The centerpiece of your 20-week prenatal visit is the anatomy ultrasound, a detailed scan that checks your baby’s organs, bones, and growth from head to toe. It’s the most thorough ultrasound of your entire pregnancy, typically lasting 30 to 45 minutes. Depending on your health history and earlier screening results, your provider may also order blood work or a cervical length measurement at the same visit.
The Anatomy Scan: What It Covers
The 20-week ultrasound (sometimes called the morphology scan, performed anywhere from 18 to 23 weeks) is a systematic check of nearly every visible structure in your baby’s body. The sonographer works through a standardized checklist, capturing images of each area so your provider can review them afterward. Here’s what they’re looking at:
- Skull and brain: The shape and integrity of the skull, plus internal brain structures including the fluid-filled spaces (ventricles), the cerebellum at the back of the brain, and the tissue that surrounds it.
- Face: The profile, eye sockets, and upper lip. This is where cleft lip, if present, is usually spotted.
- Spine: Scanned both lengthwise and in cross-section to check that the vertebrae have closed properly.
- Heart: Rate, rhythm, and the four chambers. The sonographer also checks the outflow tracts, the major vessels carrying blood out of the heart. Heart defects are among the most common birth defects, so this part of the exam tends to be the most detailed.
- Lungs and diaphragm: The shape of the chest cavity and the muscle separating it from the abdomen.
- Abdomen: The stomach, liver, kidneys, bladder, and abdominal wall. They confirm the umbilical cord inserts normally into the belly.
- Limbs: All four limbs are measured and checked for normal bone shape and joint movement, from the upper arms and thighs down to the hands and feet.
If the baby is in a difficult position or won’t cooperate, you may be asked to walk around, drink something cold, or come back for a follow-up scan. An incomplete scan doesn’t mean something is wrong. It just means the sonographer couldn’t get a clear view.
Growth Measurements
Four key measurements tell your provider whether the baby is growing on track for gestational age. These are the head width (biparietal diameter), head circumference, abdominal circumference, and femur length. At 20 weeks, typical averages are roughly 48 mm for head width, 176 mm for head circumference, 148 mm for abdominal circumference, and 33 mm for femur length. Your provider plots these on a growth chart and compares them to standard ranges. A single measurement slightly outside the expected range is rarely cause for concern on its own, but a pattern of measurements that fall consistently small or large may prompt closer monitoring.
Placenta and Amniotic Fluid
The scan also evaluates two things that aren’t part of the baby but are critical to the pregnancy: the placenta and the amniotic fluid.
Your provider checks where the placenta is attached inside the uterus. If it’s sitting low and covering or nearly covering the cervix (called placenta previa), they’ll note it. Many low-lying placentas migrate upward as the uterus grows, so a follow-up scan later in pregnancy often resolves the finding without any intervention.
Amniotic fluid volume is assessed by measuring the deepest pockets of fluid visible on ultrasound. A normal amniotic fluid index for a single pregnancy after 20 weeks falls between 5 and 24 cm. Too little fluid (oligohydramnios) or too much (polyhydramnios) can signal issues with the baby’s kidneys, swallowing, or other systems and will prompt additional testing.
Cervical Length Check
Some providers measure your cervix at the 20-week visit using a transvaginal ultrasound probe, particularly if you have risk factors for preterm birth such as a prior preterm delivery or a history of cervical procedures. This is a quick, painless internal scan separate from the abdominal anatomy ultrasound.
A cervix shorter than 3 cm between 20 and 24 weeks is strongly associated with preterm delivery. In one study, over 93% of women with a cervical length under 3 cm at this stage delivered before full term, with an average delivery around 35 weeks. If your cervix measures short, your provider may recommend treatment options like progesterone supplements or a cervical cerclage to help prolong the pregnancy.
Soft Markers and What They Mean
Sometimes the anatomy scan reveals minor findings called “soft markers.” These are subtle variations that can appear in perfectly healthy babies but show up slightly more often in babies with chromosomal conditions like Down syndrome or trisomy 18. Common soft markers include a bright spot on the heart (echogenic intracardiac focus), small cysts in the brain’s choroid plexus, mildly dilated kidneys, bright-appearing bowel, a thickened fold of skin at the back of the neck, and shorter-than-expected arm or thigh bones.
An isolated soft marker, meaning just one finding with no other abnormalities, carries a very low risk. A thickened nuchal fold and bright bowel have the strongest statistical link to Down syndrome among isolated markers, while a single choroid plexus cyst or echogenic focus in an otherwise normal scan is generally considered reassuring. If your scan shows one or more soft markers, your provider will weigh them alongside your earlier screening results (first-trimester blood work, cell-free DNA testing, or the quad screen) to decide whether further testing like amniocentesis makes sense.
Blood Tests Around 20 Weeks
The quad screen, a blood test that measures four substances produced by the pregnancy, can be drawn anytime between 15 and 22 weeks. If you haven’t had it yet, your provider may offer it at your 20-week visit. It screens for Down syndrome, trisomy 18, and neural tube defects like spina bifida. Many women who had cell-free DNA screening earlier in pregnancy skip the quad screen, so this test isn’t universal.
Routine glucose screening for gestational diabetes typically happens later, between 24 and 28 weeks. However, if you have risk factors (a family history of diabetes, a high BMI, or glucose showing up in your urine at earlier visits), your provider may move that test up to the 20-week window.
How to Prepare
You may be asked to drink two to three glasses of water about an hour before the ultrasound and avoid using the bathroom until after the scan. A full bladder pushes the uterus into a better position for imaging, especially early in the appointment. Once the sonographer has the initial images, they’ll usually let you empty your bladder partway through.
Wear a two-piece outfit so you can easily expose your belly. The gel used during the scan is water-based and wipes off easily, but it can feel cold. The appointment itself is painless. If a transvaginal cervical length measurement is needed, the internal probe causes mild pressure but no significant discomfort. Plan for the visit to take about an hour total, including wait time, the scan, and any discussion with your provider afterward.
What Happens If Something Looks Abnormal
Most anatomy scans come back completely normal. When something does look unusual, the next steps depend on the finding. Structural abnormalities like a heart defect or spina bifida typically lead to a referral to a maternal-fetal medicine specialist for a more detailed ultrasound and counseling about what to expect. Soft markers or borderline findings often lead to a conversation about diagnostic testing. Amniocentesis, which involves drawing a small sample of amniotic fluid, can confirm or rule out chromosomal conditions with high accuracy. It’s offered when ultrasound findings, blood screening results, or both suggest an elevated risk.
Some findings, like a low-lying placenta or a slightly small measurement, simply require a repeat scan in a few weeks to see how things progress. Your provider will explain the specific finding, what it could mean, and whether any follow-up is recommended.