Pregnancy
Anatomy Scan (20-Week Ultrasound): What It Checks and What to Expect
The anatomy scan, usually called the 20-week ultrasound, is a detailed ultrasound done between 18 and 22 weeks of pregnancy. The sonographer works through a checklist of your baby's brain, heart, spine, organs and limbs, measures growth, and checks the placenta and amniotic fluid. Most scans come back normal, and when something does need a second look, finding it now gives you and your care team months to plan.
Here's what the appointment is really like, what they're looking at on the screen, and what to ask before you leave.
What the appointment feels like, start to finish
Check-in. You'll go to your OB's office, a hospital imaging department, a standalone imaging center, or a maternal-fetal medicine (MFM) practice. Most places don't need a full bladder at 20 weeks, but read your instructions, because a few still ask for one.
On the table. You lie back with your belly exposed, and the sonographer spreads warm (or not so warm) gel and presses a probe across your skin. The pressure gets firm, especially low on the pelvis. It can be uncomfortable, but it shouldn't hurt. If you get dizzy or nauseated lying flat, say so; tipping onto your side for a minute usually fixes it.
The long middle part. Plan on 30 to 60 minutes, sometimes more. The sonographer is clicking, measuring and freezing images, and the room may be quiet for long stretches. A common worry is that silence means bad news. It almost never does. Sonographers are concentrating, and in many practices they aren't allowed to discuss findings at all.
Results. A doctor reviews the images. Some offices have the physician come in that day; others go over results at your next prenatal visit.
Guests. Many offices allow one support person. Ask ahead, since policies differ, and some practices limit video recording during the medical part so the sonographer can focus, then let you take photos at the end.
What are they actually checking?
Your baby's body
- Head and brain: skull shape and the main brain structures
- Face: the lips (to look for cleft lip) and the profile
- Spine: the full length, to look for spina bifida
- Heart: the four chambers, the major blood vessels and the heart rate
- Chest: lungs and diaphragm
- Abdomen: stomach, kidneys, bladder, and the abdominal wall where the cord attaches
- Arms, legs, hands and feet
- Umbilical cord: normally two arteries and one vein
- Sex, if the view is clear and you want to know
Growth
The sonographer measures the head, belly and thigh bone and uses them to estimate size and weight, then compares those numbers with your due date.
The pregnancy itself
- Placenta location: whether it sits low or covers the cervix (placenta previa)
- Amniotic fluid: whether the amount looks normal
- Cervix: many practices measure cervical length, because a short cervix is linked to preterm birth
"They couldn't see everything." Is that bad?
No. This is one of the most common things people hear, and it's rarely about a problem. The baby may be curled up, facing your back, or sitting low in the pelvis. You might be asked to walk the hallway, drink some water, or roll side to side. If views are still incomplete, you'll come back for a follow-up scan, usually a few weeks later.
Image quality can also be limited by a higher body weight. That's a physics problem, not a judgment, and a repeat scan or a transvaginal view sometimes helps.
What are "soft markers"?
Soft markers are small findings that are usually normal variations but are sometimes linked to a slightly higher chance of a chromosome condition. Examples include:
- A bright spot in the heart (echogenic intracardiac focus)
- A small cyst in the brain's choroid plexus
- Mild widening of the kidneys' collecting area (renal pelvis dilation)
- A shorter-than-expected arm or thigh bone
On their own, most soft markers aren't concerning, especially if you've already had normal cell-free DNA screening. A common reaction is to go home and search every term on the report at midnight. Try asking first, before you leave: "Does this change anything for us, or is it just something you noted?"
Common findings and what happens next
| Finding | What it usually means | Typical next step |
|---|---|---|
| Low-lying placenta or previa | Placenta near or over the cervix; often moves up as the uterus grows | Repeat ultrasound in the third trimester; avoid sex if advised |
| Short cervix | Higher chance of preterm birth | Your provider may recommend vaginal progesterone or other treatment |
| Baby measuring small or large | May be normal or may need monitoring | Growth ultrasound in a few weeks |
| Possible heart difference | Needs a closer look | Fetal echocardiogram |
| Structural difference found | Varies widely | Referral to MFM, genetic counseling, specialist consultation |
When will I see maternal-fetal medicine?
