used. The measurement around the baby’s belly taken in an across view at the point, where a main vein divides and including the baby’s stomach is also a method that offers an accuracy of about 7 to 15 days. Lastly, the length of the baby’s thigh bone not including cartilage is measured.
In the Third Trimester
The methods in this stage are similar to those in the second trimester, specifically the measurements of the baby’s belly and thigh bone. These measurements show accuracy with an error rate of around 18 to 35 days and 21 days, respectively. Re-testing and follow-up are necessary as dating becomes less accurate in the third trimester.
Possible Complications of Sonography Assessment of Gestational Age
A comprehensive review done in 2009 determined that having an ultrasound during pregnancy is safe. However, certain challenges may arise related to the accuracy of determining how far along the pregnancy is, and how this might affect care for the mother or her baby. It’s also crucial to know that if a pregnancy is developing in the wrong place, like in a fallopian tube (known as an ectopic pregnancy), a false image of pregnancy may appear in the uterus during an ultrasound. If this is misdiagnosed, it may lead the healthcare team to miss the ectopic pregnancy, which may result in serious complications.
What Else Should I Know About Sonography Assessment of Gestational Age?
Knowing the correct age of the unborn baby, known as gestational age, is crucial for many aspects of prenatal care. It helps the doctor understand if the baby is developing normally and if it would be able to survive outside the womb. This information is also used to determine the due date, which can influence future decisions around childbirth. For instance, it assists in deciding if a cesarean section (c-section) might be necessary or whether labor might need to be induced early. Moreover, accurately figuring out the age of the baby helps in tracking its growth rate and size. So, the importance of having the correct gestational age cannot be understated.