The 20-week scan is one of the most important ultrasound examinations of your pregnancy. It is a chance to look in detail at how your baby is developing, and also at the placenta, the umbilical cord and other aspects of your pregnancy health.
This guide explains what the scan involves, when it is best performed, how our detailed examination differs from the routine NHS scan, and what happens if a finding is identified.
Key points
- The 20-week scan is much more than an examination to identify fetal abnormalities.
- Our detailed 20–22-week assessment examines your baby's anatomy, growth and development, as well as placental health and pregnancy risk.
- We assess the fetal organs, look for findings associated with genetic conditions, and examine the placenta and umbilical cord.
- We also offer additional screening for preterm birth and placental complications.
- Most examinations are reassuring, but a detailed scan can also identify pregnancies that may benefit from additional care before complications develop.
What is the 20-week anomaly scan?
The 20-week pregnancy scan assesses your baby's developing organs and anatomy, evaluates fetal growth and examines important aspects of the placenta and pregnancy. It is often called the 20-week anomaly scan, the mid-pregnancy scan or the detailed fetal anatomy scan.
At The Pregnancy Clinic, our consultant-led examination goes beyond the assessment of fetal anatomy alone. We take a comprehensive approach to evaluating your baby's development and identifying findings that may be associated with chromosomal or genetic conditions. We also assess fetal growth, placental position, umbilical cord abnormalities and factors that may indicate an increased risk of complications later in pregnancy.
Our aim is not simply to identify abnormalities, but to provide a detailed assessment of your pregnancy and help plan appropriate care for the weeks ahead.
When should I have my 20-week scan?
The routine NHS fetal anomaly screening scan is generally offered between 18+0 and 20+6 weeks of pregnancy. At The Pregnancy Clinic, we prefer to perform the detailed fetal anatomy examination at approximately 20–22 weeks.
By this stage, several important fetal structures have developed enough to allow a more detailed assessment. In particular, the fetal brain has undergone further development, allowing better views of certain structures within the brain. The fetal heart and its major blood vessels can also be examined in greater detail.
Although many major abnormalities can be identified earlier in pregnancy, an examination at 20–22 weeks provides an excellent opportunity for a comprehensive assessment of fetal anatomy.
How is our detailed 20-week examination different from the NHS anomaly scan?
The NHS Fetal Anomaly Screening Programme offers a structured ultrasound examination designed to screen for 11 specified conditions only. These include major abnormalities affecting the brain, spine, heart, abdominal wall, kidneys and skeleton, as well as physical findings associated with Edwards' and Patau's syndromes. The routine scan also assesses fetal measurements and placental position.
At The Pregnancy Clinic, we provide a detailed, consultant-led assessment of the wellbeing of your pregnancy. This includes:
- A detailed examination of fetal anatomy, with a much wider scope than the 11 conditions in the NHS programme.
- An assessment of fetal growth and development.
- An assessment of placental location, and also of placental development and function, using uterine artery Doppler.
- A detailed assessment of genetic markers.
- Measurement of cervical length to assess the risk of preterm birth.
| Routine NHS anomaly scan | Detailed scan at The Pregnancy Clinic | |
|---|---|---|
| Timing | 18+0 to 20+6 weeks | Approximately 20–22 weeks |
| Fetal anatomy | Screens for 11 specified conditions | Detailed, systematic assessment of all major organs |
| Genetic markers | Findings associated with Edwards' and Patau's syndromes | Comprehensive assessment of genetic and soft markers |
| Placenta | Placental position | Position, appearance and function (uterine artery Doppler) |
| Cervical length | Not routinely included | Included, to assess the risk of preterm birth |
What does the detailed 20-week scan assess?
1. Detailed assessment of fetal anatomy
The main part of the examination is a systematic assessment of your baby's developing organs and structures. This includes the brain and skull, face and neck, spine, heart, lungs and diaphragm, abdominal organs, kidneys and urinary tract, and limbs and skeleton. Each organ is examined using appropriate views to assess its development and identify any suspected structural abnormality.
2. Assessment of ultrasound markers associated with genetic conditions
An important part of our examination is looking for ultrasound findings that may indicate an increased likelihood of a chromosomal or genetic condition. The findings at this stage can be:
- Normal: no genetic or soft markers.
- Genetic markers or structural abnormalities present.
