An obstetric anomaly scan (often called the 20-week scan) is a highly detailed ultrasound performed in the second trimester of pregnancy. Its primary purpose is to examine the physical structure of your baby from head to toe. The radiographer will check that your baby's organs, bones, limbs, brain, and heart are developing normally.
The scan also checks the position of the placenta, the blood vessels in the umbilical cord, and the volume of amniotic fluid surrounding your baby.
Common Indications for an Obstetric Anomaly Scan
- Routine screening for structural anomalies between 18 and 22 weeks of pregnancy.
- Checking the position of the placenta (to rule out placenta previa).
- Monitoring amniotic fluid levels.
- Verifying the growth and age of the baby.
- Checking the cervix length.
What exactly does an obstetric anomaly scan show?
The scan shows a detailed grey-scale image of your baby's internal organs, including the brain, heart, kidneys, stomach, and bladder. It also shows the spine, ribs, limbs, hands, and feet. If you wish to know and the baby is in a suitable position, it can usually show the baby's sex.
How should I prepare for an obstetric anomaly scan?
You do not need to fast. You should drink about 2 to 3 glasses of water one hour before the scan and try not to urinate. A partially full bladder helps push your womb upward, making it easier for the radiographer to see the baby clearly. Do not overfill your bladder to the point of pain.
Do I need to stop taking medication?
No. Continue taking all your pregnancy supplements (such as folic acid and iron) and any other prescribed medications as usual.
What happens when I arrive for my scan?
You will go into a private scan room. The radiographer will ask you to lie flat on your back on a scan couch and expose your abdomen. They will adjust the pillows to make you comfortable.
How is the procedure performed?
The radiographer will apply clear, warm gel to your lower abdomen. They will slide a hand-held probe (transducer) over your skin, applying gentle pressure to get clear views. You will see black-and-white images of your baby on a screen. The radiographer will take multiple measurements of your baby’s head, abdomen, and thigh bone.
How long does an obstetric anomaly scan take?
This is a very thorough scan that takes about 30 to 45 minutes. The radiographer has a long checklist of structures to check. If the baby is lying in a difficult position, it may take slightly longer, or you may be asked to walk around to encourage the baby to move.
Is there any discomfort?
You will feel some pressure on your abdomen as the radiographer slides the probe. This is generally not painful, but the pressure can feel a bit firm when trying to see specific structures. Let the radiographer know if it becomes uncomfortable.
Are there risks?
The scan uses no ionising radiation, and diagnostic ultrasound has no independently confirmed harmful effect when used appropriately. The sonographer should still use the lowest output and shortest examination time needed, and use spectral Doppler only when clinically justified.
Can children have this scan?
This scan is for pregnant patients.
Can I have this scan if I am pregnant?
Yes. It is a recommended structural screening examination in pregnancy, but timing and content should follow the maternity team's protocol and local access.
When will I get my results?
The radiographer will often explain what they are seeing during the scan. A final written report will be compiled by a radiologist and sent to your referring doctor or handed to you shortly after the scan.
What if the scan finds something?
An anomaly scan cannot detect every structural, genetic, developmental, or functional condition, and some views may be incomplete because of fetal position or the acoustic window. A suspected finding usually needs expert review, repeat imaging, fetal echocardiography, genetic counselling/testing, or MRI depending on the concern. Screening ultrasound alone does not “confirm” every diagnosis.
What are the important limitations?
Ultrasound does not use ionising radiation, but a useful result still depends on the clinical question, the operator, the equipment, patient anatomy, and whether the target can be reached by sound waves. Gas, bone, body habitus, pain, movement, and a limited acoustic window can hide disease. A normal scan does not automatically exclude the suspected condition; persistent or worsening symptoms need clinical reassessment and sometimes repeat ultrasound, CT, MRI, laboratory tests, or another specialist test.
Ultrasound should be used for a medical purpose, with output and examination time kept as low as reasonably achievable—especially in pregnancy and with Doppler. Internal scans and procedures require explanation, consent, privacy, and a chaperone according to patient preference and local policy.
Questions to ask your care team
- What exact question should this ultrasound answer, and could anything important remain unseen?
- Is this a screening, diagnostic, surveillance, or procedure-guidance examination, and will the result change care?
- What preparation is required, and should I continue all medicines unless my own clinician gives different instructions?
- When will I receive the signed report, who will explain it, and what symptoms should prompt urgent assessment rather than waiting?
Sources and further reading
- RadiologyInfo.org: Obstetric ultrasound
- AIUM: Prudent clinical use and safety of diagnostic ultrasound
Conclusion
An obstetric anomaly scan is a safe, routine part of pregnancy care that provides reassurance about your baby’s development. To make the scan easier, wear two-piece clothing (like a top and skirt/trousers) so you can easily expose your abdomen.