Anatomy Scan Deep Dive: Understanding Your 20-Week Ultrasound

The 20-week anatomy scan, also known as the anomaly scan, is one of the most comprehensive and important ultrasounds during pregnancy. This detailed examination provides crucial insights into your baby's development and helps ensure everything is progressing normally. For many expectant parents, this scan is also the moment they discover their baby's gender. Understanding what this scan involves can help you feel more prepared and make the most of this significant milestone in your pregnancy journey.

What Is the 20-Week Anatomy Scan?

The 20-week anatomy scan is a detailed ultrasound examination typically performed between 18 and 22 weeks of pregnancy, with the optimal timing being around 20 weeks. This scan is offered to all pregnant women on the NHS as part of routine antenatal care and represents the most thorough assessment of your baby's physical development during pregnancy.

Unlike the earlier 12-week scan which focuses primarily on dating and basic viability, the anatomy scan provides a comprehensive evaluation of your baby's organs, bones, and overall structure. The sonographer will systematically examine your baby from head to toe, checking that all major body systems are developing correctly.

This scan typically takes 30-45 minutes to complete, though it may take longer if your baby is in a difficult position or if detailed measurements are needed. The examination is performed abdominally using ultrasound gel and a transducer moved across your bump.

Why Is the Anatomy Scan So Important?

The 20-week anatomy scan serves several critical purposes that make it an essential part of your antenatal care:

Detecting Structural Abnormalities

The primary purpose of this scan is to identify any structural abnormalities or birth defects. Whilst most babies develop normally, the scan can detect conditions such as spina bifida, heart defects, cleft lip and palate, or limb abnormalities. Early detection allows for appropriate planning and specialist care when needed.

Assessing Growth and Development

The sonographer will take detailed measurements to ensure your baby is growing appropriately for the gestational age. These measurements help identify babies who may be smaller or larger than expected, which could indicate underlying issues requiring monitoring.

Examining Placental Function

The scan assesses the placenta's position, structure, and blood flow. This is crucial for identifying conditions like placenta praevia (where the placenta covers the cervix) or placental insufficiency, which could affect your baby's growth and your delivery plans.

Checking Amniotic Fluid Levels

The amount of amniotic fluid (liquor) around your baby is carefully assessed. Too little (oligohydramnios) or too much (polyhydramnios) can indicate various conditions and may require additional monitoring.

Gender Determination

If you wish to know your baby's gender, this can usually be determined accurately at the 20-week scan, provided your baby is in a suitable position. However, this is optional, and you can choose not to find out if you prefer to wait until birth.

What Does the Detailed Examination Include?

The anatomy scan involves a systematic examination of your baby's entire body. Here's what the sonographer will assess:

Head and Brain

The examination includes:

• Shape and size of the head
• Brain structures including the cerebellum, ventricles, and choroid plexus
• Facial features including eyes, nose, and lips
• Presence and structure of the skull bones

Spine

Every vertebra is examined to ensure:

• Proper formation and alignment
• Complete closure of the spinal column
• No signs of spina bifida or other neural tube defects

Heart

This is one of the most detailed parts of the examination:

• Four-chamber view of the heart
• Major blood vessels (aorta and pulmonary artery)
• Heart rhythm and rate
• Position within the chest

Abdomen

The sonographer examines:

• Stomach presence and position
• Kidneys and bladder
• Liver and gallbladder
• Bowel development
• Umbilical cord insertion

Limbs

All arms and legs are checked for:

• Presence of all bones (femur, tibia, fibula, humerus, radius, ulna)
• Appropriate length measurements
• Hand and foot structure
• Normal positioning and movement

Growth Measurements

Key measurements taken include:

• Biparietal diameter (head width)
• Head circumference
• Abdominal circumference
• Femur length

Preparing for Your Anatomy Scan

Proper preparation can help ensure the best possible images and a smooth scanning experience:

Before Your Appointment

• Timing: Unlike the 12-week scan, you don't need a full bladder. In fact, an overly full bladder can sometimes make examination more difficult
• Clothing: Wear comfortable, loose-fitting clothes that allow easy access to your abdomen
• Partner: Your partner or support person is usually welcome to attend
• Questions: Prepare any questions you'd like to ask about your baby's development
• Expectations: Understand that this is primarily a medical examination, though you'll see detailed images of your baby

What to Bring

• Your maternity notes
• A list of any questions or concerns
• Your mobile phone or camera (check hospital policy about taking photos)
• Tissues (the gel can be messy)

What Happens During the Scan?

