Your second trimester ultrasound, also called the anatomy scan or mid-pregnancy scan, is a detailed imaging exam that checks your baby's development and major organ formation between 18 and 22 weeks of pregnancy. This scan typically lasts about 45 minutes and is one of the most detailed looks you'll have at your baby before birth.
During the appointment, a trained sonographer will use a wand called a transducer to move across your belly, creating detailed images of your baby's brain, heart, spine, kidneys, and other organs. You'll also learn the baby's sex (if you want to know), get confirmation of your due date, and find out about placenta position and amniotic fluid volume. While this scan catches major structural issues and some chromosomal concerns, it's important to understand that no ultrasound finds every condition—even in ideal circumstances, it detects 50–70% of major anomalies.
Table of Contents
- When You'll Have the Scan and What It's Designed to Do
- Everything the Sonographer Examines
- The Measurements That Confirm How Far Along You Are
- Finding Out Your Baby's Sex
- What Serious Conditions Can Be Detected
- Critical Limitations—What the Scan May Miss
- Understanding Soft Markers and What They Mean
- What Happens if the Scan Finds Abnormalities
- Factors That Affect How Clear and Accurate Your Scan Is
- Preparing for Your Appointment and What to Bring
- Frequently Asked Questions
When You'll Have the Scan and What It's Designed to Do
The anatomy scan happens between 18 and 22 weeks of gestation, according to Cleveland Clinic. This timing allows the baby to be large enough for detailed imaging while still fitting in the frame for a complete view. The scan typically lasts approximately 45 minutes, though some take longer if the sonographer needs additional views.
The scan assesses fetal development halfway through pregnancy by evaluating fetal growth, organ formation, and detecting certain congenital conditions; it also measures placental position and amniotic fluid volume, according to research in PubMed Central. This is not a quick peek—it's a systematic review of your baby's anatomy from head to toe.
Your doctor uses these measurements and observations to make sure your pregnancy is progressing as expected. The appointment is typically ordered between weeks 16 and 18 so results come back before your third trimester begins. If you've had complications in a previous pregnancy or your first trimester screening suggested risks, your doctor may recommend this scan move forward sooner.
The images and measurements become part of your medical record and are used alongside other screening tests. You'll lie on an exam table while the sonographer applies gel to your skin—the gel helps sound waves move from the transducer into your body without air gaps. The room is often darkened so you can see the screen more clearly, and most facilities allow partners to attend.
The sonographer will take measurements, images, and video clips that your obstetrician will review. Many parents find this appointment deeply emotional. You're seeing detailed images of your baby's face and movements, often for the first time clearly. At the same time, this is the appointment where potential issues surface, so it can bring both joy and anxiety. It's normal to feel both.
Everything the Sonographer Examines
The sonographer examines the fetal brain (including multiple planes), heart (with four-chamber and outflow tract views), kidneys, spine, face, chest, digestive system, abdominal wall, limbs, umbilical cord attachment, and placental dimensions, according to PubMed Central research. This systematic review means nothing gets skipped. Each structure gets its own careful look, and images are compared against standard reference measurements for your stage of pregnancy.
The heart gets special attention because congenital heart defects are among the most common birth anomalies. The sonographer looks at the four chambers (the right and left sides of the heart) and checks that blood is flowing the right direction through the outflow tracts. While the scan catches some heart defects, others remain invisible on ultrasound even in skilled hands.
The brain is imaged in multiple planes—different angles and cross-sections—to check overall size, shape, and the development of major brain structures. The cerebellum, ventricles, and choroid plexus all get measured and observed. Structural brain abnormalities can sometimes be seen, though many conditions only show up later in development or after birth.
The spine is scanned from the neck down to the tailbone, checking for gaps, curvature, or other structural problems. Spina bifida and other spinal cord issues can sometimes be detected, though not all cases appear on ultrasound. The sonographer will count the vertebrae and note the appearance of the spine throughout its length. The kidneys, stomach, and bowel are checked for presence, size, and normal fluid patterns.