- Soft markers associated with chromosomal conditions, for example Down's syndrome.
3. Assessment of fetal growth
The 20-week scan is an important opportunity to assess your baby's size and growth. Measurements of your baby's head, abdomen and thigh bone (femur) are used to calculate the estimated fetal weight (EFW), which is then converted to a percentile for the stage of pregnancy:
- Normal weight: EFW between the 10th and 90th percentile.
- Small for gestational age: EFW below the 10th percentile.
- Large for gestational age: EFW above the 90th percentile.
4. Detailed examination of the placenta
The placenta plays a central role in supplying oxygen and nutrients to your baby. We assess placental location and appearance, and look for findings that may be associated with pregnancy complications. The findings can be:
- Normal: normal (high) location and normal function, with normal Doppler findings.
- Abnormal location: a low-lying placenta, or placenta praevia (covering the cervix).
- Abnormal attachment: placenta accreta spectrum.
- Abnormal function: placental insufficiency.
For further information, read: Low-Lying Placenta at 20 Weeks: Will It Move Up?
5. Examination of the umbilical cord and screening for vasa praevia
The umbilical cord connects your baby to the placenta and normally contains three blood vessels: two arteries and one vein. The vein carries oxygen-rich blood from the placenta to your baby, and the arteries carry blood and waste products from your baby back to the placenta. The findings can be:
- Normal: two arteries and one vein.
- Single umbilical artery: one of the two arteries is absent.
- Abnormal cord insertion: the cord is attached to the edge of the placenta (marginal cord insertion) or to the membranes rather than the placenta (velamentous cord insertion).
- Vasa praevia: unprotected fetal blood vessels running near the cervix.
6. Assessment of amniotic fluid volume
Amniotic fluid surrounds your baby and is important for normal movement, growth and development. By 20–22 weeks, most of the amniotic fluid is produced by your baby's urine. Major abnormalities of amniotic fluid are uncommon at this stage and, when present, are more likely to be associated with a less favourable outcome. The findings can be:
- Normal amniotic fluid volume.
- Reduced fluid (oligohydramnios): seen with prelabour rupture of the membranes, severe placental insufficiency or fetal growth restriction, or major abnormalities of the baby's kidneys.
- Increased fluid (polyhydramnios): uncommon at this stage, but may be an early sign of certain conditions, for example affecting the baby's gut (although this usually presents later in pregnancy), neuromuscular or genetic conditions. In some cases, no cause is found.
7. Cervical length and screening for preterm birth
An important aspect of the mid-pregnancy scan is estimating the risk of spontaneous preterm birth, which can be assessed by measuring the length of the cervix. The cervix is the lower part of the uterus, which normally remains closed during pregnancy. A short cervix in the second trimester is associated with an increased risk of spontaneous preterm birth. Cervical length is most accurately measured using a transvaginal (internal) ultrasound scan. The findings from the scan can be:
- Normal cervical length.
- Short cervical length: associated with an increased risk of preterm birth, which may require treatment with either progesterone or a cervical stitch (cerclage).
8. Uterine artery Doppler and the placental circulation
The uterine arteries are the blood vessels supplying the uterus and placenta. Uterine artery Doppler uses ultrasound to measure the resistance to blood flow in these arteries, using the pulsatility index (PI). The findings can be:
- Normal blood flow and resistance.
- Increased resistance with reduced blood flow, which may be associated with pre-eclampsia and fetal growth restriction.
For further information, read: Uterine Artery Doppler at 20 Weeks: What Does Increased Resistance Mean?
Can the 20-week scan exclude all fetal abnormalities?
No. Even a detailed specialist ultrasound examination cannot identify every fetal abnormality or genetic condition. Some abnormalities are too subtle to detect, while others develop or become apparent later in pregnancy.
Examples include small heart defects, certain brain abnormalities, some blockages of the gut, and genetic conditions that do not cause visible structural changes. Ultrasound also cannot exclude all metabolic, neurological or developmental conditions.
A normal examination is reassuring, because the structures that have been adequately assessed show no suspected abnormality. However, it cannot guarantee that your baby will be born without a medical condition.
What happens if an abnormality is suspected?
