The anatomy scan is a detailed ultrasound your doctor typically orders between weeks 18 and 22 of pregnancy to map your baby's development and check for structural conditions. It measures your baby's body parts, examines organs, checks amniotic fluid levels, and assesses placenta position—giving you a clear picture of your pregnancy's physical health while often offering the chance to learn your baby's sex if you choose.
This appointment is not a test for genetic conditions like Down syndrome (that comes from separate screening). It is a structural roadmap: an ultrasound tech spends 20–30 minutes systematically examining your baby head to toe while your doctor or maternal-fetal medicine specialist reviews the images and explains what they see. Most pregnancies pass this scan without findings that change the plan; when something is spotted, it usually leads to closer monitoring or a conversation about next steps, not an emergency.
Table of Contents
- What the Anatomy Scan Checks
- Measurement and Dating
- What Happens During the Appointment
- Understanding Measurements and Growth Percentiles
- Detecting Structural Conditions
- Sex Determination and Accuracy
- When Follow-Up Imaging Is Needed
- Limitations of the Anatomy Scan
- Preparing for Abnormal Findings
- What Happens After the Anatomy Scan
- Frequently Asked Questions
What the Anatomy Scan Checks
The anatomy scan examines your baby's major organs, bones, and body structures using sound waves that create detailed images on a screen. The tech measures the head, checks the brain's ventricles (fluid chambers), looks at the face and spine, examines all four chambers of the heart, traces blood flow through the heart and umbilical cord, inspects the stomach, kidneys, and bladder, and measures the longest bone in the leg to estimate gestational age.
The scan also confirms the position and appearance of the placenta and measures the amount of amniotic fluid surrounding your baby. These measurements establish a baseline for how your pregnancy is progressing. Your baby's measurements are compared to standardized growth charts for weeks of pregnancy. A head that measures smaller or larger than expected, for example, might mean your dates are off, your baby is growing differently, or—rarely—something that needs follow-up imaging.
The tech is looking for structures that are present and in the right place, not for every possible abnormality; some birth variations are too small to see on ultrasound, and others do not show on ultrasound at all. Your baby's heart receives careful scrutiny because congenital heart defects are among the most common birth conditions.
The tech checks that all four chambers are formed, that blood flows in the right direction through the major vessels, and that the chambers appear to be the right sizes relative to each other. A normal heart rhythm and rate—typically 120 to 160 beats per minute in mid-pregnancy—is reassuring. The spine gets traced from the neck down through the tailbone to check that the vertebrae are closed and aligned.
An open spine (spina bifida) is one of the few conditions visible at this scan. The brain's structure is checked: the cerebellum, the fluid-filled spaces, and the tissue density. Brain measurements are less precise than you might expect, and many neurological conditions do not appear on ultrasound at all. The face is examined in profile and from the front.
The lips and palate are checked for clefts. The eye sockets are measured. The nasal bone is noted (some conditions show a smaller or absent nasal bone, though this alone is not diagnostic). The ears are checked for position and structure.
Measurement and Dating
Measurements taken at your anatomy scan help confirm how far along you are and track whether your baby is growing at an expected rate. The most commonly used measurements are head circumference (the distance around the baby's skull), femur length (the longest thigh bone), abdominal circumference, and cerebellum diameter. Each measurement has an expected range for your gestational age.
If your baby measures consistently larger or smaller than your dates suggest, your doctor may adjust your due date—particularly if this is your first ultrasound. Dating adjustments based on early ultrasounds (before 14 weeks) are considered most accurate. Adjustments made at the anatomy scan (18–22 weeks) are less reliable but still used if measurements are significantly off.
Growth estimates can be off by up to two to three weeks even when everything is normal. Genetic variation means some babies are constitutionally smaller or larger; their parents' heights and genetics matter. If measurements suggest your baby is smaller than expected, your doctor may recommend follow-up ultrasounds later in pregnancy to make sure growth is continuing.
If measurements are larger, monitoring also typically follows, though large baby size alone does not necessarily require intervention. Measurements can appear different depending on the angle, the quality of the image, the skill of the tech, and how your baby is positioned. A measurement that seems slightly off on one pass may be completely normal on another.
Your doctor interprets the full picture, not one measurement in isolation. The amniotic fluid volume is assessed using a simple visual scoring system. Too much fluid (polyhydramnios) or too little (oligohydramnios) can sometimes signal concerns, but mild variations are usually watched rather than acted on. Fluid volume changes throughout pregnancy and is measured again at later ultrasounds if your doctor thinks it is relevant.
