20-Week Ultrasound (Anatomy Scan): What to Expect

The 20-week anatomy scan — also called the mid-pregnancy ultrasound or anomaly scan — is the most comprehensive ultrasound you’ll have during pregnancy. Performed between weeks 18 and 22, it gives your provider a detailed look at your baby’s developing organs, bones, and overall growth. For many parents, it’s also the appointment where they first learn whether they’re having a boy or girl. For a full overview of this period, see our second trimester guide.

📌 Key Takeaway: The anatomy scan checks baby’s brain, heart, spine, kidneys, limbs, and placenta position. It takes 30–45 minutes and is performed abdominally. If the sonographer can’t get clear views, you may need a follow-up. Sex determination at 20 weeks is approximately 99% accurate — let your sonographer know your preference at the start. Planning a gender reveal party? This is often when you get the information you need.

What the Anatomy Scan Checks

This ultrasound is far more than a photo opportunity. Your sonographer systematically examines every major structure:

Baby’s Body

Placenta and Fluid

Growth Measurements

The sonographer measures several dimensions to confirm baby is growing on track:

• BPD (biparietal diameter) — head width
• HC (head circumference) — around the skull
• AC (abdominal circumference) — belly measurement
• FL (femur length) — thigh bone

These measurements are compared against standard growth charts. A small difference from the “average” is normal — babies vary in size just like adults.

📊 Key Data: According to a 2020 study in Ultrasound in Obstetrics & Gynecology, the anatomy scan detects approximately 50% of major congenital anomalies. Detection rates are higher for some conditions (95% for anencephaly) and lower for others (25% for heart defects, which is why some providers recommend a fetal echocardiogram for high-risk pregnancies).

How to Prepare

Before the Appointment

• Eat normally — unlike some blood tests, you don’t need to fast
• Drink water — a moderately full bladder helps image quality in early scans, but at 20 weeks it’s less critical. Your provider will let you know
• Wear comfortable clothing — a two-piece outfit makes it easier to expose your belly
• Plan for 45–60 minutes — the scan itself takes 30–45 minutes, plus waiting time
• Decide about baby’s sex — let the sonographer know upfront whether you want to find out

During the Appointment

The sonographer applies warm gel to your abdomen and moves a transducer (wand) across your belly. You’ll see your baby on a screen in real-time.

💡 Tip: The sonographer may be quiet for long stretches while concentrating on measurements. This doesn’t mean something is wrong — they’re simply focused. They’ll share observations after completing all measurements, or your provider will discuss results at your next visit.

You may be asked to:

• Shift to your side to get a better angle
• Walk around briefly to encourage baby to change position
• Come back for a follow-up if baby’s position obscures key structures (this happens in about 10–15% of scans)

Can You Find Out the Sex?

Yes — the anatomy scan is when most parents learn baby’s sex. At 20 weeks, the external genitalia are developed enough for visual identification.

Accuracy: Approximately 99% when performed by an experienced sonographer.

Note: Sex determination depends on baby’s position. If baby’s legs are crossed or they’re facing away from the transducer, the sonographer may not get a clear view. In that case, you may be able to find out at a follow-up scan.

⚠️ Important: If you’ve already had NIPT (non-invasive prenatal testing) or CVS, you already know the chromosomal sex with >99% accuracy. The ultrasound provides visual confirmation.

What Happens if Something Is Found

Most anatomy scans come back completely normal. However, if the sonographer identifies a potential concern:

1. Don’t panic. Many findings are “soft markers” — minor variations that are usually harmless
2. Your provider will explain. After the scan, your OB-GYN or midwife will review results and discuss next steps
3. Further testing may be recommended. Options include a detailed ultrasound (level 2), fetal echocardiogram, amniocentesis, or MRI

Common “Soft Markers” (Usually Not a Problem)

💡 Tip: If you receive concerning findings, ask your provider: “What is the likelihood this is a problem?” and “What are the next steps?” Having a clear action plan reduces anxiety.

Questions to Ask at Your Anatomy Scan

• Is baby growing on track for my due date?
• Where is my placenta positioned?
• Is my amniotic fluid level normal?
• Are there any findings I should know about?
• Do I need any follow-up scans?
• Can you tell baby’s sex? (if you want to know)

Use our Week-by-Week Tracker to see what else is happening at week 20.

FAQ

What if the baby won’t cooperate during the scan?

This is common — babies are active and may not stay in a position that allows clear views. The sonographer may ask you to walk around, change positions, or drink cold water to encourage movement. If key structures can’t be seen, you’ll be asked to return for a follow-up scan in 1–2 weeks. This is routine and not a cause for concern.

Is the anatomy scan the same as a 3D/4D ultrasound?

No. The anatomy scan is a medical diagnostic exam using standard 2D ultrasound. Some providers may briefly switch to 3D for facial or surface views, but the purpose is medical assessment, not keepsake photos. Elective 3D/4D ultrasounds at boutique studios are not a substitute for the anatomy scan.

Can the anatomy scan detect Down syndrome?

The anatomy scan can identify some physical markers associated with Down syndrome, but it is not a diagnostic test. NIPT blood testing (available from week 10) is more accurate for chromosomal screening. If the anatomy scan shows concerning markers, your provider may recommend additional testing.

What happens if I miss the 20-week window?

The optimal window is 18–22 weeks because baby is large enough to see structures clearly but small enough to have room to move. If you’re past 22 weeks, the scan can still be performed but some views may be harder to obtain. Talk to your provider about scheduling.

References

• American College of Obstetricians and Gynecologists. “Ultrasound Exams.” acog.org
• Society for Maternal-Fetal Medicine. “Second Trimester Ultrasound.” smfm.org
• Mayo Clinic. “Fetal Ultrasound.” mayoclinic.org
• Healthline. “What to Expect at Your 20-Week Ultrasound.” healthline.com
• NHS. “20-Week Screening Scan.” nhs.uk

Written by

Dr. Rachel Nguyen

Board-Certified OB-GYN, Medical Reviewer

Dr. Nguyen is a board-certified OB-GYN with 15 years of experience in maternal-fetal medicine. She serves as medical reviewer for Pregnancy Guide, ensuring all content reflects current clinical evidence and ACOG best practices.