This detailed ultrasound scan, sometimes called the mid-pregnancy or anomaly scan, is usually carried out when you're between 18 and 21 weeks pregnant.

The 20-week screening scan is offered to everybody, but you do not have to have it if you do not want to. The scan checks the physical development of your baby, although it cannot pick up every condition.

The 20-week screening scan is carried out in the same way as the 12-week scan. It produces a 2D black and white image that gives a side view of the baby. The NHS screening programme does not use 3D or colour images.

The scan is a medical examination. You'll be asked to give your permission for it to be carried out. Make sure you understand what's going to happen, and feel free to ask any questions.

What does the scan look for?

The 20-week screening scan looks in detail at the baby's bones, heart, brain, spinal cord, face, kidneys and abdomen.

It allows the sonographer to look for 11 rare conditions. The scan only looks for these conditions, and cannot find everything that might be wrong. You can find more information on each of these conditions, including treatment options, in these leaflets from GOV.UK:

• anencephaly
• open spina bifida
• cleft lip
• diaphragmatic hernia
• gastroschisis
• exomphalos
• serious cardiac abnormalities
• bilateral renal agenesis
• lethal skeletal dysplasia
• Edwards' syndrome, or T18
• Patau's syndrome, or T13

In most cases, the scan will show that the baby appears to be developing as expected, but sometimes the sonographer will find or suspect something different.

If there's a condition, will the scan find it?

Some conditions can be seen more clearly than others. For example, some babies have a condition called open spina bifida, which affects the spinal cord.

This can usually be seen clearly on a scan, and will be detected in around 9 out of 10 babies who have spina bifida. Some other conditions, such as heart defects, are more difficult to see. The scan will find about half (5 out of 10) of babies who have heart defects.

Some of the conditions that can be seen on the scan, such as cleft lip, will mean the baby may need treatment or surgery after they're born.

In a small number of cases, some very serious conditions are found – for example, the baby's brain, kidneys, internal organs or bones may not have developed properly. In some very serious, rare cases where no treatment is possible, the baby will die soon after they're born or may die during pregnancy.

What happens at the 20-week scan?

Most scans are carried out by specially trained staff called sonographers. The scan is carried out in a dimly lit room so the sonographer can get good images of the baby.

You'll be asked to lie on a couch, lower your skirt or trousers to your hips and lift your top to your chest so your abdomen is uncovered.

The sonographer or their assistant will tuck tissue paper around your clothing to protect it from the gel, which will be put on your tummy.

The sonographer then passes a handheld probe over your skin to examine the baby's body. The gel makes sure there's good contact between the probe and your skin. A black and white image of the baby will appear on the ultrasound screen.

Having the scan does not hurt, but the sonographer may need to apply slight pressure to get the best views of the baby. This might be uncomfortable.

The sonographer needs to keep the screen in a position that gives them a good view of the baby. The screen may be directly facing them, or at an angle.

Sometimes the sonographer doing the scan will need to be quiet while they concentrate on checking your baby. But they'll be able to talk to you about the pictures once they've completed the check.

The appointment for the 20-week screening scan usually takes around 30 minutes.

Sometimes it's difficult to get a good picture if the baby is lying in an awkward position or moving around a lot, or if you're above average weight or your body tissue is dense. This does not mean there's anything to worry about.

You may need to have a full bladder when you come for the appointment. The doctor or midwife looking after you will let you know before you come. If you're not sure, you can contact them and ask.

Can I find out the sex of my baby?

If you want to know your baby's sex, you should ask the sonographer at the start of the scan, so they know that they need to check.

Some hospitals may refuse to tell you sex of the baby. Speak to your sonographer or midwife to find out more.

Can my partner or a friend come to the scan with me?

Yes. The 20-week screening scan can sometimes find the baby has a health condition. You may like someone to come with you to the scan appointment.

Most hospitals do not allow children to attend scans as childcare is not usually available. Ask your hospital about this before your appointment.

Can the scan harm me or my baby?

There are no known risks to the baby or you from having an ultrasound scan, but it's important to think carefully about whether to have the scan or not.

It may provide information that may mean you have to make some important decisions. For example, you may be offered further tests that have a risk of miscarriage, and you'll need to decide whether or not to have these tests.

Do I have to have this scan?

No – it's your choice whether to have it or not. Some people want to find out if their baby has a condition, and some do not. If you choose not to have the scan, your pregnancy care will continue as normal.

When will I get the results of the scan?

The sonographer will be able to tell you the results of the scan at the time.

What if the scan shows something?

Most scans show that the baby seems to be developing as expected. If any condition is found or suspected, the sonographer may ask for another member of staff to look at the scan and give a second opinion.

Scans cannot find all conditions, and there's always a chance that a baby may be born with a health issue that scans could not have seen.

Will I need any further tests?

If the scan shows there might be something, you may be offered another test to find out for certain. If you're offered further tests, you'll be given more information about the tests so you can decide whether or not you want to have them.

You'll be able to discuss this with your midwife or consultant. If necessary, you'll be referred to a specialist, possibly in another hospital.