20 Weeks Anatomy or Anomaly Scan

The 20-week anatomy scan (anomaly scan) is the most comprehensive pregnancy ultrasound, examining your baby’s development in detail from head to toe. Performed between 18-22 weeks, this thorough scan checks brain, face, spine, heart (4 chambers), kidneys, bladder, limbs, and all major organs.

We also measure baby’s growth, assess placenta position, and check amniotic fluid levels. This scan can detect structural abnormalities like spina bifida, heart defects, kidney problems, and cleft lip. It’s also when most parents find out their baby’s gender (if desired).

Our expert sonographers take time to explain each part of the scan, answer questions, and provide clear images. You’ll receive a detailed report and multiple photos. Essential scan for every pregnancy at our Brentwood clinic.

 

Comprehensive Examination:

1. Head & Brain:

• Skull shape and bones
• Brain structures:
  • Lateral ventricles (fluid spaces)
  • Cerebellum (back of brain)
  • Cavum septum pellucidum (midline structure)
  • Choroid plexus
• Head circumference (HC) measured
• Biparietal diameter (BPD) measured

2. Face:

• Profile view (nose, lips, forehead, chin)
• Lips (cleft lip detection)
• Nose and nostrils
• Eyes (position and orbits)
• Palate (limited view – cleft palate harder to see)

3. Spine:

• Complete spine from neck to sacrum
• All vertebrae checked
• Spina bifida screening
• Skin covering confirmed intact
• Alignment checked

4. Chest:

• Heart (most detailed assessment):
  • Four chambers clearly visible
  • Heart valves assessed
  • Great vessels (aorta, pulmonary artery)
  • Heart position (left side of chest)
  • Heart rhythm regular
  • Outflow tracts examined
• Lungs appearance checked
• Diaphragm position

5. Abdomen:

• Stomach (position and size)
• Kidneys (both sides, size, fluid collections)
• Bladder (present and fills/empties)
• Abdominal wall (gastroschisis/exomphalos screening)
• Bowel appearance
• Liver position
• Umbilical cord insertion point

6. Limbs:

• Arms and legs (all bones visible)
• Hands and feet visible
• Fingers and toes counted (if position allows)
• Bone measurements:
  • Femur length (FL) – thigh bone
  • Humerus – upper arm
  • Radius/ulna – forearm
  • Tibia/fibula – lower leg

7. Growth Measurements:

• Head circumference (HC)
• Abdominal circumference (AC)
• Femur length (FL)
• Estimated fetal weight (EFW)
• Growth percentile calculated
• Compare measurements to ensure proportionate growth

8. Placenta:

• Position in uterus
• Low-lying? (placenta praevia check)
• Anterior/posterior location
• Not covering cervix
• Appearance normal

9. Amniotic Fluid:

• Amount assessed (too much/too little)
• Amniotic Fluid Index (AFI) or
• Maximum vertical pocket (MVP)
• Normal amount for gestation

10. Umbilical Cord:

• Three vessels (2 arteries, 1 vein) confirmed
• Insertion point on placenta
• No abnormalities visible

11. Gender (Optional):

• Baby’s sex determined if you want to know
• High accuracy at 20 weeks
• Can choose not to know
• We’ll ask your preference

Timing:

Optimal Window:

• 18-22 weeks gestation
• 20 weeks ideal (best detail + baby not too big)
• Before 18 weeks: Baby may be too small
• After 22 weeks: Baby larger, harder to see everything

Why This Timing:

• Organs sufficiently developed to assess
• Anomalies detectable
• Baby still has space to move
• Amniotic fluid good for viewing
• Time for decisions if abnormality found

What We’re Checking For:

Common Detectable Conditions:

Brain & Spine:

• Anencephaly (absent skull/brain)
• Spina bifida (open spine)
• Hydrocephalus (fluid on brain)
• Major brain malformations

Heart:

• Structural heart defects
• Four chamber abnormalities
• Vessel abnormalities
• Position abnormalities

Face:

• Cleft lip
• Cleft palate (limited view)
• Facial abnormalities

Abdomen:

• Exomphalos (abdominal contents outside)
• Gastroschisis (bowel outside abdomen)
• Kidney abnormalities
• Bladder problems
• Diaphragmatic hernia

Limbs:

• Missing or shortened limbs
• Skeletal dysplasias
• Talipes (club foot)

Chromosomal Markers:

• Soft markers that might suggest chromosomal issues
• Discussed if found
• Usually insignificant alone

Important Limitations:

Not Everything Can Be Seen:

• Some conditions develop later
• Some are too subtle to detect
• Dependent on baby position
• Dependent on image quality
• Not a guarantee of perfect health

Cannot Always See:

• Small heart defects
• Some brain conditions
• Internal palate
• All chromosomal conditions
• Small abnormalities

Position Issues:

• Baby lying face-down (spine blocks view)
• Baby’s back towards us
• Hands covering face
• Limited fluid
• Maternal obesity reduces clarity

The Scan Process:

What to Expect:

• Duration: 30-45 minutes
• Detailed examination: Every part checked systematically
• May take time: Need baby in right positions
• Silent periods: Sonographer concentrating
• Explanation provided: Shown what we’re seeing
• Questions welcome: After examination

During Scan:

• Lie comfortably on couch
• Gel applied to abdomen
• Systematic examination of all structures
• Measurements taken
• Images captured
• May need to change position
• May need break and walk around if baby won’t move

