Facebook Instagram Whatsapp
Book Your Discovery Call

The 20 week scan

The 20 week scan

The 20 week scan, what to expect?

Between 18 weeks and 20 weeks and 6 days of your pregnancy, you’re invited to attend the mid-pregnancy scan, commonly known as the 20-week anomaly scan. This examination offers a detailed look at your baby’s development, focusing on areas such as the head, brain, spinal cord, heart, abdomen, kidneys, limbs, bones, and face. In the UK, around 95% of pregnant women attend the 20-week scan, making it one of the most widely accepted screening procedures. This high uptake underscores how valuable many parents find the scan for assessing their baby’s well being during pregnancy.

In most instances, this scan provides reassurance that your baby is developing as expected. However, if any concerns arise, I will guide you through the next steps, ensuring you have the support and information needed to make informed decisions. It’s important to understand that while this scan is comprehensive, it may not detect all conditions. Factors such as the baby’s position, maternal size, and previous abdominal surgeries can influence the clarity and outcomes of the scan. Additionally, ultrasound is a screening tool rather than a diagnostic test, meaning that some conditions may be missed, and in some cases, findings may raise concerns that later turn out to be unfounded. Therefore some women choose to decline scans which is ok.

Conditions screened during the 20 week scan

This ultrasound scan assesses for several conditions. Some are incompatible with life and others need immediate surgery at birth. According to NHS England, in 2021, 1 in 59 life babies were born with a congenital anomaly and 63% of them were diagnosed antenatally. Age (over 40) and deprivation levels were found to be risk factors. The conditions screened for during this scan are not all the complications that a baby can be born with. Other minor anomalies can only be picked up at birth. The ones screened for at the scan include (prevalence in brackets from the GOV.UK website):

Serious cardiac abnormality ( 8 in 10000 or 0.08%)

Significant heart defects that may need medical or surgical treatment post-birth. The detection rate is 50%.

Openings in the upper lip and/or the roof of the mouth, which can affect feeding and speech but are treatable with surgery. The detection rate for this condition is 75%. 

The spinal column doesn’t close completely, potentially leading to mobility and neurological challenges. The detection rate for this condition is 90%. 

A genetic disorder causing profound developmental
issues; unfortunately, many affected babies do not survive beyond
infancy (also checked at the 12-week scan). The detection rate is 95%.

  •  A severe condition where parts of the brain and skull do not develop. Sadly, babies with anencephaly cannot survive. Detection rate for this condition is 98%. 

The baby’s abdominal wall has an opening, causing intestines to extend outside the body, necessitating surgical intervention after birth. The detection rate is 98%.

Diaphragmatic hernia (4 in 10000 or 0.04%)

An opening in the diaphragm allows abdominal organs to move into the chest cavity, impacting lung development. The detection rate is 60%.

Abdominal organs develop in a sac outside the abdomen, requiring surgical correction. The detection rate is 80%

A genetic condition leading to serious health problems, with a low survival rate past the first few months (also checked at the 12-week scan). The detection rate id 95%.

A rare condition where both kidneys are absent, leading to severe complications. The detection rate is 84%. 

Bone development disorders resulting in atypical growth and potential physical challenges. The detection rate is 60%.

To find out more about these conditions check out the government.uk website below.

More information about screening conditions

Understanding detection rates and prevalence

The effectiveness of detecting these conditions varies. For example, open spina bifida is identified in approximately 90% of cases during this scan, and gastroschisis has a detection rate of 98%. On the other hand, some conditions, such as serious cardiac abnormalities, are more challenging to detect, with about 50% being identified. These detection rates reflect the varying ability of the ultrasound to identify specific anomalies, which is influenced by factors like the baby’s position, maternal body type, and the skill of the sonographer.

When thinking about prevalence — that is, how likely your baby is to have a certain condition — it’s helpful to consider other risks we face in everyday life. For example, do you avoid treating yourself to a McDonald’s meal or a drink, knowing it could increase your chances of developing a chronic illness? The truth is, those risks are higher than you might think. In the UK, 9% of people aged 45-54 have diabetes, and 24% of 25-34 year olds are classified as obese. It’s important to understand risk in context — some things feel scarier than they really are, while others may surprise you with how common they can be.

What else is checked during the 20 week scan?

 Beyond screening for specific conditions, the 20-week scan also provides valuable insights into other aspects of your pregnancy. This includes assessing the placenta, monitoring blood flow, and identifying factors that may require additional care. However, it’s important to note that not all findings will necessarily indicate a problem, and in some cases, results may lead to further testing, which can create anxiety or uncertainty.