The abdominal wall is inspected to rule out openings or bulges. The umbilical cord is examined where it attaches to the placenta and to the baby's abdomen to ensure it's in the normal position, not over a defect. The face is imaged to look for cleft lip and palate, which can often be seen at this stage.
Limbs are measured for length and symmetry, and the sonographer confirms that all four limbs are present with the expected number of bones. Gender determination happens during this scan if parents want to know and positioning allows a clear view.
The Measurements That Confirm How Far Along You Are
Standard biometric measurements include head circumference, femur length, and abdominal circumference to confirm gestational age and appropriate fetal growth, according to research published in PubMed Central. These three measurements are the backbone of the anatomy scan's dating function. They're compared against growth charts to see whether your baby is measuring on track, ahead, or behind.
Head circumference measures across the baby's head from one side to the other. This measurement is accurate to within plus or minus one to two weeks at this stage of pregnancy. If your due date was set by last menstrual period but the ultrasound measurements show a different age, your doctor may adjust your due date based on the ultrasound.
Femur length—the thigh bone—is measured from the hip to the knee. It's one of the most reliable measurements for dating at the mid-pregnancy scan. A femur length that's noticeably shorter or longer than expected for the dates can prompt further evaluation or follow-up ultrasounds to track growth. Abdominal circumference reflects how the baby's belly is growing and is useful for spotting growth restrictions or overgrowth.
Combined with the head circumference and femur length, it gives a three-dimensional picture of whether growth is even across the body or unbalanced. Significant gaps between measurements might warrant more frequent monitoring in the third trimester. If your measurements place the baby significantly ahead or behind, your doctor might order another scan in a few weeks to track the pattern.
A baby measuring two weeks behind at the mid-scan might still be healthy if growth is progressing steadily, or it might signal a problem that needs monitoring. Conversely, a baby measuring ahead might be genuinely older or larger, or might have other conditions affecting size. These same measurements are used to calculate amniotic fluid volume and to confirm placenta placement.
They provide the baseline against which all future growth is measured. Together with the organ-by-organ review, they paint a complete picture of how your baby is developing at the halfway point of pregnancy.
Finding Out Your Baby's Sex
When the fetus is in an optimal viewing position at 20 weeks, gender assignment is highly accurate at approximately 100% for male determination and nearly 100% for female determination, according to research in PubMed Central. The anatomy scan is the ideal time and place for sex determination because the baby is large enough for clear imaging and the external genitalia are well-developed.
Male determination is slightly more accurate because the presence of the scrotum and penis is unmistakable when clearly visible. Female determination relies on seeing the labia and clitoris, which requires a perfect angle and clear imaging. Both are very reliable at 20 weeks when positioning is good, though no ultrasound is 100% certain. The sonographer needs a clear view of the baby's genital area, and sometimes the baby's position makes that impossible.
If the baby is curled up or facing the placenta, a clear picture might not be possible despite multiple attempts. In those cases, you might be asked to come back for a follow-up scan in a week or two, or the determination might be listed as "unable to determine." Some parents want to know the sex immediately, while others prefer to wait.
Tell the sonographer your preference before the scan starts so they know whether to look, whether to tell you if they happen to see it, and whether to put the information in a sealed envelope for you to open later. Miscommunication here can lead to accidental reveals you didn't want. It's worth noting that sex assigned on ultrasound is based on external genitalia appearance, and this may not match a child's eventual gender identity.
The scan tells you about anatomy, not identity. The images and measurements are recorded, and the sex determination is just one piece of the appointment. If you're carrying multiples, each baby's anatomy is scanned and each baby's sex can potentially be determined. With twins, position becomes even more critical because two babies mean twice as many angles to get right. Some twin pregnancies require multiple scans to get complete information on both babies.
What Serious Conditions Can Be Detected
The scan can detect major structural abnormalities including cleft lip and palate, ventricular septal heart defects, and some cases of congenital heart disease; it screens for chromosomal markers associated with Down syndrome, according to research published in PubMed Central. Detecting these conditions during pregnancy gives you and your medical team time to plan and prepare.