If Baby Uncooperative:

• Walking around helps
• Eating/drinking something sweet
• Come back later same day
• Reschedule (rare)
• We’re patient and flexible

What You’ll See:

• Amazing detailed images
• Baby’s face (if positioned well)
• All limbs moving
• Heartbeat
• Baby sucking thumb/yawning/moving
• Gender (if you want)

Gender Determination:

Accuracy at 20 Weeks:

• Very high (>95% accurate)
• Much clearer than 16 weeks
• Clear differences visible
• Rarely uncertain

What We Look For:

• Boy: Penis and scrotum visible
• Girl: Three lines (labia)
• Uncertain: Rarely, if poor position

Your Choice:

• Tell us your preference at start
• Want to know: We’ll tell you
• Don’t want to know: We’ll be careful not to show
• Changed your mind: Let us know!

After the Scan:

Normal Scan:

• Good news shared immediately
• All structures appear normal
• Growth appropriate
• Measurements within normal range
• Detailed report provided
• Multiple photos given

If Concerns Found:

Soft Markers:

• Minor findings sometimes seen
• Often insignificant
• May suggest further monitoring
• Usually nothing to worry about
• Explained clearly

Abnormality Detected:

• Explained sensitively and clearly
• Images shown and discussed
• Further scans may be needed
• Referral to fetal medicine specialist
• Options discussed
• Support provided
• Never rushed

Follow-Up:

• Detailed report for midwife/GP
• Referral letters if needed
• Further appointments arranged
• Contact details provided
• Questions answered

Growth Assessment:

Measurements Taken:

• Head circumference (HC)
• Abdominal circumference (AC)
• Femur length (FL)

Estimated Fetal Weight:

• Calculated from measurements
• Plotted on growth chart
• Percentile shown
• Compared to expected range

Growth Percentiles:

• <10th percentile: Small for dates (may need monitoring)
• 10th-90th: Normal range
• >90th percentile: Large for dates

Proportionate Growth:

• All measurements should be similar percentile
• Disproportionate growth may indicate issue
• Head vs body size compared

Placenta Position:

Normal Positions:

• Anterior: Front of uterus
• Posterior: Back of uterus
• Fundal: Top of uterus
• Lateral: Side of uterus

Low-Lying Placenta:

• Placenta praevia: Covering cervix
• Common at 20 weeks (20-30% of scans)
• Usually moves up as pregnancy progresses
• Rescan at 32 weeks if low
• Only 1-2% still low at term

No Immediate Concern:

• Even if low at 20 weeks
• Follow-up scan arranged
• Rarely causes problems
• Most resolve spontaneously

Amniotic Fluid:

Normal Amount:

• Enough for baby to move
• Not too much (polyhydramnios)
• Not too little (oligohydramnios)

Too Much Fluid:

• May indicate maternal diabetes
• May indicate swallowing problems in baby
• May be normal variant
• Further monitoring needed

Too Little Fluid:

• May indicate kidney problems
• May indicate membrane rupture
• May indicate placental insufficiency
• Needs investigation

Preparation:

Before Scan:

• No fasting required
• Comfortable full bladder (not bursting)
• Wear two-piece clothing (easy access to abdomen)
• Can bring partner/children
• Allow 45-60 minutes

What to Bring:

• Maternity notes
• Previous scan reports
• List of any concerns
• Notebook for questions

Special Considerations:

Twins/Multiple Pregnancy:

• Each baby examined fully
• Takes longer (allow 60+ minutes)
• Each baby measured
• Placenta positions noted
• Growth comparison

Previous Abnormality:

• Extra attention to relevant area
• May take longer
• Specialist assessment if needed
• Reassurance if all normal

High BMI:

• Image quality can be reduced
• May need transvaginal scan for some structures
• Takes longer
• Most structures still visible

Photos & Report:

What You Get:

• Multiple printed photos
• Baby’s face (if good view)
• Gender shot (if wanted and visible)
• Detailed written report
• Measurements chart
• Growth percentile
• All findings explained

Report Includes:

• All structures checked
• Measurements with percentiles
• Placenta position
• Fluid amount
• Any findings or recommendations
• Next steps if needed

Why This Scan is Essential:

Medical Importance:

• Detects 50-60% of major abnormalities
• Allows planning for delivery
• Enables specialist care if needed
• Provides reassurance
• Confirms normal development

Emotional Importance:

• See baby in detail
• Bond with baby
• Find out gender (if wanted)
• Reassurance baby developing well
• Excitement of seeing baby move

Not a Guarantee:

• Normal scan doesn’t guarantee perfect baby
• Some conditions can’t be detected
• Some develop later
• Provides reassurance based on what can be seen

What Happens if Abnormality Found:

Immediate:

• Sensitive explanation
• Images discussed
• Severity explained
• Options outlined

Next Steps:

• Referral to fetal medicine specialist
• Further detailed scans
• MRI may be recommended
• Genetic testing may be offered
• Multidisciplinary team discussion

Your Choices:

• Continue pregnancy with monitoring
• Specialist delivery planning
• Termination (if severe abnormality)
• Time to make informed decision
• Support throughout

Important Message: Most scans are completely normal. This scan is about:

• Checking baby is growing well
• Confirming normal development
• Planning for delivery
• Reassurance for you