Ultrasound in pregnancy

Expert reviewer Miss Sangeeta Agnihotri, Consultant in Maternal Medicine, Obstetrics and Gynaecology
Next review due April 2020

An ultrasound uses high-frequency sound waves and their echoes to create pictures of your growing baby. You’ll be offered ultrasound scans throughout your pregnancy to monitor your baby's growth and check that everything’s going well.

Most routine scans will be done by a sonographer – a technician who’s trained to do ultrasound scans. They’ll talk you through each step of the process. Your care may be different from what’s described here as it will be personalised to meet your needs.

A pregnant woman having her blood pressure taken by a nurse

About ultrasound

Ultrasound pictures (scans) are black, white and grey. They’re displayed on a monitor for you to see during your procedure.

There are different reasons for doing ultrasound scans at different stages during your pregnancy. Everyone is offered two routine scans, which are:

  • a dating scan to check your due date
  • a fetal anomaly scan to check that your baby is developing normally

You might be offered more scans if you have any health problems that might affect your pregnancy such as diabetes. You might also have more scans if you’re having twins to monitor how they’re getting on. Your midwife or obstetrician (a doctor who specialises in pregnancy and childbirth) will let you know if you need any extra scans.

Types of ultrasound

Dating and screening scan

You’ll be offered an ultrasound scan before you’re 14 weeks pregnant to check when your baby is due. Your midwife or doctor can tell how far into your pregnancy you are by measuring your baby's crown–rump length. This is the length of your baby from the top of their head to their rump (bottom). On average, your baby is about 8.4cm long at 14 weeks, and they’ll have all of their organs, muscles and bones. This scan will also tell you if you're expecting more than one baby.

If you want to, your sonographer or doctor can also screen you for Down's syndrome at this scan. They’ll use ultrasound to measure how thick the back of your baby’s neck is, which is called the nuchal translucency test. They might also ask you to have a blood test.

Fetal anomaly scan

When you’re between 18 and 20 weeks (and six days) pregnant, you’ll have another scan to check that your baby is developing well. Your doctor or sonographer will check your baby's major organs, arms and legs, and measure the size of their head, waist and thigh bone.

Your doctor or sonographer will also check the position of the placenta, which provides vital nutrients and oxygen to your baby. If it’s lying unusually low in your womb, this is called a marginal or low-lying placenta. This will usually sort itself out before your baby is born, but if it doesn't, you might have what’s called placenta praevia. This means that your placenta is lying so low that it’s next to, or covering your cervix (the neck of your womb). You’ll be offered further scans later on in your pregnancy if you have this. You might need to have a caesarean delivery – an operation to deliver your baby through your abdomen (tummy). Ask your doctor or midwife for more information.

If your sonographer notices anything different about your baby’s heart in the fetal anomaly scan, they’ll arrange for you to have another specialised scan. A fetal cardiology specialist will do this and will look closely at your baby’s heart to see if everything is OK. See our section on fetal echocardiogram below for more information.

Extra ultrasound scans in pregnancy

If your routine scans or antenatal appointments find a possible problem with your baby or the placenta, you may need to have other ultrasound scans. For example, you might have more ultrasound scans if:

  • your fetal anomaly scan showed you have a low-lying placenta
  • you have diabetes
  • your midwife thinks your baby may be in a breech position (bottom-down rather than head-down)
  • you have high blood pressure
  • your baby looks smaller or larger than usual
  • you’re having more than one baby

Doppler ultrasound

A Doppler ultrasound monitors circulation and flow in your blood vessels. It can check the flow of blood in the umbilical cord, as well as the blood flow through your baby’s heart and blood vessels. It can also check how well your placenta is working, which will affect your baby's growth and development. This isn’t a routine test – your doctor will only offer it to you if they think there might be a problem with the placenta.

Fetal echocardiogram

A fetal echocardiogram is a type of Doppler ultrasound to examine your baby's heart before they’re born. You’ll usually have it after 18 to 21 weeks of your pregnancy. It’s a safe test for your baby and is pain-free for you. You’ll only be offered a fetal echocardiogram if a routine scan shows anything unusual or if your baby’s at risk of heart problems. This might be due to your family history, for example. Your doctor or sonographer will use ultrasound to have a careful look at your baby’s heart. It takes longer than a standard ultrasound – usually at least 45 minutes.

Other procedures

Ultrasound is used in other procedures that you might have when you’re pregnant. For example, your doctor might use it to guide them if you have an amniocentesis test. Or they may use it to help them collect samples of tissue from your placenta for chorionic villus sampling. For more information about these tests, see FAQ: Other tests for your baby.

Preparing for your ultrasound

Your midwife will arrange your ultrasound scans, and you usually have them in an out-patient department at your local hospital.

