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Updated on
October 23, 2023

20-Week Ultrasound Explained

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20-Week Ultrasound Explained.
20-Week Ultrasound Explained

Around 20 weeks pregnant, you’ll most likely have the most-anticipated screening you’ll get during pregnancy. It’s the Level 2 full-body anatomy scan, which is part of the second-trimester battery of screens, as well as an amazing opportunity to look at your baby.

We asked Dr. Robyn Horsager-Boehrer, Chief of Obstetrics and Gynecology at UT Southwestern Medical Center’s William P. Clements Jr. University Hospital, why the 20-week ultrasound is so important.

An ultrasound done around 20 weeks offers patients and doctors a chance to get a good view of the developing fetus,” she says. “Different parts of the fetus are measured and give an assessment of the growth of the fetus. We also obtain images of different organs and structures of the baby like the heart and brain to look for any anomalies that may be present.”

What Happens During the 20-Week Ultrasound?

The ultrasound tech does a complete scan looking at baby’s body: the brain and spine, face, abdomen, limbs and all four chambers of the heart. They are also measuring everything to make sure the baby is growing at the right pace for their gestational age. “The measurements are plotted against nomograms and give a sense of the overall growth of the fetus,” says Dr. Horsager-Boehrer.

Lastly, the scan will also look at your uterus and placenta to make sure all’s well with mom too. “The uterus is examined to see if there are any anomalies like a uterine septum that might impact the pregnancy,” Dr. Horsager-Boehrer says. “Uterine fibroids are benign uterine masses that may also impact pregnancy outcomes and delivery plans. Examination of the placenta provides a lot of information. We look to see where the placenta has attached to the uterus, making sure it isn’t covering the cervix or in the area of a previous cesarean section scar. We also check where the umbilical cord is attached to the placental mass.”

Your provider may also measure your amniotic fluid levels. “The volume of amniotic fluid represents a balance between fetal functions like swallowing and urine production and placental function,” says Dr. Horsager-Boehrer. “Low levels can be associated with placental dysfunction or genitourinary problems like the absence or malfunction of kidneys or a blockage in the urethra that prevents urine from leaving the bladder. Increased levels may represent some sort of blockage in the GI tract. More often, amniotic fluid problems are found in the last trimester of pregnancy and not at the 20-week ultrasound.”

If you’re curious, you can learn the baby’s gender now. (If you want to keep it a secret, be sure to tell your tech or they will start calling the baby he or she.)

What do you have to do to get ready? Basically nothing. Drinking some water beforehand can help them get a better look, but you can just show up. Prepare to spend some time, though. There’s a lot going on in there! The scan takes about 30 to 45 minutes if baby’s in a good position, or even longer if you have to wait for them to pose.

Other 20-Week Screening Tests

Around this time—between 16 weeks pregnant and 20 weeks pregnant—you’ll also be offered other screening tests.

First, let’s break down the difference between a screening test and a diagnostic test.

  • A screening tells you the chances that your baby has a disorder. Screening tests are generally based on blood samples from mom and ultrasounds of the baby.
  • Diagnostic tests tell you whether your baby actually has the disorder. Think “diagnosis.” These require extracting cells from the placenta or fetus as in an amniocentesis or chorionic villus sampling (CVS).

Most women are only offered screens during their pregnancy, unless they are in a high-risk group and their doc recommends going straight to diagnostic tests.

The thing about screening tests is that they aren’t that accurate. They cast a wide net, and a “positive” result does not mean that your baby actually has something—it means they are at a higher risk of having a disorder than the general population. Likewise a “negative” result means that they are at a lower risk, not that they don’t have it.

So what kind of screens are you in store for? The two straight-forward screening tests are:

Quad screen: This measures the quantity of four substances in your blood to determine your baby’s risk of having a neural tube defect, Down syndrome or another genetic condition. “The quad test is another way to screen for Trisomy 21 which measures different factors in the maternal blood,” explains Dr. Horsager-Boehrer. “It’s done in the second trimester, which allows for women to get care beyond 14 weeks when nuchal translucency screening is an option.”

Genetic carrier screens: It turns out you can be a carrier for a disease that you don’t have. Some ethnicities have higher risk of carrying certain genetic conditions: Ashkenazi Jews are more likely to carry Tay-Sachs and African Americans are more likely to carry sickle cell. So the genetic carrier screen is often given based on your ethnicity. In late February 2017 the American College of Obstetricians and Gynecologists (ACOG) started recommending that all patients be offered some kind of carrier screen, but these guidelines have not been widely adopted. That said, both parents have to be carriers for a child to be at risk, so your doctor may not even recommend these screens.

Also, you don’t have to have to have any of these screens. Some families opt out. Others want to know everything so they can be as prepared as possible. The choice to go forward with screening and diagnostic testing is a personal one, says Dr. Horsager-Boehrer: “You should understand whether you are at particular risk for being a carrier for certain conditions that can be screened for, and then decide what you would do if you find that you (and potentially your partner) are carriers. Would you pursue invasive prenatal testing that comes with a small chance of pregnancy loss? If the fetus is affected, have you thought about what that means for you? It’s important to give this some consideration before accepting testing. Often I see patients and partners trying to make these decisions under circumstances that aren’t optimal and are very rushed.” So take your time, and remember that there is no right or wrong answer—it’s up to each individual family.


Babylist Staff

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