What Is the 20-Week Anomaly Scan — And Should You Get a Private One Too?

25. March 2026

The NHS anomaly scan, commonly referred to as the 20-week scan, is typically performed between 18 and 21 weeks of pregnancy, though it can be conducted up to 22+6 weeks in some trusts. The examination takes approximately 30 minutes and uses ultrasound waves to create detailed images of your baby’s anatomy. This screening test serves multiple purposes beyond simply checking for abnormalities. The sonographer will take specific measurements of your baby, including the head circumference, abdominal circumference, and femur length. These measurements help confirm that your baby is growing normally and provide an estimated fetal weight for that stage of pregnancy. The scan also examines the pregnancy environment, checking the position of your placenta and measuring the amount of amniotic fluid surrounding your baby. In some cases, if the placenta cannot be clearly visualised through an abdominal scan, you may be offered an internal (vaginal) scan for a clearer view.

The NHS anomaly scan screens for 11 rare but serious conditions. Knowing what these conditions are helps you understand what the scan can and cannot detect. The sonographer examines your baby for physical differences that might indicate: Anencephaly affects brain development, whilst open spina bifida involves incomplete closure of the spine. Cleft lip, a facial difference, can often be detected during the scan. Diaphragmatic hernia occurs when abdominal organs move into the chest cavity, and gastroschisis involves abdominal wall defects where organs develop outside the abdomen. Exomphalos presents as abdominal contents protruding through the umbilical cord area. Serious cardiac anomalies encompass various heart defects, though detection rates vary significantly. Bilateral renal agenesis means both kidneys are absent, whilst lethal skeletal dysplasia refers to severe bone development disorders. The scan also screens for two chromosomal conditions: Edward’s syndrome (Trisomy 18) and Patau’s syndrome (Trisomy 13). These conditions involve extra chromosomes that cause multiple physical and developmental abnormalities.

Whilst the anomaly scan provides valuable reassurance for most families, it’s crucial to understand its limitations. Detection rates vary significantly between different conditions. According to NHS guidance, conditions like spina bifida can be detected in approximately 90% of cases, as they’re usually clearly visible on ultrasound. However, other conditions prove more challenging to identify. Heart defects, despite being among the screened conditions, have variable detection rates during routine anomaly scanning. This variation in detection rates reflects the technical challenges of examining different anatomical structures through ultrasound. The scan cannot detect all possible abnormalities or guarantee a perfectly healthy baby. Many conditions, including some genetic disorders, developmental delays, and subtle structural differences, may not be apparent at 20 weeks or may not be detectable through ultrasound imaging alone. It’s also important to note that the examination focuses on major structural abnormalities rather than functional assessments. For instance, whilst the sonographer can examine the four chambers of the heart, detailed cardiac function assessment requires specialised techniques not routinely included in standard anomaly scanning.

Preparation for your anomaly scan typically involves arriving with a full bladder, achieved by drinking approximately one pint of water about an hour before your appointment. You should remove any belly button piercings as these can interfere with the ultrasound equipment. The examination takes place in a dimly lit room where you’ll lie on a couch with your clothing moved to expose your abdomen. Clear ultrasound gel is applied to your skin, and the sonographer uses a transducer to capture images of your baby. During the lengthy examination, the sonographer will remain concentrated and relatively quiet whilst systematically examining your baby’s anatomy. They’ll image your baby’s brain, face, spine, heart, diaphragm, stomach, kidneys, bladder, arms, hands, legs, and feet. The cord insertion site and placental position are also assessed. To obtain optimal views, you may need to change position, empty your bladder, or even take a walk and return later if your baby is in a position that limits visibility of certain structures.

Many parents consider private scanning to supplement their NHS care, particularly for earlier reassurance or additional detail. Private clinics often offer more time per appointment, advanced equipment, and additional services like detailed 3D/4D imaging. Some private providers offer early anomaly scanning from 10 weeks onwards, though structural assessment at this embryonic stage remains limited compared to the standard 20-week timeframe. Early scanning may identify some major structural abnormalities but cannot replace the comprehensive 20-week examination when fetal development is more advanced. Private scanning can complement NHS care by providing additional reassurance, more frequent monitoring for specific concerns, or detailed imaging when the NHS scan raises questions requiring further investigation. However, it’s essential to understand that private scanning doesn’t necessarily detect conditions missed by NHS screening, as the fundamental limitations of ultrasound technology remain regardless of the setting. The key advantage of private screening often lies in combining structural scanning with advanced genetic testing, such as non-invasive prenatal testing (NIPT), which can screen for chromosomal conditions from as early as 10 weeks with very high detection rates for Down’s syndrome.

Most anomaly scans provide reassuring results, showing that babies are developing as expected. When concerns arise, the NHS provides clear pathways for further investigation through fetal medicine specialists who can offer detailed diagnosis, counselling, and management options. For parents considering additional private screening, the decision should be based on individual circumstances, risk factors, and personal preferences rather than concerns about NHS care quality. The NHS anomaly scan represents gold-standard screening using evidence-based protocols developed through extensive research and clinical experience. If you’re considering complementary private screening, focus on services that offer genuine additional value rather than duplicating NHS examinations. Early genetic screening combined with structural assessment, for instance, can provide earlier reassurance about both chromosomal and structural development. At London Pregnancy Clinic, we understand that pregnancy brings both excitement and anxiety. Our comprehensive early screening programmes, including the SMART Test from 10 weeks, complement your NHS care by providing earlier genetic and structural assessment. Our experienced sonographers and fetal medicine specialists work alongside your NHS team to ensure you receive the most complete picture of your baby’s health throughout your pregnancy journey. The 20-week anomaly scan remains a cornerstone of excellent pregnancy care, providing vital information about your baby’s development. Whether you choose NHS scanning alone or complement it with private screening, understanding what these examinations can and cannot detect empowers you to make confident decisions about your pregnancy care.