What Happens During the Appointment
You will arrive with a full bladder, since a full bladder helps the ultrasound waves transmit clearly in early scans—though by week 20, the baby is large enough that this matters less than it did at earlier ultrasounds. You will change into a gown and lie on an exam table.
The tech will apply warm gel to your belly and press the probe (transducer) firmly against your skin, angling it to see different parts of your baby. The entire appointment typically takes 20–45 minutes. For the first 20–30 minutes, the tech will systematically scan your baby, taking measurements and images. You will see the ultrasound screen and will often be shown your baby's face, profile, hand, foot, and sometimes the heart beating.
Some techs are very chatty; others are more reserved. This is not usually because they see something wrong—techs are trained not to comment on findings until the doctor reviews the images. After the tech finishes scanning, the images are reviewed by your doctor or a maternal-fetal medicine specialist. The doctor may call you back in to point out structures on the screen, show you your baby's position, and discuss findings.
If your baby is in a good position, you may learn the sex during this scan, though some practices wait for you to ask or take a different approach. You will not feel the ultrasound. The probe may press uncomfortably if your bladder is extremely full, and the gel is cool. If your baby is moving, you may feel movement but will not sense the ultrasound waves themselves.
Some offices take printed pictures or record a video clip for you to keep. Others provide digital images or a keepsake video for an extra fee. Ask ahead what your practice offers.
Understanding Measurements and Growth Percentiles
Your baby's measurements are plotted on a growth chart, and a percentile is often calculated. A 50th percentile measurement means your baby is average for gestational age. A 10th percentile means your baby measures smaller than 90 percent of babies at the same age; a 90th percentile means larger than 90 percent. These percentiles help your doctor decide whether follow-up is needed.
The width of the range is important: babies measuring in the 3rd to 97th percentile are all within normal limits. Babies in the 1st or 99th percentile are unusual but can still be healthy. Genetic factors explain much of the variation: parents who are very tall tend to have babies measuring larger; parents who are shorter tend to have smaller babies.
If measurements are clustered in a very low percentile—say, all in the 2nd to 5th percentile—your doctor may watch more closely for signs of poor growth (intrauterine growth restriction). If multiple measurements are spread across a very wide range—some very small, some very large—your doctor might recommend earlier follow-up ultrasounds to make sure growth is consistent.
Measuring your baby repeatedly across the weeks of pregnancy is more informative than a single measurement. A baby measuring in the 10th percentile at week 20 and again in the 12th percentile at week 28 is likely constitutionally small and healthy. A baby measuring in the 50th percentile at week 20 and dropping to the 5th percentile by week 30 shows a change in pattern and requires attention.
Growth estimates become less accurate as pregnancy advances. An estimate given at week 20 might be off by several ounces; an estimate given at week 36 might be off by a pound or more. Your baby's actual weight will not be known until birth.
Detecting Structural Conditions
The anatomy scan can detect some birth conditions but not all. Conditions with clear structural changes—like cleft lip, spina bifida, or major organ malformations—may show on ultrasound. Others are invisible to ultrasound or show very subtle signs that require expert interpretation. Many genetic conditions and developmental differences show no signs at all on mid-pregnancy ultrasound.
Major structural conditions sometimes spotted at anatomy scan include heart defects with obvious structural changes (like transposition of the great arteries), large kidney abnormalities, bladder outlet obstruction, severe spina bifida, cleft lip and palate, limb differences, and skeletal dysplasias (bone development disorders). Lung and diaphragm abnormalities sometimes show. Some abdominal wall defects are visible. Conditions less visible or invisible on ultrasound include Down syndrome (requires blood screening or genetic testing), most autism differences, most hearing conditions, cerebral palsy, cystic fibrosis, sickle cell disease, and many other genetic and metabolic disorders.
Down syndrome shows subtle markers on ultrasound—like a thickened nuchal fold or an absent nasal bone—but these markers are not diagnostic. A baby with markers may be born without the condition; a baby without markers may have it. Genetic testing (amniocentesis or noninvasive prenatal testing) is needed for diagnosis. Your doctor will explain any findings using clear language. If a condition is suspected, the next step is usually more detailed imaging—often a specialized ultrasound with a maternal-fetal medicine specialist—or genetic testing to confirm the diagnosis and understand what it means for your baby.
