The anatomy scan: what it checks and what to expect
A detailed ultrasound around the middle of pregnancy that looks at how your baby is developing. In the UK it is called the 20-week scan or anomaly scan.
When is the anatomy scan?
The NHS usually offers it between 18 and 21 weeks, and in some cases up to 23 weeks. In the US it is usually done at around 18 to 22 weeks. It takes about 30 minutes, or longer if the baby is in a position that makes the pictures hard to get. Having it is your choice.
What happens during the scan
You lie down and a cold gel goes on your tummy. A sonographer moves a handheld probe over it to see your baby on a screen. This may be a little uncomfortable. They may go quiet while they concentrate, and they may ask a colleague to look too. Neither of those means anything is wrong.
What the scan checks
The sonographer looks at the baby’s bones, heart, brain, spinal cord, face, kidneys and tummy, and estimates their size. They also check the placenta, the amount of amniotic fluid and how many babies there are.
The NHS scan looks specifically for 11 conditions:
- Anencephaly
- Open spina bifida
- Cleft lip
- Diaphragmatic hernia
- Gastroschisis
- Exomphalos
- Serious heart problems
- Bilateral renal agenesis
- Severe skeletal dysplasia
- Edwards’ syndrome
- Patau’s syndrome
What the scan can’t tell you
It can’t find every condition, and some show up more clearly than others. A normal result is reassuring, but it isn’t a guarantee. Sometimes the view isn’t clear enough because of how the baby is lying, and you may be asked to come back for another look.
If something is found
You’ll be told after all the checks are done. Most people hear that their baby shows no signs of a condition. If something does show up, you may be offered more scans or tests, such as amniocentesis, and a referral to a specialist who can say whether your baby definitely has a condition. You can take time to ask questions and decide what you want to do next.
If your placenta is low
The placenta’s position is checked at this scan. The NHS calls it low-lying if it is less than 2 cm from the cervix, and placenta praevia if it completely covers the cervix.
This often sorts itself out as the womb grows. If your placenta is very low, you’ll be offered another scan at around 32 weeks. Tommy’s says the placenta has moved to the upper part of the womb by then in 9 out of 10 people. About 1 in 200 still have placenta praevia at the end of pregnancy. Tell your maternity team straight away about any bleeding.
Can you find out the sex?
Often, if the baby is in a good position. It isn’t the purpose of the scan, and some hospitals won’t tell you, so ask ahead. Compare every way to find out, or read about week 20.
Questions people ask
How long does the anatomy scan take?
Usually about 30 minutes. It can take longer if the baby is moving or is in a position that makes the pictures hard to get.
Is the anatomy scan the same as the 20-week scan?
Yes. The UK usually calls it the 20-week scan or anomaly scan, and the US usually calls it the anatomy scan.
Can the anatomy scan miss something?
Yes. The NHS says the scan cannot find all conditions, and some are seen more clearly than others.
References
- NHS: 20-week scan
- ACOG: Ultrasound exams
- NHS: What complications can affect the placenta?
- Tommy’s: Low-lying placenta (placenta praevia)
- NHS: Your antenatal appointments
Advice differs between countries, so follow your own care team’s plan. How we check this.