Overview of Sonography 3rd Trimester and Placenta Assessment, Protocols, and Interpretation
Ultrasound is a tool often used during a woman’s third trimester of pregnancy, which is from week 28 to week 42. The ultrasound is used for several things including determining how many babies the woman is carrying and their position (presentation), checking for growth problems, and looking at the placenta (the organ that provides oxygen and nutrients to the growing baby) and the amount of fluid that surrounds the baby (amniotic fluid).
The use of ultrasound in the third trimester differs from the earlier stages of pregnancy due to different goals and focus areas.
At the moment, there isn’t a universally accepted method for monitoring the progress of the pregnancy during this time. Ultrasounds are not typically done in the third trimester unless the woman has not had any previous ultrasounds, or if there is a history of complications or concerns raised during the initial or second-trimester ultrasounds.
What doctors look for during these ultrasounds depends on the reason for the examination. If there are any issues or unusual findings, more in-depth studies are carried out to gain a better understanding of the situation.
Anatomy and Physiology of Sonography 3rd Trimester and Placenta Assessment, Protocols, and Interpretation
In the last three months of pregnancy, an ultrasound scan can be done either through the vagina (transvaginally) or over the belly (trans-abdominally). Doctors usually prefer the belly approach, as it could be potentially less risky. We worry that vaginal scanning may cause severe bleeding if the placenta is near the birth canal opening and we’re not aware of it.
The vaginal scan can provide a closer look at the structures because the probe is nearer to them. The flip side of this is that the high-frequency sound waves used for this kind of ultrasound might hamper viewing deeper or lower down structures inside and outside the uterus (womb).
The uterus is generally found sandwiched between the bladder in front and the colon, which is behind it. From the top downwards, you can identify the fundus (the top part), the body, and the cervix (the neck of the uterus) of the uterus. In the final trimester of pregnancy, one should be able to clearly see the baby’s body and heart activity.
The amniotic fluid, the water the baby floats in, appears as a dark area within the uterus, which has walls of the same brightness. The location of the placenta, the tissue that connects mom and baby for nutrient and oxygen exchange, can vary. It looks different from the uterus wall and has the umbilical cord leading from it to the baby’s belly.
Why do People Need Sonography 3rd Trimester and Placenta Assessment, Protocols, and Interpretation
In a pregnant woman who’s not showing any symptoms, having an ultrasound in the third trimester (the last three months of pregnancy) can help check for several things. These include:
- What the baby looks like inside (fetal anatomy).
- If there are any problems with the baby (fetal anomalies).
- How far along in the pregnancy she is (gestational age).
- How much the baby has grown (fetal growth).
- The baby’s position (fetal presentation).
- Any suspicion of more than one baby (suspected multiple gestations).
- The position of the placenta, the organ that provides nutrients to the baby (placental location).
- Any weakness in the neck of the womb (cervical insufficiency).
If the pregnant woman has any symptoms, an ultrasound can help detect numerous conditions. Some of these include:
- A difference between how big the womb feels and how far along the pregnancy is (discrepancy between uterine size and calculated gestational date).
- A lump in the pelvis (pelvic mass).
- A pregnancy that’s not in the womb (ectopic pregnancy).
- If the baby has died in the womb (fetal death).
- Irritation of the womb (vaginal bleeding, abdominal or pelvic pain).
- Rare forms of cancer of the womb (hydatidiform mole).
- Abnormalities in the womb, baby, or amniotic fluid (the fluid that surrounds the baby in the womb).
- Situations where the placenta, the organ that provides nutrients to the baby, has separated prematurely from the womb (placental abruption) or is located near or covering the opening to the birth canal (placenta previa).
- Situations where the bag of water around the baby has burst before labor starts (premature rupture of membranes) or labor has started too early (premature labor).
Ultrasound can also be used to help with certain procedures, like drawing fluid from around the baby (amniocentesis), stitching the neck of the womb (cervical cerclage placement), or turning the baby’s position when they’re facing in the wrong direction (external cephalic version).
Ultrasound can be particularly helpful in the third trimester for pregnant women who have not been able to access prenatal care earlier. These women may be more likely to experience the above-mentioned problems, and an ultrasound can help spot them early or prepare for them, reducing the risk of serious issues for the mother and baby.
In some cases, a more detailed check of the baby or the placenta may be needed. This might involve specialized tests that are not covered in this basic explanation.
When a Person Should Avoid Sonography 3rd Trimester and Placenta Assessment, Protocols, and Interpretation
There are only a few reasons for not doing an ultrasound through the belly (transabdominal ultrasound) in the last three months of pregnancy. The one reason that everyone agrees on is that if the mother-to-be does not want the ultrasound, it should not be done. This is called the principle of patient autonomy, which basically means respecting the patient’s decisions about their own health care.
