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When to Get Your Anomaly Scan: A Guide for Expecting Parents

The anomaly scan, also known as the 20-week scan or anatomy scan, is a detailed ultrasound examination performed during mid-pregnancy. It's a crucial step to assess your baby's development and screen for any structural anomalies. Understanding the optimal timing and purpose of this scan can help you prepare for this important appointment.

Published: October 27, 2023•Updated: October 27, 2023

When to Get Your Anomaly Scan: A Guide for Expecting Parents

Congratulations on your pregnancy! As you progress through this exciting journey, you'll encounter several important medical check-ups and scans designed to ensure the health and well-being of both you and your baby. One of the most significant is the Anomaly Scan.

At Param Maternity & Surgical Nursing Home, we understand you have questions, and we're here to provide clear, reassuring information about this vital scan.

What is an Anomaly Scan?

The Anomaly Scan is a detailed ultrasound examination performed during the middle of your pregnancy. Its primary purpose is to carefully assess your baby's structural development and screen for any congenital anomalies (birth defects).

You might also hear it referred to by other names, such as:

  • Mid-pregnancy scan
  • 20-week scan
  • Anatomy scan
  • Fetal morphology scan
  • Level II ultrasound (though this can sometimes imply a more targeted scan by a specialist)

During the scan, a sonographer will systematically examine various parts of your baby, including the head, face, spine, chest, heart, abdomen, kidneys, bladder, and limbs. They will also check the umbilical cord, placenta, amniotic fluid, and your uterus and cervix.

Why is This Scan So Important?

The anomaly scan serves several crucial purposes:

  • Screening for Structural Anomalies: This is the main goal – to look for major structural birth defects that might affect your baby's health.
  • Assessing Fetal Growth: Measurements of your baby's head, abdomen, and thigh bone help confirm growth is on track and estimate gestational age.
  • Confirming Fetal Number: If not already known, the scan confirms if you're carrying one baby or multiples.
  • Checking Fetal Viability: The sonographer will confirm your baby's heartbeat.
  • Placental Position: The location of your placenta is checked, especially its relation to the cervix, to screen for conditions like placenta previa.
  • Amniotic Fluid Volume: The amount of fluid surrounding your baby is assessed.
  • Maternal Health: The scan also evaluates your uterus for any abnormalities like fibroids.

What Structures Are Examined?

During the scan, the sonographer will meticulously examine:

  • Head, Face, Neck: Including the brain structures (cerebellum, ventricles), midline falx, and upper lip.
  • Chest: A detailed four-chamber view of the heart and its major outflow tracts.
  • Abdomen: Presence and size of the stomach, kidneys, bladder, and the umbilical cord insertion site.
  • Spine: To ensure the spine is intact along its length.
  • Extremities: Presence of all four limbs, hands, and feet.
  • Gender: If clearly visible and you wish to know, the baby's sex can often be determined.

When is the Best Time to Have Your Anomaly Scan?

The optimal window for the anomaly scan is during the mid-trimester, specifically between 18 and 22 weeks of gestation.

Leading medical organizations worldwide provide similar recommendations:

  • In India, FOGSI (Federation of Obstetric and Gynaecological Societies of India) recommends a targeted scan for anomalies between 18-20 weeks.
  • The National Institute for Health and Care Excellence (NICE) in the UK recommends the scan between 18 weeks 0 days and 20 weeks 6 days to "maximise the detection of fetal anomalies."
  • The American College of Obstetricians and Gynecologists (ACOG) in the US recommends a standard obstetric ultrasound, including fetal anatomy evaluation, between approximately 18 and 22 weeks.
  • The World Health Organization (WHO) recommends one ultrasound scan before 24 weeks for all pregnant women to improve detection of fetal anomalies.

Why This Specific Timing?

This timeframe is chosen for several important reasons:

  • Fetal Development: By 18-22 weeks, your baby's organs are sufficiently developed and large enough for detailed assessment.
  • Amniotic Fluid: There's usually an adequate amount of amniotic fluid, which acts like a clear window, allowing for better image quality.
  • Time for Decisions: If a significant anomaly is detected, this window allows enough time for further diagnostic tests (like amniocentesis or fetal MRI) and for parents to consider all available options and make informed decisions with their healthcare team.
  • Technical Feasibility: Performing the scan too early (before 18 weeks) can make it difficult to visualize all structures clearly. Waiting too late can also be challenging due to fetal crowding, increased bone calcification (which can block ultrasound waves), and potentially reduced amniotic fluid.

What the Anomaly Scan Can and Cannot Detect

It's important to understand that while the anomaly scan is a powerful screening tool, it does not detect every single anomaly. Its overall sensitivity varies depending on many factors.

