The 20-Week Scan

What is the 20-week scan?

The 20-week scan is the routine mid-pregnancy ultrasound that looks in detail at how your baby has formed. The NHS calls it the 20-week screening scan or anomaly scan, and says it looks for 11 specific conditions and checks your baby's growth. It is usually done between 18 and 21 weeks.

The national screening handbook for England is more exact: the scan is performed between 18+0 and 20+6 weeks, and "the screening pathway must be completed by 23+0 weeks". It also means that if the scan cannot be finished, the repeat has to be done by 23 weeks. If you are between 18 and 23 weeks and have not had one, the NHS says to tell your midwife or GP.

What does the 20-week scan check?

The sonographer works through a fixed checklist of your baby's bones, heart, brain, spinal cord, face, kidneys and tummy, and also checks your placenta and the blood flow in your uterus. The NHS lists the 11 conditions the scan screens for:

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

How reliable is it?

It finds some conditions far more reliably than others. The national screening leaflet Screening tests for you and your baby gives two examples: open spina bifida "will be detected by the scan" in around 9 out of 10 babies who have it, while "the scan will find about half (50%) of those babies who have heart conditions". A clear scan is reassuring, not a guarantee — the same leaflet says there is "always a chance that a baby may be born with a condition that could not be seen on scan".

Other findings can turn up too. The handbook says conditions outside the 11 can be detected and that all significant findings are reported, adding that anyone who would not want to know about the 11 conditions "or other unexpected findings should consider not having the scan". Some findings are recorded but treated as normal variants; soft markers on a scan explains which ones lead to referral. The scan also records where the placenta sits — see low-lying placenta follow-up scans if yours is low.

What happens at the 20-week scan?

The NHS says the scan usually takes around 30 minutes and carries no known risk to you or your baby. To prepare, it suggests you:

• arrive early and wear loose clothing that makes your tummy easy to reach
• take out a belly piercing
• bring paper maternity notes if you have them
• have a full bladder only if you have been told to
• check in advance whether pictures cost money, and whether you can bring children — most hospitals do not allow them

The room is dimly lit. The probe is moved over gel on your tummy, which may be uncomfortable, and the sonographer may be quiet or may ask a colleague to look. The national leaflet explains the silence: "It is important for them to be able to concentrate so they can perform a good quality scan."

If they cannot get clear pictures

If your baby is moving a lot or lying awkwardly, you may be asked to walk about or eat something and come back after a short break. The handbook says a single repeat scan must be offered and completed by 23+0 weeks where image quality was compromised by your body mass index, fibroids, abdominal scarring or the baby's position. If that second scan cannot be completed either, the leaflet says you will not be offered another screening scan, and your baby will have a physical examination after birth.

Will they tell you the sex at the 20-week scan?

Usually, if it is easy to see and you ask — but it is not what the scan is for. The national leaflet says: "Finding out the sex of your baby is not the purpose of this scan, but most hospitals will tell you if it's easy to see. You will not be offered another scan if the sex of your baby is not seen." The handbook confirms the programme "does not require fetal sex to be determined".

The NHS says you may be asked during the scan whether you would like to know. If you would rather keep it a surprise, say so at the start; Ireland's HSE gives the same advice for its anatomy scan. Every route to finding out, with its accuracy, is set out in when can you find out the sex.

What happens if the 20-week scan finds something?

You are told at the end of the scan, and most people are told their baby has no signs of any of the conditions. If something is seen, the NHS says you may be offered more scans and tests, such as amniocentesis or chorionic villus sampling, and will be referred to a specialist who can explain them before you decide. The handbook requires those referrals to be timely, to a local or specialist centre.

For information and support while you decide, the NHS points to Antenatal Results and Choices (ARC), which covers screening results and how you might feel if you are told your baby has, or might have, a condition.

The mid-pregnancy scan in other countries

Every country covered here offers an equivalent scan; the name and the exact window differ.

A few differences worth knowing. ACOG says that "if the fetus is in a good position, it may be possible to tell the sex". In Australia, Pregnancy, Birth and Baby says around 1 or 2 in every 100 morphology scans find a possible anomaly, and that Medicare covers the scan though there may be out-of-pocket costs. In Ireland, the HSE says that if a problem is found you will be offered an appointment with a fetal medicine specialist. Health New Zealand says its scan takes about 30 minutes and "cannot highlight all problems". For each country's wider screening programme, see the US, Australia and the UK. Later checks on your baby's size are covered in growth scans, and the week itself in 20 weeks pregnant.

Frequently asked questions

What is the 20-week scan called?

It has several names. The NHS uses "20-week screening scan" and "anomaly scan". In Australia it is the morphology scan, in Ireland the anatomy or anomaly scan, in New Zealand the fetal anatomy scan, and in the US ACOG calls it a standard ultrasound exam.

Can you find out the gender at the 20-week scan?

Usually, if the baby is lying well and your hospital's policy allows it. The national screening leaflet says most hospitals will tell you if the sex is easy to see, but that finding it out is not the purpose of the scan, and no extra scan is offered if it cannot be seen.

How accurate is the 20-week scan for boy or girl?

When a sonographer is confident, it is very accurate. A 2014 Australian study that followed 640 fetuses scanned in a private practice found every sex prediction made after 14 weeks was correct. The catch is confidence: if the view is poor, the sonographer may not be able to say.

Can I get pictures at my 20-week scan?

Often, but check first. The NHS advises checking with the hospital or clinic before your appointment whether you will need to pay for pictures of your baby. In Australia, Pregnancy, Birth and Baby says you can ask for a copy of the images from your morphology scan.