If we identify a possible abnormality, we will explain the finding, its potential significance and the options for further assessment. You will have a detailed consultation with our fetal medicine consultant to understand the nature of the finding, what it means for your pregnancy, and whether it may be associated with other developmental problems or genetic conditions. Where these are excluded, the outcome of the pregnancy may be normal.
We will explain whether further investigations, such as fetal echocardiography, fetal MRI or invasive testing, may be needed, and we will liaise with your local NHS Fetal Medicine Unit for onward care and further management of your pregnancy.
Why choose The Pregnancy Clinic for your 20-week scan?
Our 20-week examination is designed to provide a comprehensive assessment of your baby's development and important aspects of your pregnancy health.
- Consultant-led expertise: your scan is performed with specialist fetal medicine expertise, allowing detailed assessment of fetal anatomy and interpretation of complex findings.
- Detailed fetal heart assessment: a systematic examination of the fetal heart and major blood vessels, with additional Doppler assessment where appropriate.
- Assessment beyond fetal anatomy: fetal growth, placental position, umbilical cord abnormalities, cervical length and uterine artery Doppler.
- Specialist assessment of genetic markers: we interpret soft markers in the context of your previous screening results and the overall fetal examination.
- Personalised pregnancy care: we take time to explain the findings, discuss any concerns and recommend further investigations or monitoring when needed.
Our aim is to provide a detailed, reassuring and informative examination, supported by specialist expertise and individualised counselling.
Related pregnancy guides
Frequently asked questions
When is the best time for the 20-week scan?
The NHS offers the routine anomaly scan between 18+0 and 20+6 weeks. We prefer to perform our detailed examination at about 20–22 weeks, when the baby's brain and heart can be seen in greater detail.
What does the 20-week scan check?
It checks your baby's organs and anatomy, growth, the placenta and umbilical cord, and the amniotic fluid. Our detailed scan also includes genetic markers, cervical length and uterine artery Doppler.
How is your scan different from the NHS anomaly scan?
The NHS scan screens for 11 specified conditions. Our consultant-led examination has a wider scope and also assesses placental function, genetic markers and the risk of preterm birth.
Can the 20-week scan detect every abnormality?
No. Some abnormalities are too subtle to see or only develop later in pregnancy. A normal scan is reassuring but cannot guarantee that your baby will be born without a medical condition.
What happens if a problem is found?
We explain the finding and its significance, offer a detailed consultation with our fetal medicine consultant, discuss any further tests, and liaise with your local NHS Fetal Medicine Unit.
Book your detailed 20-week scan at The Pregnancy Clinic
We offer comprehensive consultant-led fetal anatomy examinations at approximately 20–22 weeks. Our assessments combine a detailed examination of your baby's development with specialist evaluation of fetal growth, placental health and important pregnancy risk factors.
Whether you are seeking a detailed assessment of your pregnancy or specialist advice about a previous ultrasound finding, we provide personalised care in a calm and unhurried setting.
Book your 20-week pregnancy scan →This article provides general educational information about the assessments offered at The Pregnancy Clinic. The examination and any additional investigations are tailored to individual clinical circumstances. The Pregnancy Clinic provides planned appointments only; for urgent concerns, please contact your maternity unit, NHS 111 or your GP.
Clinical references
- NHS. NHS Fetal Anomaly Screening Programme: 20-week screening scan.
- Salomon LJ, et al. ISUOG Practice Guidelines (updated): performance of the routine mid-trimester fetal ultrasound scan. Ultrasound Obstet Gynecol. 2022;59:840–856.
- Carvalho JS, et al. ISUOG Practice Guidelines (updated): fetal cardiac screening. Ultrasound Obstet Gynecol. 2023;61:788–803.
- Society for Maternal-Fetal Medicine. Consult Series #57: Evaluation and management of isolated soft ultrasound markers for aneuploidy in the second trimester. Am J Obstet Gynecol. 2021;225(4):B2–B15.
- National Institute for Health and Care Excellence (NICE). Antenatal care. NICE guideline NG201.
- National Institute for Health and Care Excellence (NICE). Preterm labour and birth. NICE guideline NG25.
- Royal College of Obstetricians and Gynaecologists (RCOG). Placenta Praevia and Placenta Accreta: Diagnosis and Management. Green-top Guideline No. 27a. BJOG. 2019;126(1):e1–e48.
- NHS England. Saving Babies' Lives Care Bundle, Version 3. 2023.