Ultrasound in pregnancy

About ultrasound in pregnancy

An ultrasound uses high-frequency sound waves and their echoes to create moving three-dimensional (3D) or four-dimensional (4D) images of your growing baby. The pictures (scans) are black, white and grey and will be displayed on a monitor for you to see during the procedure.

Ultrasound scans in pregnancy are usually performed by a sonographer or an obstetrician. An obstetrician is a doctor who specialises in pregnancy and childbirth. A sonographer is a technician who is specially trained to take ultrasound scans.

There are different reasons for doing ultrasound scans at different stages during your pregnancy. All pregnant women are offered two routine scans – a ‘dating scan’ to primarily check your due date, and a ‘fetal anomaly scan’ to check that your baby is developing normally. You may be offered additional scans if you’re at a higher risk of medical problems or have a family history of certain medical conditions that may affect your pregnancy.

Dating scan

You will normally have an ultrasound scan between eight and 14 weeks of your pregnancy to check when your baby is due. This can help you to monitor important milestones during your pregnancy. This scan will also tell you if you're expecting more than one baby. If you have the scan between 11 and 13 weeks of getting pregnant, your baby can also be screened for Down's syndrome.

Fetal anomaly scan

You will have another scan to check your baby's development between 18 and 20 weeks of your pregnancy. During this scan, your obstetrician or sonographer will check for abnormalities. He or she will check your baby's heart, brain, kidneys, liver and spine, and measure the size of their arms, legs and head.

Your sonographer will also check the position of the placenta, which provides vital nutrients and oxygen-rich blood to your baby. If the placenta is lying unusually low in your womb, this is called a marginal, or low-lying placenta. This will usually resolve before your baby is born, but if it doesn't, it is called placenta praevia and you may need to have a caesarean delivery – an operation to deliver your baby through your abdomen (tummy).

Other ultrasound scans in pregnancy

Ultrasound is used in other procedures that you may be offered during your pregnancy. For example, your obstetrician or sonographer may use an ultrasound to guide a fine needle through your abdomen to collect a sample of amniotic fluid that surrounds your baby for an amniocentesis test or to collect tissue samples from your placenta for chorionic villus sampling.

You may have other scans during pregnancy if your routine scans or antenatal appointments suggest there may be a problem with your baby or the placenta. For example, you might have more ultrasound scans if:

  • your 20-week scan showed you have a low-lying placenta
  • you have diabetes and are at risk of having a large-for-gestational-age baby
  • your midwife thinks your baby may be in a breech position (bottom-down rather than head-down)
  • you have vaginal bleeding

Doppler ultrasound

A Doppler ultrasound monitors flow in your blood vessels and is sometimes used to check how well your placenta is functioning as this can affect your baby's growth and development. This isn’t a routine test – you will only be offered it if your obstetrician thinks there might be a problem with the placenta.

Fetal echocardiogram

Fetal echocardiogram is a type of Doppler ultrasound to examine your baby's heart before birth. It's usually done at around 18 to 23 weeks by scanning through your abdomen. You will only be offered a fetal echocardiogram if a routine scan shows abnormalities, or if your baby is at risk of having heart problems, such as congenital heart disease.

Preparing for your ultrasound

Your GP or midwife will arrange your ultrasound scans. You usually have the scan in an outpatient department in hospital.

Your obstetrician or sonographer will explain how to prepare for your procedure. In early pregnancy you may need to have a full bladder, so you will be asked to drink fluids about an hour before the scan. A full bladder will help to lift your large bowel out of your pelvis so that your womb (uterus) can be seen more easily.

Usually only one person is allowed to accompany you into the ultrasound room but check with your obstetrician or sonographer.

Your obstetrician or sonographer will discuss with you what will happen before, during and after your procedure, and any discomfort you might have. This is your opportunity to understand what will happen, and you can help yourself by preparing questions to ask about the risks, benefits and any alternatives to the procedure. This will help you to be informed, so you can give your consent for the procedure to go ahead, which you may be asked to do by signing a consent form.

What happens during an ultrasound scan

An ultrasound scan usually takes 10 to 15 minutes to perform. A Doppler scan or fetal echocardiogram may take longer depending on the investigation.

The ultrasound scanner looks a bit like a home computer system. There is a hard drive, keyboard and a display screen. There is a sensor that your obstetrician or sonographer will hold and 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 vagina or abdomen depending on how many weeks pregnant you are. Both the routine dating scan and the fetal anomaly scan are usually abdominal scans.

Vaginal scan

This method is used if the scan is being done in early pregnancy (before about 12 weeks) when the embryo is very small. A vaginal scan gives a better view compared to an abdominal scan at this stage.