Ultrasound done through the vagina (transvaginal ultrasound) is not usually advised during the last three months of pregnancy. This is particularly true if doctors do not already have recorded information about where the placenta (the organ that provides oxygen and nutrients to your baby) is located. The reason for caution is that there could be a condition known as placenta previa, where the placenta is too close to the opening of the womb, which can cause bleeding during the ultrasound. It’s safer to get a transabdominal ultrasound first to check the placenta’s location, and only then consider a transvaginal ultrasound if medically needed.
That being said, a transvaginal ultrasound can be safe during the last part of pregnancy because of the natural angle of the cervix and vaginal canal. The risk of causing serious bleeding has not been confirmed by all studies and the opinions vary about it.
Equipment used for Sonography 3rd Trimester and Placenta Assessment, Protocols, and Interpretation
For a proper ultrasound check-up, two different types of tools, called probe transducers, are needed. The first tool, a curvilinear probe which uses low frequency sound waves (1-6 MHz), is best for examining the outside of the abdomen (also known as a transabdominal assessment).
The other tool, a high-frequency endocavitary probe, uses stronger sound waves (7.5-10 MHz) and it is designed for checking the insides by going through the vagina (also known as transvaginal evaluation).
There needs to be a protective cover on the probe when used for transvaginal examination. It is crucial to clean the probe after each use as per the standard cleaning instructions of your healthcare institution to prevent infections.
Preparing for Sonography 3rd Trimester and Placenta Assessment, Protocols, and Interpretation
When doctors need to study the inside of your abdomen, they may use a process known as a transabdominal ultrasound. For this test, it’s usually better for the patient to have a full bladder. Why? Because this makes it easier for the doctor to see deeper inside your body. When the bladder is full, it works like a sort of ‘window’, letting the ultrasound see clearer images of structures like the uterus, the placenta in pregnant women, and the amniotic fluid.
However, if your doctor advises a transvaginal ultrasound – which takes a closer look at the female reproductive system – it’s better to empty your bladder before the test. When the bladder is empty, the doctor gets a better view of specific structures, such as the cervix, which is the lower part of the uterus. Also, having an empty bladder can make the procedure more comfortable for you.
How is Sonography 3rd Trimester and Placenta Assessment, Protocols, and Interpretation performed
When checking a pregnant woman’s belly using ultrasound, the doctor will use a tool called a curvilinear probe, set with the right settings for pregnancy. This probe can be placed in two different directions. For the first, the probe’s marker points towards the head for a vertical image. For the second, the marker points to the right side for a horizontal image.
In some cases, the doctor may need to do a vaginal ultrasound. For this, the woman lies down with her legs apart. The probe is inserted into the vagina with the marker pointing up for vertical images, and turned to face the right for horizontal images.
To check the baby and the mother’s health, the doctor will take several measurements. These include the size of the baby’s head, the length of its leg bones, and the size of its stomach. These measurements can help estimate the baby’s age and weight. However, these estimates might not be very accurate in the third trimester (last three months of pregnancy). Movement of the baby can usually be seen on the ultrasound and should be noted. However, if the baby isn’t moving, there may be different reasons why, and this could be due to the baby’s sleep cycle or other factors.
The doctor will also check the baby’s heartbeat using a mode on the ultrasound machine. The heart rate should be between 110 and 160 beats per minute. If the heartbeat doesn’t fall within this range, further checks may be needed.
The position of the baby is also very important, especially as the woman gets closer to her due date. Doctors will identify the baby’s head and the direction of its spine. If the baby’s head is closer to the cervix (the lower part of the womb), this is described as facing head-down. If the head is further away, it is considered to be breech, or feet-first and the specific type of breech position should be assessed.
The entire body of the baby is evaluated for any visible abnormalities. The head should be symmetrical and certain intracranial (inside the skull) structures should be visible. If any of these structures can’t be identified, further tests may be needed.
The doctor will also look in detail at the baby’s heart, lungs, and abdominal organs. Additionally, the long bones, spine, and position and movement of the body’s extremities will be examined.
The cervix (or neck of the womb), will also be assessed to see if it is open or closed. If open, the distance across it is measured. If it is found to be too short, further checks will be needed, as this could cause risks in pregnancy.
The placenta (the organ that provides oxygen and nutrients to the baby) will also be assessed. It can be found in various positions and should not be lying low in the womb close to the cervix, as it can block the baby’s exit during labor. Different measurements and checks are used to ensure the placenta is healthy.