  • High Detection Rate (over 90%): Conditions like anencephaly (absence of major parts of the brain), gastroschisis (intestines outside the body), and open spina bifida.
  • Moderate Detection Rate (50-90%): Major cardiac abnormalities, diaphragmatic hernia, and cleft lip.
  • Lower Detection Rate (under 50%): Some subtle cardiac defects, microcephaly (small head), or certain genetic syndromes without obvious structural markers.

Factors that can influence detection rates include:

  • Maternal Body Mass Index (BMI): A higher BMI can make it harder to get clear images.
  • Fetal Position: If your baby is in an unfavorable position, some views might be obscured.
  • Amniotic Fluid Volume: Too little fluid can reduce image quality.
  • Operator Skill and Equipment: The experience of the sonographer and the quality of the ultrasound machine play a role.

Are There Any Risks?

You can be reassured that diagnostic ultrasound has been used for decades and is considered very safe for both you and your baby. There are no known adverse effects from exposure to standard diagnostic ultrasound.

However, there can be a psychological impact:

  • Anxiety from Uncertain Findings: Sometimes, the scan might show a "soft marker" (a minor finding that is usually normal but can sometimes be associated with a higher risk of a condition) or a suspected anomaly that later turns out to be normal. This can cause significant worry.
  • False Reassurance: While rare, a false negative (missing an existing anomaly) can provide false reassurance.

What Happens After the Scan?

  • Normal Results: If everything looks normal, you'll likely be reassured and continue with your routine antenatal care.
  • Incomplete Scan: If the sonographer couldn't get clear views of all necessary structures (e.g., due to baby's position), you might be asked to return for a repeat scan, often within 1-2 weeks. This is common and usually doesn't mean there's a problem.
  • Suspected Findings: If a potential anomaly or a significant soft marker is identified, you will be referred to a specialist, such as a Fetal Medicine Unit or a Maternal-Fetal Medicine (MFM) specialist. They may recommend further investigations:
    • Repeat, More Detailed Scan: A tertiary or Level III scan.
    • Fetal Echocardiography: A specialized ultrasound of your baby's heart if a cardiac issue is suspected.
    • Fetal MRI: Magnetic Resonance Imaging may be used for clearer images of certain organs, especially the brain.
    • Invasive Diagnostic Testing: Procedures like amniocentesis may be offered to test for chromosomal abnormalities or specific genetic conditions.
    • Counselling: You will receive comprehensive counselling from a multidisciplinary team to discuss the diagnosis, prognosis, and all available management options.

Important Things to Know for Your Scan

  • No Full Bladder Needed: Unlike some early pregnancy scans, you generally do not need a full bladder for the anomaly scan.
  • Duration: The scan typically takes 20-45 minutes, but it can sometimes take longer if your baby is in an awkward position or if more detailed views are required.
  • Results: The sonographer will usually discuss the findings with you immediately after the scan. A detailed report will then be sent to your referring doctor.
  • Sex Determination: If your baby's position allows and you wish to know, the sonographer can usually determine the sex.
  • Not Mandatory, But Highly Recommended: While you have the choice, the anomaly scan is strongly recommended by medical professionals due to its crucial role in assessing fetal health.

The anomaly scan is a significant milestone in your pregnancy journey. It provides valuable information about your baby's development and helps your healthcare team plan the best possible care for you and your little one. If you have any concerns or questions, please don't hesitate to discuss them with your doctor or the sonographer.

Frequently Asked Questions

1. What is the exact recommended week to have the anomaly scan?

The optimal window for the anomaly scan is typically between 18 and 22 weeks of gestation. FOGSI (Federation of Obstetric and Gynaecological Societies of India) recommends 18-20 weeks. NICE (UK) recommends 18 weeks 0 days to 20 weeks 6 days, while ACOG (US) recommends approximately 18 to 22 weeks.

2. Why is the scan done between 18 and 22 weeks and not earlier or later?

This timing is ideal because the fetus is large enough and its organs are sufficiently developed for detailed assessment. There's also usually adequate amniotic fluid for clear imaging. This window allows enough time for any necessary follow-up diagnostic tests and for parents to consider management options if a significant anomaly is detected. Earlier scans may not visualize all structures clearly, and later scans can be challenging due to fetal crowding and increased bone calcification.

3. Is it too late to have an anomaly scan after 22 weeks?

While some guidelines extend the window to 22 weeks, performing the scan much later can be challenging. Fetal crowding, increased bone calcification causing acoustic shadowing, and potentially reduced amniotic fluid can make it harder to get clear images. Additionally, detecting anomalies later in pregnancy may limit the time available for further diagnostic steps and management options.

4. Is it possible to have the scan before 18 weeks?

While an ultrasound can be performed earlier, it is not the optimal time for a detailed anomaly scan. Before 18 weeks, some fetal structures may not be fully developed or large enough for clear visualization, potentially leading to incomplete or inconclusive results.