Cleft lip and palate can often be seen when the baby's face is clearly visible. This finding allows you to connect with specialists before birth, learn what repair looks like, and understand what support your baby will need immediately after delivery. Cleft repair is a well-established procedure with good outcomes, and knowing in advance means you can have the conversation with your medical team.
Certain heart defects—particularly ventricular septal defects (holes between the heart chambers) and some forms of congenital heart disease—can be detected during the anatomy scan. Some heart conditions require immediate intervention at birth, while others are managed over time. Knowing which type your baby has allows your delivery hospital to prepare appropriate resources and specialists. Spina bifida and open neural tube defects are sometimes visible on the anatomy scan.
If detected, your medical team can plan for specialized delivery and immediate postnatal care. These conditions vary widely in severity, and early detection allows for informed decision-making and appropriate preparation. The scan screens for soft markers—variations in anatomy that are more common in babies with Down syndrome and other chromosomal conditions. These might include a thickened area at the back of the neck, extra fluid in the kidneys, or an echogenic bowel.
A single soft marker does not mean your baby has a chromosomal condition, but it may prompt further testing if multiple markers are present. Severe kidney problems, intestinal obstructions, and severe growth restriction can sometimes be detected. However, not all of these conditions are visible on the mid-pregnancy ultrasound, and some only become apparent as pregnancy advances or after birth. The scan is thorough but not all-seeing.
Critical Limitations—What the Scan May Miss
While ultrasound detects 50–70% of major anomalies in ideal settings, many conditions including cardiac defects, skeletal dysplasias, and some limb abnormalities may remain undetected, and some conditions (such as intestinal obstruction) may not appear until the third trimester, according to research in PubMed Central. This is the most important fact to understand about the anatomy scan: a normal result does not guarantee a healthy baby.
Many congenital heart defects do not show up on the anatomy scan, even when imaged by a highly skilled sonographer. Some defects only become apparent as the baby grows, and others are only visible after birth when blood flow patterns change. Complex heart conditions in particular can be invisible on ultrasound because they rely on fetal circulation patterns that look normal before birth.
Skeletal dysplasias—conditions affecting bone development—can be subtle and are frequently missed on the anatomy scan. Some are not visible until the third trimester when bones have ossified further, and some only become apparent after delivery when X-rays are taken. A baby with normal-looking long bones on ultrasound might still have a skeletal condition. Limb abnormalities such as clubfoot are sometimes visible, but not always.
The position of the baby's feet matters; if both feet are tucked or facing away from the transducer, an abnormality might be missed. Some limb length differences only become apparent as growth continues through the third trimester. Intestinal obstructions, atresia (missing portions of the bowel), and some digestive tract conditions often don't show up until the third trimester or after birth.
The ultrasound shows the bowel is present and contains fluid, but functional abnormalities may not be apparent. Some conditions only become obvious when the baby starts feeding after delivery. Genetic conditions that don't produce visible structural changes—like cystic fibrosis or many metabolic disorders—will not show up on ultrasound. Infections acquired during pregnancy may not cause visible abnormalities at 20 weeks, though some do. Some children are born with conditions that were completely invisible on every prenatal ultrasound.
Understanding Soft Markers and What They Mean
The scan may identify "soft markers"—variations like thickened nuchal fold or echogenic bowel—that appear in normal fetuses but occur more frequently in fetuses with chromosomal abnormalities; most isolated soft markers are not associated with Down syndrome, according to PubMed Central research. A soft marker is not a diagnosis. It's a variation that warrants conversation and sometimes further testing.
A thickened nuchal fold—extra fluid at the back of the baby's neck—is the soft marker discussed most often. This measurement is taken in a specific way at a specific age, and norms vary by exact gestational age. Many healthy babies have a thickened nuchal fold, and many babies with Down syndrome do not. Finding this marker alone typically does not change management, though your doctor will discuss what it means.
