Understanding Your Anatomy Scan at 20 Weeks: What It Checks, Measurements & What to Expect

Last updated: September 8, 2026

Medically reviewed by Dr. Humaira Latif, MBBS (KMU Peshawar), Gynae & Obstetrics Specialist | Ultrasound Specialist | 14+ Years of Clinical Experience

Reaching 20 weeks of pregnancy is an important milestone. Around this time, many pregnant women are offered a detailed ultrasound commonly called the 20-week anatomy scan, anomaly scan, or mid-trimester ultrasound.

Unlike an early pregnancy ultrasound that mainly focuses on pregnancy dating, location, viability and early development, the anatomy scan takes a much closer look at your baby’s developing body.

The examination assesses fetal growth and systematically evaluates important structures including the brain, face, spine, heart, abdomen, kidneys, bladder, limbs and other visible anatomy. The placenta and amniotic fluid are also assessed.

International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) guidance describes the mid-trimester scan primarily as an examination of fetal anatomy, while routine measurements help assess fetal size and growth.

Important: A normal anatomy scan is reassuring, but no ultrasound can detect every possible fetal condition.

What Is a 20-Week Anatomy Scan?

The anatomy scan is a detailed second-trimester ultrasound examination performed to assess your baby’s anatomy and development.

It is commonly performed around 18–22 weeks, although exact timing varies according to local practice, clinical circumstances and the healthcare system. For example, NHS screening is generally offered between 18 and 21 weeks.

At this stage, your baby is large enough for the sonographer to examine many individual structures in considerably more detail.

The scan may assess:

• Baby's head and brain
• Face and lips
• Spine
• Heart
• Chest
• Diaphragm
• Stomach
• Kidneys
• Bladder
• Abdominal wall
• Arms and legs
• Hands and feet
• Placenta
• Amniotic fluid
• Fetal size and growth

The exact examination protocol can vary between countries, institutions and individual pregnancies.

Why Is the 20-Week Anatomy Scan Important?

The main purpose is to look for structural abnormalities and assess fetal development.

Some congenital abnormalities are easier to identify during the second trimester because the baby’s organs and body structures are sufficiently developed and large enough to evaluate.

ISUOG notes that, when performed by experienced professionals, most clinically important structural anomalies can be detected during the mid-trimester examination.

The scan can also provide important information about:

• Fetal growth
• Fetal anatomy
• Placental location
• Amniotic fluid
• Number of babies
• Certain pregnancy-related findings

However, it is essential to understand that this is a screening examination, not a guarantee that the baby has no medical problems.

What Does the 20-Week Anatomy Scan Check?

1. Baby’s Brain

The sonographer obtains specific views of the fetal brain.

Depending on the examination protocol, assessment can include structures such as:

• The lateral ventricles
• Midline structures
• Cavum septi pellucidi
• Cerebellum
• Cisterna magna
• Overall brain appearance

The fetal brain is assessed in different ultrasound planes because different views provide information about different anatomical structures. Some major abnormalities of the central nervous system may be identified during this examination.

2. Baby’s Spine

The spine is carefully examined from different views.

The examiner looks at:

• Alignment of the vertebrae
• Continuity of the spine
• Appearance of the spinal canal
• Overlying skin
• Relationship between the spine and surrounding structures

One important reason for examining the spine is to look for signs suggestive of open neural tube defects such as spina bifida.

3. Baby’s Face

The face may be examined for:

• Facial profile
• Upper lip
• Nose
• Facial symmetry
• Orbits, where appropriate
• Palate-related findings that can be assessed sonographically

Some facial abnormalities can be detected during the anatomy scan, although ultrasound cannot exclude every facial or palate abnormality.

4. Baby’s Heart

The fetal heart is one of the most important parts of the anatomy scan. Depending on the protocol and image quality, the examiner evaluates views such as:

• Four-chamber view
• Cardiac position
• Heart size and rhythm
• Ventricular and atrial structures
• Outflow tracts
• Major vessels
• Other cardiac features according to the screening protocol

The fetal heart can be technically difficult to assess because it is small and constantly moving. Therefore, a normal screening ultrasound does not rule out every congenital heart defect. If there is a suspected abnormality or a significant risk factor, your doctor may recommend a specialist fetal echocardiogram or targeted cardiac assessment.

5. Baby’s Abdomen

The abdominal examination may include assessment of:

• Stomach
• Abdominal wall
• Umbilical cord insertion
• Bowel appearance
• Kidneys
• Bladder

The examiner checks whether the organs appear appropriately located and developed for the gestational age.

6. Kidneys and Bladder

Both kidneys are usually assessed. The sonographer also looks for the fetal bladder. A normally filling bladder can often be visualized during the examination. The appearance of the kidneys, urinary tract and bladder can provide important information about fetal development.

7. Arms, Legs, Hands and Feet

The long bones are assessed, including:

• Humerus
• Femur

The examiner also evaluates the general appearance and movement of the limbs. The hands and feet may be visualized when technically possible. The purpose is not simply to count fingers and toes. Rather, the examination evaluates overall limb development and identifies major structural abnormalities.

What Measurements Are Taken During the Anatomy Scan?

One of the most confusing parts of an ultrasound report is the collection of abbreviations and measurements.

ISUOG recommends routine fetal biometry using measurements such as BPD, HC, AC and FL for assessment of fetal size.

Does every measurement have to match exactly? No. Ultrasound measurements are estimates rather than perfectly precise measurements. Small differences between measurements do not automatically mean something is wrong. Your doctor interprets the measurements according to gestational age, dating method, growth pattern, percentile, previous ultrasound measurements, maternal and pregnancy factors, and overall fetal anatomy. The overall pattern is usually more important than one isolated number.