Sex Determination and Accuracy
Many parents choose to learn their baby's sex at the anatomy scan, and the accuracy is generally high by week 20. An experienced ultrasound tech can determine fetal sex with about 95–99% accuracy when the baby is in a favorable position and there is good visualization of the genital area. The accuracy is lower earlier in pregnancy (before week 14) and depends partly on the tech's skill and your baby's position.
The determination is based on the visible anatomy: a scrotum and penis are signs of a male baby; a vulva and clitoris are signs of a female baby. Ambiguous genitalia (where the anatomy does not clearly show typical male or female appearance) is rare and may be difficult to interpret on ultrasound; further evaluation may follow birth.
Some babies are in positions where the genital area is not clearly visible during your appointment, so the tech may not be able to make a determination. If learning sex is important to you, ask your tech to look carefully and to tell you if they are confident. Some practices offer a rescanning option if the view was not clear.
Biological sex determined by ultrasound is based on anatomy, not on chromosomes or gender identity. It is one piece of information about your baby. Some parents choose not to learn sex at the anatomy scan. Ask your tech and doctor ahead of time if you prefer not to hear or see this information.
When Follow-Up Imaging Is Needed
If your anatomy scan raises questions, your doctor will discuss next steps. Sometimes a finding needs nothing more than monitoring. Sometimes more detailed imaging is recommended. Sometimes genetic testing helps clarify what a finding means. A second opinion ultrasound is commonly done when the anatomy scan shows something unexpected or unclear. A maternal-fetal medicine specialist may be asked to review the images and perform their own scan.
This usually happens at a high-risk pregnancy clinic or a large hospital and does not mean something is definitely wrong; it means more expert eyes are helpful. Advanced imaging options include high-resolution ultrasound, 3D ultrasound (which builds a three-dimensional image), and fetal MRI (magnetic resonance imaging). MRI is sometimes recommended for certain conditions—like brain or spine findings—where it provides clearer detail than ultrasound.
MRI uses no radiation and is considered safe in pregnancy, though it is more expensive and less widely available than ultrasound. Genetic testing options depend on what is seen. If Down syndrome is suspected based on markers, cell-free DNA testing (a blood test) can be done at any point but is most accurate after week 10.
Amniocentesis (taking a sample of amniotic fluid) can be done after week 15 and gives a definitive answer. Your doctor will explain the risks, benefits, and accuracy of testing options specific to your situation. Most anatomy scan findings do not change the plan for the remainder of pregnancy or delivery. Some findings lead to more frequent monitoring, delivery planning, or preparation for a baby who may need specialized care after birth.
Limitations of the Anatomy Scan
The anatomy scan provides a detailed structural picture but is not perfect. It misses some conditions, can be falsely reassuring, and sometimes flags something that later turns out to be normal variation. Understanding these limits helps you interpret your results. No ultrasound can see everything. Some birth conditions show no signs on ultrasound because they affect brain chemistry, metabolism, genes, or systems that are not visible on imaging.
Down syndrome, autism, cerebral palsy, hearing loss, and cystic fibrosis are conditions that typically show no signs on mid-pregnancy ultrasound. A normal anatomy scan does not rule these out. Image quality depends on maternal factors outside anyone's control: the thickness of abdominal tissue, the amount of amniotic fluid, the position of the placenta, and your baby's position.
A thick abdominal wall or posterior placenta can make imaging harder. A baby in a position that hides the heart or brain can make some structures hard to see. These limitations sometimes lead to incomplete scans that need to be repeated. Interobserver variation means different doctors or techs may interpret the same image slightly differently.
A measurement can look different depending on the angle. Some findings are subjective: is a ventricle measurement mildly enlarged or normal? Is the heart rate slightly fast or completely normal? These judgment calls matter, which is partly why second opinions are sometimes helpful. Some conditions change over time. A subtle marker present at week 20 may resolve by week 28.
A condition not yet visible at week 20 may develop and become visible later. Later ultrasounds (at weeks 28–30) sometimes add information that the anatomy scan did not reveal. False positives happen: something looks concerning at the anatomy scan but turns out to be normal after follow-up imaging or at birth. False negatives also happen: the anatomy scan looks completely normal, but the baby is born with a condition that was not visible on ultrasound.