Echogenic bowel means the baby's bowel appears brighter on ultrasound than usual. This can be a normal variation, or it can be associated with chromosomal conditions like Down syndrome or with congenital infections. Most babies with echogenic bowel are born healthy. Your doctor will consider whether other markers or risk factors are present before recommending testing.
Choroid plexus cysts—small fluid-filled spaces in the brain—are sometimes found on the anatomy scan. These are usually harmless and often disappear on their own by the third trimester. They can be associated with chromosomal conditions when multiple other markers are present, but an isolated cyst in an otherwise normal anatomy scan typically does not change management.
Renal pyelectasis—extra fluid in the baby's kidney—is another soft marker that can look concerning but is often benign. Some babies born with mild pyelectasis never have a kidney problem. Others develop reflux or obstruction that requires postnatal monitoring or intervention. The sonographer will note the measurement and your doctor will decide whether follow-up scans are needed.
If multiple soft markers are present, your doctor is more likely to recommend additional testing like amniocentesis or non-invasive prenatal testing (NIPT). A single isolated marker in an otherwise normal anatomy scan with low prior risk typically does not change management, though you should discuss your specific situation with your obstetrician.
What Happens if the Scan Finds Abnormalities
If structural abnormalities or soft markers are detected, amniocentesis (performed between 18–24 weeks) or non-invasive prenatal testing (NIPT) may be recommended to assess chromosomal status and guide further care decisions, according to research in PubMed Central. Abnormal findings don't automatically mean your baby has a serious condition, but they do warrant investigation. Amniocentesis involves inserting a needle through the abdomen into the amniotic sac to collect fluid containing fetal cells.
These cells can be tested for chromosomal conditions like Down syndrome, Edwards syndrome, and Patau syndrome, as well as other genetic conditions. The procedure carries a small risk of miscarriage (less than 1 in 400 in most settings), and your doctor will discuss whether this risk is acceptable given what was found on ultrasound. Non-invasive prenatal testing (NIPT) analyzes fetal DNA fragments in your blood and is safer than amniocentesis because it carries no miscarriage risk.
If your ultrasound found findings concerning for Down syndrome or other chromosomal conditions, NIPT can help clarify whether a chromosomal condition is actually present. Some abnormal ultrasound findings are associated with chromosomal conditions; others are not. A specialized fetal echocardiogram—a detailed ultrasound of just the heart—may be recommended if the anatomy scan raised concerns about cardiac structure or function.
This exam takes longer than the standard scan and provides images that a pediatric cardiologist can review. Many heart conditions found this way are manageable or correctable after birth. If a severe structural abnormality like anencephaly (absence of major parts of the brain) or severe renal dysplasia is found, your medical team will discuss prognosis, management options, and what to expect at delivery and after.
These conversations are deeply personal and require time and support. Your obstetrician should connect you with specialists and counselors who can help you process the information. For less severe findings, your doctor will typically recommend follow-up ultrasounds in the third trimester to monitor progression and gather more information. Some conditions resolve on their own; others remain stable; some progress. Serial imaging helps your team track what's happening and plan delivery and postnatal care accordingly.
Factors That Affect How Clear and Accurate Your Scan Is
The accuracy of findings depends on the sonographer's skill and experience, equipment quality, and maternal factors such as obesity, which can reduce image quality and the ability to visualize all structures clearly, according to research in PubMed Central. Not all anatomy scans are created equal. The quality of images and thoroughness of the exam depend on multiple factors.
Sonographer training and experience matter significantly. A sonographer who specializes in obstetric ultrasound and performs many anatomy scans will have better skills at getting optimal images and spotting subtle abnormalities than someone who performs ultrasounds across many different areas of medicine. If you're at a specialized fetal imaging center, the sonographers likely have extensive focused training.
Equipment quality affects what can be seen. Newer, high-resolution ultrasound machines produce clearer images than older equipment. Some specialized fetal imaging centers have equipment specifically designed for detailed obstetric imaging, which may show more detail than standard office ultrasounds. The quality of equipment can affect the detection rate for subtle abnormalities. Maternal weight and body composition affect image quality.