 

Placental position

Click to flick
The most important element is the placenta. If you are thinking of declining all scans, having an abnormally positioned placenta is what puts you and your baby at risk the most. But again this needs to be considered against the risks of having the condition.
Find out more

Umbilical Cord Anatomy and Uterine Artery Dooplers

Click to flick.
The umbilical cord is checked to ensure it has three vessels (two arteries and one vein), which is the normal structure. The dooplers meassure the blood flow in the umbilical cord and if high levels are found then further scans will be offered to exclude a Small for Gestational Age (SGA). This is part of the recommendations from the saving babies bundle that aims to increase the detection of SGA babies.

Umbilical Cord Insertion

Click to flick.
Sometimes the cord inserts too close to the edge of the placenta called velamentous cord insertion wich increases the chances of it ripping during pregnancy or labour. Marginal and velamentous cord insertion may have an association with fetal growth restriction, therefore a fetal growth scan at 32 weeks is recommended.

Amniotic Fluids Levels

Click to flick.
Too much (polyhydramnios) or too little (oligohydramnios) fluid could indicate an underlying issue, such as gestational diabetes, fetal swallowing difficulties, or placental insufficiency. This can also be picked up on a later scan following a measurement of your belly by myself or your chosen midwife.

Cervical Lenght

Click to flick.
In accordance with the Saving Babies lives bundle, this will only be checked if you are at risk of preterm delivery, such as if you have has a previous preterm birth or more than 1cm of your cervix has been removed after an abnormal smear test.

Finding Out the Sex of Your Baby

Click to flick.
If you would like to know the sex of your baby, the sonographer can usually check depending on your baby's position. However, no scan is 100% accurate when determining the baby’s sex. If you decline the 20 week scan you can always arrange a private blood test (NIPT) that would tell you the sex of the baby as well as any chromosomal abnormalities.

Baby's Movements and General Wellbeing

Click to flick.
While the scan is not designed to assess movement in detail, the sonographer will check whether the baby is moving normally.

Growth Measurements

Click to flick.
Fetal measurements at 20 weeks—including head circumference, abdominal circumference, and femur length—are checked to ensure that your baby is growing as expected.

Fetal Echogenic Bowel, Dilated Renal Pelvises and Talipes

Click to flick.
These are somewhat common findings on the 20 weeks scan that can sometimes cause more anxiety, follow up scans and interventions, but do not always result in an abnormality after the birth.

Due Date Confirmation, Viability, Twins and Down Syndrome checks if you missed or declined the first trimestre scan.

Click to flick.
If you missed or declined the first trimester scan, this 20 week ultrasound scan can be an opportunity to make sure your due date matches the baby’s growth, that you have not had a missed miscarriage, that you are not carrying twins and that your baby does not seem to have Down syndrome.

Receiving Scan Results and Scan repeats

 The sonographer will discuss the scan findings with you immediately, providing clear explanations and answering any questions you may have. While most scans provide reassurance, sometimes findings can be uncertain, leading to further investigations. It is not unlikely that an inconclusive scan would lead to a repeat scan, a low lying placenta would also need a repeat scan, and abnormal dooplers would trigger 3 routine growth scans in pregnancy. This can be emotionally challenging and anxiety inducing, but I will be here (at the scan or at the other end of the phone) to offer guidance, support, and help you navigate any decisions that arise. As discussed, this does not mean that something is wrong at present, it means that further testing is needed to make sure nothing is wrong, but nothing may have been wrong in the first place!

Having definite difficult news (such as finding out that your baby has an abnormality not compatible with life) can also be challenging. Even though clients hire me to support them in pregnancy and prepare for birth, I can also support in this trying time and be present during a medical termination giving labour support.   

Occasionally, obtaining clear images during the scan can be challenging due to factors like the baby’s position or maternal characteristics. If this occurs, you will be offered a follow-up scan, ideally by 23 weeks, to complete the assessment. In the rare event that a comprehensive view isn’t achievable, rest assured that I am qualified to complete a thorough physical examination of your baby called NIPE after birth to identify any concerns.

Lastly, it should not be forgotten that this screening scan also has false positives, meaning that you may be told that your baby has an abnormality but he/she doesn’t. A case like this was recently in the news. 

Declining the 20-Week Scan

 While the 20-week scan is a valuable opportunity to check on your baby’s development, declining the 20-week scan is entirely your choice. Some parents decide to decline the scan, feeling that it wouldn’t change their plans for the pregnancy, or because they want to avoid the potential anxiety associated with uncertain findings. Others may have personal, religious or cultural reasons for opting out. There are also potential risks of the exposure to ultrasound waves and I discuss this on this post. Obstetrician and researcher Sarah Buckley has a thought provoking article about it here. 