Preparing for Abnormal Findings
If your anatomy scan finds something unexpected, you will need time to process the news and gather information. Most practices will explain the finding, discuss what it means, and outline next steps. If you feel rushed or unclear, ask questions and ask for written information or a follow-up appointment. Finding out your baby has a diagnosed condition in utero gives you time to prepare—to learn about the condition, connect with specialists, arrange referrals, and plan for birth and after.
This is valuable time, even when the news is difficult. Some conditions are manageable; others are life-limiting. Some babies need treatment or surgery shortly after birth; others need monitoring and management over years. Your care team can help you understand the specifics of your baby's situation. If you are considering ending a pregnancy based on an anatomy scan finding, genetic counseling and maternal-fetal medicine consultation are important.
Genetic counselors can explain what a diagnosis means and help you understand the range of outcomes. Some diagnoses are more serious than parents initially understand; others are less serious. Getting clear information matters. If a finding might require specialized care at delivery, your delivery location may need to change. Some babies with certain conditions are safer delivering at a hospital with a neonatal intensive care unit rather than a smaller facility.
This should be discussed with your doctor well before labor. Many pregnancy support organizations exist for specific conditions. If your baby has been diagnosed with something, ask your doctor for referrals and consider connecting with other parents who have had babies with the same condition.
What Happens After the Anatomy Scan
After your anatomy scan, your care typically continues with regular check-ups and usually one more anatomy ultrasound in the third trimester (around weeks 28–30), particularly if any findings need monitoring. Some practices do a third ultrasound closer to delivery to check positioning and confirm estimated weight. If your anatomy scan was completely normal and you have no risk factors, you may not have another ultrasound unless you develop a complication or concern.
If something was found, the frequency of monitoring depends on what it is: monthly ultrasounds, weekly ultrasounds, or referral to a specialist might be recommended. Between ultrasounds, you monitor your baby's movement—usually starting around week 20 or 22. You will notice your baby kick, roll, and hiccup. While specific kick-count protocols are not standard anymore, you should know what your baby's normal pattern is and report a significant decrease to your doctor.
Most babies maintain a consistent movement pattern in the weeks between your anatomy scan and delivery. Your care provider will discuss how the anatomy scan findings, if any, affect your birth plan. Some findings do not change anything; others mean delivery at a specific hospital or with certain specialists present. Most parents feel reassured by a normal anatomy scan.
Even so, pregnancy anxiety is normal, and it is fine to call your doctor with concerns between appointments. Spotting, bleeding, severe cramping, or a dramatic decrease in movement are reasons to call. General worry does not need an ultrasound to answer; it usually needs reassurance and time. After delivery, your baby will be examined by a pediatrician looking for the same structures your ultrasound looked for, plus things that are easier to assess in person. Conditions found on ultrasound can be confirmed or ruled out, and babies sometimes have things visible at birth that did not show on prenatal ultrasound.
Frequently Asked Questions
What is the anatomy scan and why do doctors do it?
The anatomy scan is a detailed ultrasound between weeks 18–22 that maps your baby's major organs, bones, and body structures. It confirms your baby is developing structurally, updates your due date, and can spot some birth conditions that might need monitoring or planning.
Can the anatomy scan detect Down syndrome?
No. The anatomy scan can see subtle markers associated with Down syndrome (like a thickened neck fold), but these are not diagnostic. Down syndrome is confirmed through blood testing or genetic testing like amniocentesis, not ultrasound alone.
How accurate is learning the baby's sex at the anatomy scan?
An experienced ultrasound tech can determine fetal sex with about 95–99% accuracy at week 20 when the baby is in a good position. Earlier ultrasounds are less accurate, and some babies are positioned where sex cannot be clearly seen.
What does it mean if my baby measures smaller than expected on the anatomy scan?
Smaller measurements usually mean your due date was slightly off, your baby is genetically smaller, or your baby is growing at a slower rate. Your doctor will recommend follow-up ultrasounds to track growth and make sure it is consistent. Many small babies at this scan are born completely healthy.
What happens if the anatomy scan finds something abnormal?
Your doctor will explain the finding and discuss next steps, which might include more detailed imaging, genetic testing, specialist consultation, or closer monitoring. Many findings turn out to be manageable, and some resolve on their own before birth.
Is the anatomy scan safe, and does it hurt?
The anatomy scan uses ultrasound (sound waves, not radiation) and is considered safe. You will not feel it, though the probe may press firmly and the gel is cool. The appointment takes 20–45 minutes.
