The 20-Week Anatomy Scan: What They Check, How Long It Takes, the Sex

What happens at the 20-week anatomy scan?

At the anatomy scan, a sonographer uses ultrasound to look at your baby's organs and body in detail, measure their growth, and check the placenta and the fluid around them, usually between 18 and 22 weeks of pregnancy. ACOG says this standard exam checks the baby's physical development, screens for major congenital anomalies (differences present from birth) and estimates how far along you are.

ACOG adds that the exam can show the baby's position, movement, breathing and heart rate, an estimate of their size and weight, the amount of amniotic fluid, where the placenta is and how many babies there are. In the UK, the NHS calls it the 20-week screening scan or anomaly scan, and says it looks for 11 specific conditions and checks the baby's growth.

When is the anatomy scan done?

The anatomy scan is done between 18 and 22 weeks in the US and between 18 and 21 weeks in England. ACOG says you should have at least one standard ultrasound exam in pregnancy, usually at 18 to 22 weeks. ACOG's January 2026 practice advisory recommends an anatomy survey for every patient, whichever genetic screening test they chose earlier, such as the NIPT test.

The NHS says the scan is usually done before 21 weeks and can be done up to 23 weeks in some cases. GOV.UK, which publishes the NHS screening leaflets, gives the window as 18 weeks to 20 weeks and 6 days.

What do they check at the anatomy scan?

The sonographer checks your baby's heart, brain, spine, face, kidneys, bones, limbs and abdomen, plus the placenta, cord and fluid. ACOG says the 18 to 22 week ultrasound looks for major physical defects in the brain and spine, facial features, abdomen, heart and limbs.

Cleveland Clinic lists pictures and measurements of the heart; brain, head, neck and spine; kidneys and bladder; arms, legs, hands, fingers, feet and toes; lips, chin, nose, eyes and face; chest and lungs; and stomach and intestines. It says the sonographer also records the heart rate, checks blood flow in the umbilical cord and where it attaches, looks at the position of the placenta and measures the amniotic fluid.

The NHS scan looks for signs of 11 conditions:

• anencephaly
• open spina bifida
• cleft lip and palate
• diaphragmatic hernia
• gastroschisis
• exomphalos
• serious cardiac (heart) abnormalities
• bilateral renal agenesis
• severe skeletal dysplasia
• Edwards' syndrome (trisomy 18)
• Patau's syndrome (trisomy 13)

On the NHS, Edwards' and Patau's syndromes are also screened for earlier in pregnancy by the combined test, which includes the nuchal translucency scan.

The scan cannot find every condition. GOV.UK says the 20-week scan detects around 9 out of 10 babies (90%) with open spina bifida and about half (50%) of babies with heart conditions. Cleveland Clinic says the anatomy scan does not diagnose anything, and that your provider will order more tests if they are concerned about something they see.

How long does the anatomy scan take?

The NHS says the scan usually takes around 30 minutes, and Cleveland Clinic says to expect around 45 minutes. It can take longer if your baby is moving a lot or lying in a position that makes the pictures unclear. The NHS says you may be asked to walk around or eat something and come back after a short break, and Cleveland Clinic says you might be asked to move a little or have a sweet drink.

If the pictures are still unclear, the NHS offers another appointment before 23 weeks. GOV.UK says a hard-to-see view can be due to the baby's position or to you being above average weight, and that it does not mean there is anything to worry about.

The scan is done through your belly with gel and a handheld probe. The NHS suggests loose clothing, and says you will be told if you need a full bladder. You can usually bring someone with you, though the NHS says most hospitals do not allow children. ACOG says there is no evidence that ultrasound harms a developing baby, and recommends it only for medical reasons by qualified professionals.

Can you find out the sex at the anatomy scan?

You can often find out the sex at the anatomy scan if the baby is lying in a position where the genitals can be seen. Cleveland Clinic says that in most cases you can learn the sex at this scan, and ACOG says it may be possible if the baby is in a good position. Cleveland Clinic suggests speaking up beforehand if you do not want to know.

GOV.UK says finding out the sex is not the purpose of the NHS scan, though most hospitals will tell you if it is easy to see, and you will not be offered another scan if it is not.

What happens if the anatomy scan finds something?

Most people are told their baby shows no signs of the conditions checked for, the NHS says. If something is found, you may be offered more scans and tests, such as amniocentesis or chorionic villus sampling, and referred to a specialist who explains them so you can decide.

Cleveland Clinic lists the next steps a US provider may suggest: an appointment with a maternal-fetal medicine specialist, amniocentesis, chorionic villus sampling or a fetal echocardiogram. MedlinePlus describes fetal echocardiography as an ultrasound of the baby's heart that gives a more detailed picture than a regular pregnancy scan. GOV.UK notes that some conditions seen on the scan, such as cleft lip, may mean the baby needs treatment or surgery after birth.

The NHS and Cleveland Clinic both say the sonographer may be quiet during the scan, and GOV.UK explains that they need to concentrate. GOV.UK adds that they might ask a colleague for a second opinion.

When should you call your doctor or midwife?

• You are 18 to 23 weeks pregnant and have not had a 20-week scan. The NHS asks you to tell your midwife or GP.
• You have symptoms that worry you or something does not feel right. The NHS points out that the scan cannot find all conditions, so tell your midwife even after a normal scan.
• You want to understand a result or a follow-up test you have been offered.