Your sonographer or doctor will explain how to prepare for your ultrasound scan. You might need to have a full bladder as this helps to push up your womb so it’s easier to see on the scan. Make sure you have plenty to drink before the scan.

Usually, you’re allowed to take someone in with you into the ultrasound room but check with your sonographer or doctor.

What happens during an ultrasound?

An ultrasound scan usually takes up to half an hour. A Doppler scan or fetal echocardiogram may take longer depending on what your sonographer or doctor is looking at.

The ultrasound scanner looks a bit like a computer – there’s a hard drive, keyboard and a monitor. There’s a sensor that your sonographer or doctor will hold and use to examine the area. This will send out sound waves and pick up the returning echoes. Pictures of your baby will be displayed on a monitor. These are constantly updated so the scan can show your baby's movements.

You may have the ultrasound scan through your tummy (abdomen) or your vagina. It depends on how many weeks pregnant you are. Both the routine dating scan and the fetal anomaly scan are usually abdominal scans.

You might need to change into a gown if you’re having a vaginal scan.

Abdominal scan

Your sonographer or doctor will ask you to lie down on your back. They’ll rub some clear gel onto your skin on the lower part of your tummy. The gel allows the sensor to slide easily over your skin and helps to produce clearer pictures. Your sonographer or doctor will press the sensor firmly against your tummy and move it over the area. If you look at the monitor, you’ll see a picture of your baby.

It isn’t always possible to get a clear view of your baby – it depends on their position in your womb. Your sonographer or doctor might ask you to move into different positions or to take deep breaths to get a clearer picture.

When they’ve finished, your sonographer or doctor will wipe the gel off your tummy. Don’t worry if they don’t get it all as it won’t stain your clothes. You can go home when the scan is finished. Your hospital will store copies of your scan and your sonographer or doctor may give you a picture of your baby to take home with you.

Vaginal scan (transvaginal ultrasound)

Your sonographer or doctor might use this method if you’re having the scan early on in your pregnancy when your baby is still very small. It gives a better view than an abdominal scan at this stage, although vaginal scans can be used later in pregnancy too.

You’ll need to lie on your back with your knees bent and your legs slightly apart. It’s a similar position to when you have a cervical screening (smear test). Your sonographer or doctor will put a protective cover over a pen-shaped sensor. They’ll add some lubricant and then gently put it into your vagina. It might feel slightly uncomfortable as they put the sensor in but it shouldn’t be painful.

What to expect afterwards

Your sonographer or doctor might explain the details of your ultrasound scan to you during or straight after your scan. Sometimes they’ll send the results to your midwife or doctor and they’ll go through things with you in a separate appointment.

You can usually go home as soon as you feel ready and can get on with your day as usual.

What are the risks?

There aren’t any risks of having an ultrasound examination. It’s considered to be safe for you and your baby. That said, you shouldn’t have any more than is necessary.

Frequently asked questions

  • If an ultrasound scan shows that you have an ectopic pregnancy, you might need to have treatment straightaway. An ectopic pregnancy is a life-threatening condition where the fertilised egg is outside of your womb so can’t develop properly. See our separate topic on ectopic pregnancy to learn more about it.

    Usually, your midwife or doctor will suspect you have an ectopic pregnancy when you’re six to 10 weeks pregnant. This is usually because of the symptoms you describe. They’ll arrange for you to have a transvaginal ultrasound as this shows an ectopic pregnancy better than an abdominal scan. A transvaginal ultrasound can show an ectopic pregnancy as early as five weeks.

  • Amniocentesis and chorionic villus sampling are other tests that you might have during your pregnancy. These can check for genetic disorders.

    More information

    Amniocentesis involves taking a sample of amniotic fluid that surrounds your baby in the womb. You’ll usually have this when you’re past 15 weeks. There are risks of having this test – there’s a small risk that it might cause a miscarriage, for example. You’ll usually only be offered this test if you have a higher than normal risk of having a baby with a genetic or chromosomal disorder. This might be Down’s syndrome, sickle cell disease or thalassaemia, for example.

    Chorionic villus sampling involves removing tiny samples of tissue from the placenta. You’ll usually have it when you’re 11 to 13 weeks pregnant. There’s a slightly higher risk of miscarriage compared to amniocentesis (if you have the amniocentesis in your second trimester). See our topic on the Stages of pregnancy to learn about trimesters.

    It’s entirely your choice whether you have chorionic villus sampling or an amniocentesis, or neither. It’s important to discuss the benefits and risks of screening tests with your doctor or midwife before you make a decision.

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  • Reviewed by Rachael Mayfield-Blake, Freelance Health Editor, June 2017
    Expert reviewer Miss Sangeeta Agnihotri, Consultant in Maternal Medicine, Obstetrics and Gynaecology
    Next review due April 2020