Extra abdominal tissue between the skin and the uterus can make it harder to get clear images, and some structures may be harder to visualize in people with higher BMI. This doesn't mean the scan is impossible or useless, but it may take longer and some views might not be as clear. Your sonographer will do their best to work with the images they can get.
Amniotic fluid volume affects visibility. Too little fluid makes it hard to get good images; too much can also make some structures harder to see. Babies positioned against the placenta or with their faces turned away may require multiple attempts to get adequate views of certain structures. Sometimes the baby just won't cooperate. The exact gestational age matters.
A scan at 18 weeks may show less detail than one at 20 weeks, when structures are larger and more developed. A scan at 23 weeks shows even more, but by then some findings might already be visible on a standard anatomy scan at an earlier date. The standard 18–22 week window balances detail with the ability to take action if problems are found.
Preparing for Your Appointment and What to Bring
Before your anatomy scan, eat normally and arrive with a moderately full bladder—full enough that your uterus is pushed forward for better imaging, but not so full you're uncomfortable. You can empty your bladder partway if you need to. Wear clothes that are easy to remove from the waist down because you'll need to expose your belly for the transducer.
Bring your insurance card and photo ID. Bring a list of any medications you take, any previous ultrasounds you've had outside this clinic, and any family history of birth defects or genetic conditions. If you've had complications in a previous pregnancy, mention that before the scan starts. The more your sonographer knows about your history, the more thorough the exam can be.
Ask ahead of time whether your clinic will record video of the scan that you can take home or view later. Some facilities routinely provide this; others do not. Some will print still images for you; others charge a small fee. Knowing what to expect prevents disappointment or surprise at the end. Bring a partner or support person if you want one.
Many partners find the anatomy scan meaningful because it's the first detailed look at the baby. If you're anxious about potential findings, having someone there can help you process whatever is shared. Some people prefer to see the scan alone and talk with their partner later. After the scan, your sonographer will typically offer a preliminary overview of what they saw—this is not a complete summary, just basic observations.
Your obstetrician will receive a formal written report and images, review everything carefully, and contact you within a few days to discuss results. If anything concerning was found, your doctor will typically reach out sooner rather than later. Schedule any follow-up appointments your doctor recommends immediately. If further testing like amniocentesis or a fetal echocardiogram is suggested, ask your doctor for a referral and find out about timing. Don't wait to schedule these, as timing matters for some tests and procedures.
Frequently Asked Questions
Is the second trimester ultrasound mandatory?
No, all prenatal screening is optional. However, the anatomy scan is widely recommended because it provides important information about fetal development and detects many conditions early. Talk with your obstetrician about whether it's right for your situation.
Can the ultrasound harm my baby?
Ultrasound uses sound waves, not radiation, and has not been shown to harm the baby or placenta when used for medical purposes. The anatomy scan is considered very safe, and no special precautions are needed before or after.
What if the sonographer can't see everything they need to see?
If positioning or image quality doesn't allow a complete exam, your doctor will typically schedule a follow-up scan in one to two weeks when the baby may be in a better position. Some structures just require the right angle to visualize clearly.
Should I be worried if the scan finds a soft marker?
A single soft marker in an otherwise normal anatomy scan usually does not indicate a problem. Most soft markers appear in healthy babies. Your doctor will discuss what specific soft markers were found and whether additional testing would be helpful given your individual circumstances.
What does it mean if my baby is measuring ahead or behind?
Measurements can vary due to genetic factors, dating errors, or actual growth differences. A baby measuring a week or two different from expected is usually not concerning. If measurements are significantly off, your doctor may adjust your due date or recommend follow-up ultrasounds to track growth.
Can the ultrasound determine if my baby has Down syndrome?
The anatomy scan does not diagnose Down syndrome, but it can identify markers or features that are more common in babies with Down syndrome. If concerning features are found, your doctor may recommend NIPT or amniocentesis to test for chromosomal conditions.