What Is Estimated Fetal Weight?

The ultrasound machine can use several measurements to calculate an estimated fetal weight (EFW). This is not the baby’s actual measured weight. It is an estimate based on mathematical formulas using fetal biometric measurements. Therefore, a difference between the estimated ultrasound weight and the baby’s eventual birth weight is expected.

What Does the Anatomy Scan Check About the Placenta?

The sonographer will usually document the location of the placenta.

The placenta may be described as:

• Anterior
• Posterior
• Fundal
• Lateral

A placenta located toward the lower part of the uterus may require follow-up depending on its relationship to the cervix. Importantly, a placenta described as low-lying at 20 weeks does not automatically mean that it will remain low later in pregnancy. Your obstetrician will decide whether a follow-up scan is needed.

What About Amniotic Fluid?

The sonographer assesses the amount of fluid around the baby. The report may describe the fluid as normal, reduced or increased. Depending on the clinical situation and local protocol, additional measurements may be performed. An abnormal fluid assessment may require further evaluation rather than an immediate diagnosis of a particular disease.

Can the 20-Week Scan Tell You the Baby’s Sex?

Sometimes, yes. If the baby’s position allows a clear view, the sonographer may be able to determine fetal sex. However, it may not always be possible to see clearly; the baby may be positioned in a way that prevents visualization; and policies regarding disclosure vary between healthcare facilities and countries. The primary purpose of the anatomy scan is medical assessment of fetal development, not sex determination.

How Long Does a 20-Week Anatomy Scan Take?

A detailed anatomy scan commonly takes around 30 minutes, but the duration can vary. It may take longer when the baby is moving frequently, the baby’s position makes certain structures difficult to visualize, maternal body habitus limits image quality, the placenta or fetal position creates technical difficulty, or additional measurements are required. The NHS notes that the scan can take longer when the baby’s position makes obtaining clear images difficult, and a repeat appointment may be offered when necessary.

What Happens During the Scan?

Usually, you will:

• Lie on an examination couch
• Expose the abdominal area
• Have ultrasound gel applied to your abdomen
• Have the sonographer move an ultrasound probe across your abdomen
• Have different views of your baby obtained
• Have measurements taken
• Have the baby's anatomy systematically examined
• Have the placenta and amniotic fluid assessed
• Have images and measurements documented
• Receive an explanation of findings or appropriate follow-up arranged

The sonographer may become quiet while concentrating on obtaining and measuring specific images. A quiet sonographer does not necessarily mean something is wrong. A detailed ultrasound requires concentration, and the sonographer may need to carefully examine multiple structures and record precise measurements before discussing the findings.

What If the Sonographer Cannot See Everything?

This is quite common and does not automatically mean something is wrong. The baby may be facing the wrong direction, moving constantly, positioned deep in the pelvis, or holding the hands or legs in front of important structures.

If adequate images cannot be obtained, you may be asked to:

• Walk around for a short time
• Change position
• Return for another scan

ISUOG guidance states that if the examination cannot be completed adequately, it should be repeated or the patient referred for further assessment as appropriate.

What Conditions Can the 20-Week Scan Detect?

The exact screening list varies by healthcare system. For example, the NHS 20-week screening programme looks for signs of 11 specific conditions, including:

• Anencephaly
• Open spina bifida
• Cleft lip
• Diaphragmatic hernia
• Gastroschisis
• Exomphalos
• Serious cardiac abnormalities
• Bilateral renal agenesis
• Severe skeletal dysplasia
• Edwards syndrome (trisomy 18)
• Patau syndrome (trisomy 13)

This list should not be interpreted as a universal list of everything an anatomy scan can or cannot detect. Detection depends on the abnormality, gestational age, fetal position, image quality, maternal factors, equipment, expertise of the examiner, and local screening protocol.

Can a Normal Anatomy Scan Guarantee a Healthy Baby?

No. This is one of the most important things parents should understand. Ultrasound is extremely valuable, but it has limitations. Some conditions develop later in pregnancy, are too subtle to identify prenatally, cannot be diagnosed by ultrasound alone, may not be visible because of fetal position, or may require genetic testing, MRI, fetal echocardiography or other investigations. A normal anatomy scan is reassuring, but it cannot provide a 100% guarantee that every aspect of the baby’s health is normal. The NHS specifically emphasizes that the 20-week scan cannot detect every condition.

Is the 20-Week Ultrasound Safe?

Diagnostic obstetric ultrasound is generally considered safe when appropriately performed for medical care. ISUOG recommends using the ALARA principle — As Low As Reasonably Achievable — meaning ultrasound exposure should be kept as low as reasonably achievable while still obtaining the necessary diagnostic information. Ultrasound should therefore be performed for an appropriate medical indication rather than unnecessarily prolonged for entertainment purposes.

How Should You Prepare for the Anatomy Scan?

Preparation varies between clinics. Your healthcare provider may advise you about bladder filling.

Before your appointment:

• Wear comfortable clothing
• Take your pregnancy records or previous scan reports if requested
• Arrive a little early
• Follow the clinic's instructions about food, water or bladder preparation
• Take your questions with you

Do not assume that you must have a completely full bladder unless your healthcare provider specifically tells you.

Questions You Can Ask After Your 20-Week Scan

• Did the scan show all the structures the sonographer needed to see?
• Is my baby’s growth appropriate for the gestational age?
• Where is the placenta located, and do I need a follow-up scan?
• Is the amniotic fluid amount normal?
• Do I need any further tests or referrals?
• When should my next ultrasound or appointment be?

This article is for education only and does not replace personalised medical advice. Always discuss your scan results, blood tests and any concerns with your own doctor or midwife.