Some people have the anatomy scan with an MFM specialist from the start because the pregnancy is higher risk: diabetes before pregnancy, twins, a prior baby with a birth defect, or certain medications. Others are referred after a routine scan shows something that needs a detailed ("level 2") look.
In New York, much of the complex follow-up (fetal echocardiograms, specialist consults, delivery planning for babies who'll need intensive care after birth) is concentrated at hospitals designated as Regional Perinatal Centers. If you live in Suffolk, Orange or Dutchess, that can mean a longer drive for one or two visits, so ask whether any of the follow-up can happen closer to home. Find specialists in our maternal-fetal medicine directory.
Can the anatomy scan miss problems?
Yes. Ultrasound is very good but not perfect. Some conditions are hard or impossible to see, such as some heart defects, hearing loss, or conditions that develop later in pregnancy. A normal scan is reassuring, but it can't promise that the baby has no health problems.
Is ultrasound safe for the baby?
Diagnostic ultrasound done by trained professionals has a long safety record in pregnancy. ACOG recommends using it when there's a medical reason, at the lowest exposure needed. The FDA discourages "keepsake" 3D or 4D ultrasounds at commercial studios that aren't done for medical reasons.
Can the anatomy scan change my due date?
Usually not. If your due date was set by a first-trimester ultrasound, ACOG advises keeping it, because babies vary more in size later in pregnancy. A baby measuring a little ahead or behind is information about growth, not a new due date. If this is your first ultrasound of the pregnancy, it may be used to set the date, though it's less precise than an early scan.
Your anatomy scan checklist
Before you go
- Eat normally. Some people find a snack helps the baby move.
- Wear a two-piece outfit so you can expose your belly without changing.
- Decide whether you want to learn the sex, and tell the sonographer at the start. If you want it kept a surprise, say so twice.
- If you need an interpreter, ask when you book. Medical interpretation should be free for you, and that matters more than usual if results need explaining.
Questions to ask before you leave
- Was everything seen, or do I need to come back?
- Where is my placenta?
- Was my cervical length measured, and is it normal?
- Did you note any soft markers? Do they change anything?
- When and how will I get the full report?
When to call your provider
Call your provider if, around the time of your scan or afterward, you have:
- Vaginal bleeding, especially if you've been told you have a low-lying placenta
- Leaking fluid from the vagina
- Pelvic pressure, cramping, or a low, dull backache that comes and goes (possible signs of preterm labor)
- Questions or worry about a finding that wasn't fully explained
If you'd like a second opinion on a finding, search the directory for OB/GYNs and MFM specialists near you. For what's coming next, see our prenatal visit schedule.
Frequently asked questions
When is the anatomy scan done?
The anatomy scan is usually done between 18 and 22 weeks of pregnancy, most often around 20 weeks. At this stage the baby's organs are formed and large enough to see in detail.
How long does the anatomy scan take?
Plan for about 30 to 60 minutes, sometimes longer. If the baby is in a position that hides some structures, you may be asked to walk around or come back for another scan.
Can you find out the sex at the anatomy scan?
Usually, yes, if the baby's position allows a clear view. Tell the sonographer at the start whether you want to know or want them to keep it a surprise.
What is a level 2 ultrasound?
A 'level 2' or detailed anatomy ultrasound is a more in-depth scan, often done by maternal-fetal medicine specialists for higher-risk pregnancies or when a finding needs a closer look.
Sources
- ACOG — Ultrasound Exams FAQ
- ACOG — Ultrasound in Pregnancy (Practice Bulletin 175)
- MedlinePlus — Pregnancy Ultrasound
- FDA — Avoid Fetal Keepsake Images, Heartbeat Monitors
This article is for general education and is not medical advice. It does not replace an evaluation by a licensed clinician who knows your history. If you think you have a medical emergency, call 911.