Most expecting parents walk into their week 20 ultrasound thinking it’s simply a chance to see their baby and maybe find out the gender. That assumption misses the point entirely. This scan, often called the anomaly scan, is actually a comprehensive medical examination – it’s basically a head-to-toe assessment of your baby’s development that checks everything from brain structures to bone length. Think of it less like a photo opportunity and more like a detailed engineering inspection of a complex system still under construction.

The 20-week ultrasound sits at a sweet spot in pregnancy. Your baby is now large enough for detailed visualisation but small enough that the sonographer can capture everything in the field of view. It’s genuinely remarkable what they can assess in a single appointment. But what exactly happens during those 30 to 45 minutes? And what do all those measurements and abbreviations on your 20-week ultrasound report actually mean?

Key Examinations and Checks Performed During the 20-Week Ultrasound

The anomaly scan isn’t random. Sonographers follow a systematic approach, checking each body system methodically. Here’s what they’re looking for during each part of the examination.

1. Brain and Skull Assessment

The fetal brain is examined using specific ultrasound planes – primarily the transventricular and transcerebellar views. These angles allow the sonographer to assess major structures including the lateral ventricles, cerebellum, and cavum septi pellucidi. According to ISUOG, this midtrimester window between weeks 18 to 21 is critical for detecting central nervous system anomalies, with neural tube defects being among the most common malformations identifiable at this stage.

The skull shape and integrity are also checked. Measurements confirm appropriate growth for gestational age. The sonographer is looking for things like proper development of the brain hemispheres and normal fluid levels in the ventricles. If anything appears unusual, they might recommend what’s called targeted fetal neurosonography – a more detailed examination requiring specialist expertise and equipment.

2. Heart Structure Evaluation

Congenital heart defects are among the most common birth abnormalities, which is why the heart gets particularly close attention during this scan. The sonographer assesses the heart’s size, shape, and functionality using standardised protocols to visualise all four chambers and the outflow tracts. They’re checking for structural abnormalities that could require intervention after delivery.

What does this actually look like in practice? The technician captures multiple views of that tiny heart – both still images and video clips – to document the blood flow patterns and chamber structure. Doppler imaging helps assess how blood moves through the heart. If abnormalities are suspected, this documentation becomes crucial for specialist referral. The NHS confirms that identifying serious cardiac abnormalities at this stage allows proper planning for any intervention needs.

3. Spine and Neural Tube Check

Neural tube defects (NTDs) encompass conditions affecting the brain, spine, or spinal cord. The most common types are spina bifida (incomplete closure of the neural tube along the spine) and anencephaly (major portions of the brain and skull failing to develop). Both can be detected during the 20-week scan.

The sonographer systematically examines the entire length of the spine, checking that each vertebra has formed correctly and that the skin covering is intact. This assessment requires careful attention to detail – the spine must be visualised in multiple planes to ensure nothing is missed. Detection at this stage allows healthcare providers to plan for any necessary interventions and gives parents time to prepare for what lies ahead.

4. Abdominal Organs Review

The stomach, kidneys, bladder, and bowel all get examined. The sonographer checks that the stomach is in the correct position and visible (which confirms the baby is swallowing amniotic fluid normally), that both kidneys are present and appropriately sized, and that the bladder is filling and emptying properly.

Abdominal wall defects such as gastroschisis and omphalocele – where the intestines or other organs develop outside the body – can be diagnosed during this examination. Children’s Hospital Los Angeles notes that early detection significantly influences management options and outcomes, particularly concerning delivery timing and postnatal surgical planning. The umbilical cord insertion is also assessed to rule out any herniation at that site.

5. Limbs and Bone Development

All four limbs are checked for appropriate length and structure. The sonographer counts fingers and toes (though sometimes those tiny digits don’t cooperate with visibility), examines the hands for proper opening and closing movements, and measures the long bones – particularly the femur (thigh bone), which is a standard biometric measurement.

Fetal ultrasonography serves as the key tool for prenatal diagnosis of limb abnormalities. The technician looks for normal arm and leg positioning, appropriate joint movement, and symmetry between left and right sides. Accurate identification during this routine ultrasound informs parents about potential health considerations and available options.

6. Facial Features Examination

The face is examined in various planes to check for conditions such as cleft lip or palate. Facial clefts occur in approximately 1 in 700 births – making them one of the more common fetal anomalies – and prenatal identification leads to better clinical management and parent counselling.

The sonographer views the lips in transverse and coronal planes, assesses the nasal bone, and examines the profile. Ears and eyes are also checked. What drives many sonographers slightly mad is when the baby positions their hands directly in front of their face throughout the entire examination. Patience often wins out. If a clear view proves impossible, sometimes a follow-up scan is scheduled.

7. Placenta and Amniotic Fluid Assessment

The placenta’s position gets documented – particularly important for delivery planning. A placenta lying low in the uterus (placenta praevia) may require monitoring throughout pregnancy, though most low-lying placentas move up as the uterus grows. The sonographer also notes the cord insertion point and overall placental appearance.