If you choose to decline the 20-week scan, there are alternative ways to monitor your baby’s wellbeing. Routine midwifery appointments, regular blood tests, and clinical assessments offer ongoing opportunities to ensure the health of both you and your baby. As your independent midwife, I can support you in navigating your pregnancy in a way that aligns with your preferences, including providing care for a homebirth without scans, if that is your chosen path.

If you decline the scan, we will continue focusing on holistic, personalized care to ensure you feel confident and well-supported throughout your pregnancy. Whatever your decision, I’m here to support you, ensuring that you feel confident and well-informed throughout your pregnancy journey. If you choose to birth at home or decline interventions, not having scans will not affect the care you receive. As an independent midwife, I am happy to support you during a homebirth without scans or any other aspect of your pregnancy.

If you’re interested in my services, please check out the link below.

Homebirth support

Further reading

For more information about the conditions screened at the anomaly scan check these GOV.UK sitee: https://www.gov.uk/government/collections/fetal-anomalies-screening-conditions-diagnosis-treatment

https://www.gov.uk/government/publications/fetal-anomaly-screening-20-week-scan/fetal-anomaly-screening-20-week-scan

Congenital abnormalities statistics https://digital.nhs.uk/data-and-information/publications/statistical/ncardrs-congenital-anomaly-statistics-annual-data/ncardrs-congenital-anomaly-statistics-report-2021

Low lying placenta, placenta previa or placenta accreta, resources by the Royal Colleage of Obstetricians and guinecologists:

https://coga.ie/wp-content/uploads/2015/04/Low-lying-placenta-after-20-weeks.pdf

https://coga.ie/wp-content/uploads/2015/04/Low-lying-placenta-after-20-weeks.pdf

Norfolk and Norwich Trust guidelines for the 20 week anomaly scan file:///C:/Users/irene/Downloads/18-20-weeks-Fetal-Anomaly-Scan-AO39-v8.pdf

Worcestershire acute hospital trust guidelines for the 20 week anomaly scan https://apps.worcsacute.nhs.uk/KeyDocumentPortal/Home/DownloadFile/2479

National Clinical Practice Guideline The Fetal Anatomy Ultrasound (from Ireland)

https://www.hse.ie/eng/about/who/acute-hospitals-division/woman-infants/clinical-guidelines/the-fetal-anatomy-ultrasound.pdf

Diabetes prevalence https://assets.publishing.service.gov.uk/media/5a82c07340f0b6230269c82d/Diabetesprevalencemodelbriefing.pdf

Placental Position and Why It Matters

During your scan, the sonographer will check the position of your placenta to ensure it’s in a suitable location for birth. In some cases, the placenta may be lying low in the womb. If this is seen, a follow-up scan will be scheduled at 32 weeks to monitor its position (this may involve a transvaginal scan). The good news is that in most cases, the placenta moves upwards as the uterus grows.

If the placenta remains low, another scan will be arranged at 36 weeks. In 9 out of 10 cases, the placenta will have moved away from the cervix by 32 weeks. However, if it hasn’t moved by 36 weeks, you may be diagnosed with either:

  • Low-lying placenta — where the placenta is less than 2cm from the cervix.
  • Placenta previa — where the placenta fully covers the entrance to the womb.

In these cases, a planned caesarean birth is usually recommended to avoid the risk of heavy bleeding if you go into spontaneous labour. This type of bleeding is dangerous for both you and your baby.

According to the Royal College of Obstetricians and Gynaecologists, the chances of having placenta previa at the end of pregnancy are 1 in 200, and tragically, three women die each year in the UK as a result of this condition.

Important Advice

If you choose to decline scans during pregnancy, it’s crucial to seek medical attention if you experience any bleeding, no matter how small.

Pro Tip: If you’d like to reduce the need for extra scans, consider asking for a transvaginal scan during your 20-week scan if the sonographer can’t clearly see the end of your placenta. While it’s part of their guidelines to offer this, it’s not always suggested unless requested.

Placenta Accreta

Placenta accreta is a rare but serious condition where the placenta grows too deeply into the uterine wall. After birth, it cannot detach without surgery. This condition carries a high risk of heavy bleeding, and in some cases, removing the uterus may be necessary to control it.

Placenta accreta may be suspected during the 20-week scan, but it can only be confirmed after birth. It’s rare, occurring in 1 in 300 to 1 in 2000 pregnancies.

Take home message

Understanding these risks can help you make informed decisions about your care throughout pregnancy. Note that 1:200 (the changes of having a placenta previa) is low. For instance, if that was your down syndrome screening result you would be classed as low risk.