5. Does my high BMI affect when I should have the scan?

Yes, increased maternal body mass index (BMI) can sometimes reduce the quality of ultrasound images and may make it more challenging to visualize all fetal structures clearly. Your doctor will discuss this with you and may suggest strategies to optimize the scan.

6. What specific birth defects does the anomaly scan look for?

The scan screens for major structural anomalies in the baby's head (e.g., anencephaly), face (e.g., cleft lip), spine (e.g., spina bifida), chest (e.g., major cardiac abnormalities), abdomen (e.g., gastroschisis, kidney problems), and limbs. It systematically examines these areas for normal development.

7. How accurate is the anomaly scan at detecting problems?

The anomaly scan is a highly effective screening tool, but it is not 100% accurate. Its ability to detect problems varies depending on the type of anomaly (e.g., high detection for anencephaly, moderate for major cardiac defects, lower for subtle issues). Factors like maternal BMI, fetal position, amniotic fluid volume, and the sonographer's skill can also influence accuracy.

8. Can the scan detect Down syndrome?

The anomaly scan primarily looks for structural abnormalities. While it can identify 'soft markers' (minor findings) that may be associated with an increased risk of chromosomal conditions like Down syndrome, it does not directly diagnose genetic conditions. Definitive diagnosis of Down syndrome requires further invasive tests like amniocentesis.

9. Is the anomaly scan mandatory?

No, the anomaly scan is not mandatory, but it is highly recommended by medical professionals worldwide. It provides crucial information about your baby's development and helps ensure the best possible care.

10. Are there any risks to the baby from the ultrasound?

Diagnostic ultrasound has been used for many decades and is considered very safe for both the mother and the fetus. There are no known adverse effects from exposure to standard diagnostic ultrasound procedures.

11. How long does the anomaly scan take?

The anomaly scan typically takes about 20 to 45 minutes. However, it can sometimes take longer if the baby is in an unfavorable position, or if the sonographer needs more time to get clear views of all necessary structures.

12. Do I need a full bladder for this scan?

No, generally a full bladder is not required for the mid-pregnancy anomaly scan. You can usually empty your bladder before the examination.

13. What happens if the sonographer cannot see everything clearly?

If the sonographer cannot get clear views of all required structures, often due to fetal position or maternal factors, you might be asked to walk around, empty your bladder, or return for a repeat scan, usually within 1-2 weeks. This is common and does not necessarily mean there is a problem.

14. What does it mean if I am called back for a second scan?

Being called back for a second scan usually means that the sonographer was unable to get clear or complete views of all necessary structures during your initial appointment. This is often due to the baby's position or other factors, and it does not automatically indicate that there is a problem with your baby.

15. What is a "soft marker" and what does it mean if one is found?

A "soft marker" is a minor finding on the ultrasound that is usually normal but can sometimes be associated with an increased risk of a chromosomal abnormality or other condition. Many soft markers are transient and benign. If one is found, your doctor will discuss its significance, your individual risk factors, and whether further testing or monitoring is recommended.

16. If an abnormality is found, what are the next steps?

If an abnormality is suspected, you will be referred to a specialist, such as a Fetal Medicine Unit. Further detailed imaging (e.g., fetal echocardiogram, fetal MRI) or invasive diagnostic tests (e.g., amniocentesis) may be recommended. You will also receive comprehensive counselling from a multidisciplinary team to discuss the diagnosis, prognosis, and all available management options.

17. Can the scan tell me the sex of my baby? How accurate is it?

Yes, if your baby's position allows for clear visualization, the sonographer can usually determine the sex. At this stage of pregnancy, it is generally quite accurate, though it is not the primary medical purpose of the scan.

18. Does a normal anomaly scan guarantee a healthy baby?

While a normal anomaly scan is very reassuring, it does not guarantee a healthy baby. Some conditions may not be visible on ultrasound, may develop later in pregnancy, or are genetic conditions without structural markers that can be seen on a scan.

19. Will I get the results on the same day?

Typically, the sonographer will discuss the findings of the scan with you immediately after the examination. A detailed report will then be prepared and sent to your referring doctor.

20. Is this scan different from the 12-week nuchal translucency scan?

Yes, they are different. The 12-week (nuchal translucency or NT) scan is an early screening test primarily for chromosomal abnormalities and some major structural defects. The anomaly scan (18-22 weeks) is a much more detailed and comprehensive examination of your baby's entire anatomy.

21. Who performs the anomaly scan?

The anomaly scan is typically performed by a specially trained sonographer or a radiologist/obstetrician with expertise in fetal ultrasound.

22. What is the difference between a Level I and a Level II ultrasound?

A Level I ultrasound is generally a basic obstetric ultrasound. A Level II ultrasound, often used interchangeably with the anomaly scan or anatomy scan, is a more detailed and targeted examination of fetal anatomy, usually performed by a specialist or highly trained sonographer to look for specific structural details.

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