Amniotic fluid volume is assessed either subjectively or through measurements like the Amniotic Fluid Index (AFI) or maximum vertical pocket. Both too much fluid (polyhydramnios) and too little (oligohydramnios) can indicate potential complications that warrant closer monitoring throughout the remaining pregnancy.

Measurements and Development Milestones Assessed During the Anomaly Scan

Beyond the structural checks, specific 20-week ultrasound measurements establish baseline data for tracking your baby’s growth trajectory.

Standard Foetal Measurements Taken

Four key biometric measurements form the foundation of fetal size assessment:

Measurement	Abbreviation	What It Measures
Biparietal Diameter	BPD	Width of the head from one parietal bone to the other
Head Circumference	HC	Measurement around the skull at its widest point
Abdominal Circumference	AC	Measurement around the baby’s belly
Femur Length	FL	Length of the thigh bone

These measurements are compared against established normal ranges for gestational age. The combination of all four gives a reliable estimate of fetal size and can help identify potential growth issues early.

Growth Percentiles and Charts

Your baby’s measurements get plotted against population reference charts. Most reports will express growth as a percentile – meaning where your baby falls compared to other babies at the same gestational age. A baby measuring on the 50th percentile is exactly average; one on the 25th percentile is smaller than 75% of babies but still potentially perfectly healthy.

Single measurements matter less than trends. If your baby consistently tracks along the 30th percentile, that’s their normal growth pattern. What concerns clinicians is when a baby suddenly drops from the 70th to the 20th percentile – that deviation from their established trajectory warrants investigation. One slightly low measurement rarely indicates a problem.

Amniotic Fluid Volume Assessment

Amniotic fluid levels tell a story about fetal wellbeing. The fluid is produced primarily through fetal urine output after the first trimester, so normal levels suggest the kidneys and urinary system are functioning properly. Too little fluid can indicate kidney problems or premature membrane rupture. Too much can sometimes signal issues with swallowing or other conditions.

The measurement appears on your report either as an AFI number (normal range typically between 5-25 cm at this stage) or as a description of the deepest pocket of fluid. Most scans show normal levels. Abnormal findings prompt further evaluation rather than immediate concern.

Placental Position and Grading

Beyond position, the placenta receives a maturity grade. At 20 weeks, most placentas appear Grade 0 or Grade I – essentially immature and homogeneous in appearance. Higher grades (II or III) suggest early maturation and aren’t typically expected until much later in pregnancy.

The cord is checked to confirm it contains three vessels (two arteries and one vein). A two-vessel cord occurs in about 1% of pregnancies and is usually benign but may prompt additional monitoring.

Understanding Your 20-Week Ultrasound Report

That printed report full of abbreviations and numbers can feel like reading another language. Here’s how to decode it.

Common Terms and Abbreviations

Beyond the biometric abbreviations (BPD, HC, AC, FL), your 20-week ultrasound report may include:

• GA – Gestational Age (how far along the pregnancy has progressed)

• EDD or EFW – Estimated Delivery Date or Estimated Fetal Weight

• AFI – Amniotic Fluid Index

• FHR – Fetal Heart Rate

• Presentation – How the baby is positioned (breech, cephalic/head-down, transverse)

• NAD – No Abnormality Detected

• Posterior/Anterior Placenta – Position of placenta (back or front wall of uterus)

The position at 20 weeks doesn’t predict delivery presentation – babies have plenty of room to flip multiple times before labour.

Normal Range Values

Normal values vary by gestational age, but at 20 weeks, typical approximate measurements include:

• BPD: 45-50 mm

• HC: 160-180 mm

• AC: 140-165 mm

• FL: 30-35 mm

These are rough guides – your sonographer and midwife interpret the specific numbers in context. A measurement slightly outside these ranges doesn’t automatically signal a problem. What matters is the overall picture and whether measurements are proportional to each other.

Soft Markers Explained

This is where things can get unnecessarily anxiety-inducing if not explained properly. Soft markers are ultrasound findings that, on their own, are usually harmless but statistically occur slightly more often in pregnancies with chromosomal conditions like Down’s syndrome.

Examples include:

• Echogenic intracardiac focus (bright spot in the heart)

• Echogenic bowel

• Mild pyelectasis (slightly dilated kidney pelvis)

• Short femur length

• Choroid plexus cysts

The frustrating reality? These findings often appear in completely healthy babies. An isolated soft marker with otherwise normal results and a low-risk combined screening test rarely changes the overall risk significantly. But multiple soft markers or soft markers combined with other concerning findings might prompt discussion about further testing options.

When Further Testing Recommended

If the anomaly scan identifies a potential abnormality, the next steps depend on what’s found:

• Repeat scan – Sometimes a better view is needed due to baby’s position

• Specialist referral – Fetal medicine consultants review complex findings

• Amniocentesis – Testing amniotic fluid for chromosomal or genetic conditions

• Fetal echocardiography – Detailed heart scan by specialists

• MRI – For certain brain or body abnormalities

Most 20-week scans return normal results. But when they don’t, the system is designed to move quickly and provide appropriate expertise and support.

What to Expect During the Appointment

Knowing the practical details helps reduce anxiety on the day.