Understanding the scanning process helps you know what to expect:

The Examination Process

• You'll be asked to lie on your back on the examination couch
• Ultrasound gel is applied to your abdomen
• The sonographer uses a transducer to obtain images
• They will work systematically through each body system
• You may be asked to change position to get better views
• The sonographer will explain what they're looking at throughout the examination

Taking Measurements

The sonographer will pause frequently to take precise measurements. These are used to:

• Confirm your baby is growing appropriately
• Calculate estimated fetal weight
• Update your estimated due date if necessary
• Compare with population growth charts

Image Quality Factors

Several factors can affect image quality:

• Your baby's position
• Amount of amniotic fluid
• Your body composition
• Placental position

If clear images cannot be obtained, you may be asked to return for a repeat scan in a week or two.

The Gender Reveal Moment

For many parents, discovering their baby's gender is a highlight of the anatomy scan:

Accuracy of Gender Determination

At 20 weeks, gender determination is highly accurate (over 95%) when the baby is in a good position. However, the sonographer can only tell you the gender if they can see the relevant anatomy clearly.

Your Choice

You can choose whether or not to find out your baby's gender. Make sure to tell the sonographer your preference at the beginning of the scan. If you want it to be a surprise, they can carefully avoid showing or mentioning the genital area.

When Gender Can't Be Determined

Sometimes the baby's position makes it impossible to determine gender accurately. In these cases:

• The sonographer won't guess
• You may be offered another scan if you specifically want to know
• Private gender scans are available if the NHS scan is inconclusive

Understanding Your Results

The scan results will be explained to you by the sonographer or doctor:

Normal Results

Most anatomy scans show normal development. You'll receive:

• Confirmation that all major structures appear normal
• Growth measurements showing appropriate development
• Photos of your baby to take home
• An updated estimated due date if measurements suggest this is necessary

When Concerns Are Identified

If any abnormalities or concerns are detected:

• The sonographer will explain what they've seen
• You'll be referred to a fetal medicine specialist
• Further detailed scans may be arranged
• Genetic counselling may be offered
• You'll receive support and information about your options

Soft Markers

Sometimes "soft markers" are identified - these are minor variations that are usually normal but can occasionally be associated with chromosomal conditions. If soft markers are found:

• Most babies with soft markers are completely healthy
• Your risk assessment may be reviewed
• Additional monitoring may be recommended
• The findings will be explained in detail

Limitations of the Anatomy Scan

Whilst the anatomy scan is very comprehensive, it's important to understand its limitations:

What Cannot Be Detected

• All birth defects (some become apparent only later in pregnancy or after birth)
• Chromosomal conditions like Down's syndrome (though structural markers may be visible)
• Some heart defects that develop later
• Hearing or vision problems
• Learning difficulties or developmental delays
• Some genetic conditions that don't affect physical structure

Detection Rates

The scan detects approximately:

• 95% of cases of spina bifida
• 80% of major heart defects
• 90% of cases of cleft lip
• 60-70% of all major structural abnormalities

After Your Anatomy Scan

Following your scan, several things typically happen:

Immediate Follow-up

• Results are usually available immediately
• A report is sent to your GP and midwife
• Your maternity notes are updated
• Future appointments are planned based on the results

Ongoing Care

Based on your scan results:

• Routine antenatal care continues as planned
• Additional scans may be recommended if growth needs monitoring
• Specialist referrals are arranged if necessary
• Your birth plan may need to be adjusted based on findings

Emotional Impact

The anatomy scan can be emotionally significant:

• Many parents feel more connected to their baby after seeing detailed images
• Gender revelation can make the pregnancy feel more real
• Any concerns identified may cause anxiety
• Support is available if you need to discuss your feelings

Special Circumstances

Certain situations may require additional consideration:

Multiple Pregnancies

If you're carrying twins or more:

• Each baby is examined individually
• The scan takes longer to complete
• Growth discrepancies between babies are assessed
• Placental arrangements are carefully evaluated

High-Risk Pregnancies

If you have risk factors such as:

• Previous pregnancy complications
• Family history of birth defects
• Maternal medical conditions
• Advanced maternal age

Your scan may involve additional assessments or be performed by a specialist.

Previous Abnormal Results

If you've had concerning results in previous pregnancies, special attention may be paid to those specific areas, and additional emotional support will be provided.