Lastly, the doctor measures the amount of amniotic fluid (the fluid the baby floats in). Both too much and too little fluid can pose risks to the baby.
All these steps help the doctor to ensure that both the mother and baby are healthy and progressing well in the pregnancy.
Possible Complications of Sonography 3rd Trimester and Placenta Assessment, Protocols, and Interpretation
Getting an ultrasound while pregnant can sometimes carry small amounts of risk. One specific method, called a transvaginal ultrasound, could be risky if you have a condition called placenta previa. This is because it might accidentally harm the placenta and cause bleeding. However, some research suggests that this risk could be less than we think because of the way the ultrasound is done.
There’s another way to do an ultrasound called a transabdominal approach which doesn’t have these risks. That’s why this method is usually tried first, especially during the later part of pregnancy.
It’s important to note that ultrasounds can sometimes cause anxiety or slight discomfort. The ultrasound device might cause discomfort when it’s pressed against the skin or vaginal mucosa, and some pregnant women might feel anxious about the procedure.
What Else Should I Know About Sonography 3rd Trimester and Placenta Assessment, Protocols, and Interpretation?
A key first step in an ultrasound of a pregnant patient is to determine how many babies she’s carrying. Multiple pregnancies can often be riskier, so it’s important to spot these early. Mistakes can occur if the entire womb, including the top end, is not completely scanned. Always be sure to follow each baby’s head and body to accurately count them.
After we know how many babies are present, the next step is to check if the pregnancy is healthy. This involves observing heartbeats and movement. A slower than normal heartbeat could indicate problems, but it’s not the only factor in evaluating the pregnancy’s health.
It’s crucial to track how the baby is lying in the womb and which body part is closest to the birth canal, especially as the due date nears. ‘Fetal lie’ refers to the angle the baby makes in relation to the uterus. A longitudinal lie where baby and uterus align in parallel, like a perfectly positioned log in a tube, is the most common. There are also transverse lies, where the fetus lies across the uterus, and lesser seen oblique lies, where it’s situated diagonally.
Meanwhile, ‘presentation’ refers to the body part of the baby which is nearest to the cervix. It’s best when the baby’s head is positioned near the cervix (cephalic presentation) as it makes delivery smoother. Other presentations such as breech (bottom first), footling (leg first), or kneeling (knee first) can lead to more difficult labors, and thus more potential complications. If an ultrasound finds a baby’s head near the cervix, it’s important to confirm its lie before declaring it’s in the ideal cephalic position, otherwise, the result might be inaccurate.
The third trimester checks also attempt to estimate the baby’s gestational age and weight. However, these measurements are less precise than those made in the first trimester. Problems with the baby’s development may affect measurements.
It’s also vital to assess the amount of amniotic fluid, the liquid that surrounds and protects the baby in the womb. If there’s too little (oligohydramnios), it could mean the baby’s life is at risk and requires immediate medical attention. On the other hand, if there’s too much (polyhydramnios), it shouldn’t be ignored because it could cause issues for both mom and baby. However, the need for further tests isn’t as urgent. In cases of multiple pregnancies, greater quantities of amniotic fluid may signal birth defects, most of which are related to one twin absorbing more nutrients and fluid than the other.
The placenta’s location and its relation to the cervix are essential to observe as well. If an ultrasound can’t confirm the absence of a low-lying placenta (placenta previa) after exhaustive efforts, it’s safer to assume it is present. In patients with prior C-sections and possible placenta previa, it’s crucial to inspect for deeply implanted placenta (placenta accreta).
For patients that report abdominal pain, there’s a major concern about placental abruption (where the placenta detaches from the uterine wall). Ultrasounds are not perfect at detecting this as the muscle of the uterus and its blood vessels can often be mistaken for blood pools. Smaller detachments can also be dismissed as normal uterine muscle.
An ultrasound can also inspect the umbilical cord and search for vasa previa (where fetal blood vessels block the birth canal.) Warning flag conditions include a resolved placenta previa, excess placental lobe, and a cord that attaches to the membranes rather than the placenta (velamentous cord insertion).
No ultrasound can examine a baby comprehensively for all possible abnormalities. Nevertheless, big abnormalities generally pop up during a regular ultrasound. Exactly what constitutes a ‘reassurance’ in the absence of visible problems remains debated because the topic is complex. Sometimes, issues picked up during early pregnancy can resolve by the third trimester, while others only appear as the pregnancy progresses. If only one ultrasound is done, it’s recommended to do it between week 18 to 20. Specific abnormalities found should be evaluated by a sonographer with expertise in that area.