Preparation Requirements

Unlike first-trimester scans that often require a full bladder, the 20-week scan typically doesn’t. A moderately full bladder can sometimes help, but you’re usually told to use the toilet if needed. Eating beforehand is absolutely fine – there’s no fasting requirement. Some departments recommend wearing a two-piece outfit for easier access to your belly.

What actually helps more than any physical preparation is realistic expectations. This is a medical examination first. The sonographer concentrates intensely on checking every structure, often in silence. That quiet focus doesn’t mean something is wrong – it means they’re doing their job properly.

Duration and Process

Expect the appointment to last between 30 and 45 minutes, though it can run longer if the baby’s position makes certain views difficult. You’ll lie on an examination couch with your abdomen exposed. The sonographer applies warm gel (ask them to warm it if your hospital doesn’t!) and moves the transducer probe across your belly to capture different views.

Here’s a moment that sticks with most parents: that first glimpse of the profile view, with the forehead and nose clearly visible. Suddenly this becomes a recognisable face. The baby often moves during the examination – sometimes cooperatively, sometimes decidedly not. You might see sucking movements, hiccups, or those trademark kicks you’ve started feeling from the inside.

Gender Determination Options

If you want to know the sex, tell the sonographer at the start. Most hospitals in the UK will disclose the sex if parents request it, though some trusts have policies against it. The accuracy at this stage is generally high – around 95-99% depending on the baby’s position and whether the necessary anatomy is visible.

Some parents choose team yellow and ask not to be told. If that’s your preference, make it very clear before the scan begins so the sonographer avoids accidentally revealing during the examination.

Questions to Ask Your Sonographer

Don’t leave with unanswered questions. Consider asking:

• Is everything measuring as expected?

• Are there any findings that need follow-up?

• Where is my placenta positioned?

• Is my amniotic fluid level normal?

• When will I receive the written report?

• Who should I contact if I have questions later?

The sonographer may not give detailed explanations during the scan itself – they’re concentrating on the examination. Many hospitals have a debriefing period at the end where results are discussed, or they may direct you back to your midwife for a fuller conversation.

Making the Most of Your 20-Week Ultrasound Experience

This scan sits at an interesting intersection. It’s a significant medical examination, and simultaneously a powerful emotional moment in pregnancy. Balancing both perspectives helps you get the most from the experience.

Bring a support person if hospital policies permit. Having someone else present helps you remember what was said (pregnancy brain is real) and provides emotional support whether the news is straightforward or more complex. Take the printed images home – regardless of image quality, they become treasured keepsakes.

Don’t catastrophise if the sonographer seems to linger on a particular area. They may simply be struggling to get a clear view due to baby’s position. Equally, don’t assume everything is perfect just because no one mentioned concerns during the scan – wait for the full report and follow-up conversation with your healthcare provider.

And here’s something worth remembering: the anomaly scan is called the anatomy scan in many countries for good reason. While it screens for abnormalities, it’s fundamentally a comprehensive look at normal human development – all those intricate systems forming and functioning together inside you. Most scans confirm exactly that. Everything is progressing as it should.

Frequently Asked Questions

Can the 20-week ultrasound detect all birth defects?

No. The anomaly scan detects approximately 50-70% of major structural abnormalities, depending on the condition and quality of the scan. Some conditions only become apparent later in pregnancy or after birth. Certain defects are too subtle to see on ultrasound, while others affect function rather than structure. A normal scan is reassuring but cannot guarantee a completely healthy baby.

What happens if an abnormality is found during the anomaly scan?

The sonographer will document findings and typically refer you to a fetal medicine specialist for a more detailed assessment. This might happen the same day or within a few days depending on what’s found. You’ll receive counselling about the specific condition, prognosis, and available options. Support from midwives, specialist nurses, and sometimes counsellors is available throughout this process.

Is it safe to have multiple ultrasounds during pregnancy?

Diagnostic ultrasound has been used in obstetrics for over 50 years with no confirmed harmful effects on mother or baby. The equipment uses sound waves, not radiation. Professional bodies recommend ultrasound when medically indicated. Extra scans for medical reasons are considered safe, though purely social or keepsake scans in non-medical settings should be kept to a minimum.

How accurate are 20-week ultrasound measurements?

Accuracy depends on multiple factors including equipment quality, sonographer expertise, baby’s position, and maternal body habitus. Individual measurements typically have a margin of error of around 10-15%. Dating accuracy is approximately plus or minus one week at this stage. What matters most is consistency of technique and tracking growth over time rather than relying on single measurements.

Can I eat before my 20-week ultrasound appointment?

Yes. There’s no requirement to fast before a 20-week scan. Eating normally is perfectly fine and some evidence suggests a small sugary snack might even encourage baby movement, potentially making the examination easier. Staying hydrated helps too. Check any specific instructions from your hospital, but most have no dietary restrictions.

Will I receive photos from my 20-week scan?

Most hospitals provide printed thermal images at no cost or for a small charge. The number varies – typically two to four images. Some offer USB sticks with additional photos or short video clips. Private scans often include more images and sometimes 3D/4D views. Keep thermal prints away from heat and light as they fade over time; photographing them with your phone preserves them permanently.