You will be asked to lie on your back and your obstetrician or sonographer will gently insert a lubricated sensor (the size of a tampon) into your vagina. The sensor will usually be covered with a condom. Please tell your examiner if you suffer from a latex allergy, so that a suitable condom is used.

Abdominal scan

This method is usually used for scans after about 10 weeks of pregnancy.

You will be asked to lie down on your back. Your sonographer or obstetrician will rub clear gel onto your skin on your lower abdomen. The gel allows the sensor to slide easily over your skin and helps to produce clearer pictures. Your obstetrician or sonographer will hold the sensor firmly against your skin and will move it over the surface. If you look at the screen, you will see a picture of your baby.

You can go home when the scan is finished. Permanent copies of your scan will be stored on a computer, a disc or printed. Your obstetrician or sonographer may give you a printed copy of the scan to take home with you after having routine scans. Some hospitals save the scans on a DVD for you to take home – ask your obstetrician or sonographer for more information.

What to expect afterwards

Your sonographer or obstetrician may explain the details of your ultrasound scan to you during or straight after your scan. Sometimes, the results of your scan will be sent to your midwife or doctor who requested it and you will need to make an appointment to find out the results.

You will usually be able to go home when you feel ready.

What are the risks?

An ultrasound examination is painless and safe. It doesn’t use radiation or have any harmful effects that may harm your baby. It’s considered safe to use during pregnancy.

What happens if my ultrasound scan shows an ectopic pregnancy?


You will need to get treatment if you have an ectopic pregnancy because it's a life-threatening condition and the embryo won’t survive.


An ultrasound scan can show an ectopic pregnancy as early as five weeks. An ectopic pregnancy is when the embryo attaches outside your womb, usually to the fallopian tube and sometimes to an ovary or your cervix.

An embryo that attaches outside your womb can't develop normally and can damage the organ it's attached to and cause severe bleeding. It can put your life at risk so this is why a confirmed ectopic pregnancy usually needs to be ended (aborted), using either medicines or surgery.

Some ectopic pregnancies are treated using a medicine called methotrexate. This stops new cells being produced and so stops the growth of the pregnancy. Methotrexate is usually given as an injection.

Often the embryo needs to be surgically removed using keyhole or open surgery. In keyhole surgery, special instruments will be passed into your abdomen (tummy) through small cuts. These instruments are used to examine and remove the ectopic pregnancy. In open surgery, your surgeon will make a single cut into your abdomen and remove the embryo. Treating ectopic pregnancy by surgery is usually a medical emergency.

How soon can an ultrasound scan determine whether my baby is a boy or girl?


About 12 weeks after you get pregnant, your baby’s sex organs are developed. You should be able to ask about the sex of your baby at your second routine scan, which checks your baby's development. You will usually have this between 18 and 20 weeks of your pregnancy.


An ultrasound scan can check the sex of your baby. However, it’s not completely accurate because it depends on the position of your baby and the skill of your obstetrician or sonographer. You may be able to find out at your routine fetal anomaly scan.

If you need to know the sex of your baby for medical reasons, you may be offered amniocentesis or chorionic villus sampling. These tests can help determine the sex of your baby and check for a range of genetic disorders.

Amniocentesis involves taking a sample of amniotic fluid that surrounds your baby in the womb. It’s usually done after 15 weeks of pregnancy. Amniocentesis has a small risk of causing a miscarriage. This is why it will usually only be offered to you if screening tests show you have a higher risk of having a baby with a genetic disorder or chromosomal disorder such as Down’s syndrome, sickle cell disease or thalassaemia.

Chorionic villus sampling involves removing tiny tissue samples from the placenta. It’s usually done at 10 to 13 weeks of pregnancy. The procedure has a slightly higher risk of miscarriage compared to amniocentesis.

It’s important to discuss the benefits and risks of screening tests with your doctor or midwife.

What is the purpose of my first ultrasound pregnancy scan?


You will usually be offered an ultrasound scan at about eight to 14 weeks of your pregnancy. It's often called a dating scan because it's done to check how many weeks pregnant you are and to estimate your expected due date. During this scan, your baby can also be screened for Down's syndrome.


Your midwife or doctor can tell how far into your pregnancy you are by measuring your baby's length, from the top of their head to rump. This is called the crown-rump length (CRL). On average your baby is about 8.5cm long at 14 weeks of pregnancy. Your baby will have all of his or her organs, muscles and bones and by about 14 weeks, your baby’s heartbeat is strong.

The amount of fluid in a fold behind your baby's neck can be measured to assess the risk of Down's syndrome. This is called the nuchal translucency test. The more fluid that is present, the greater the chance your baby has Down's syndrome. People with Down's syndrome have an extra chromosome 21. The condition causes characteristic physical and mental problems, such as learning difficulties and heart problems.

If the test indicates a higher risk, you will be offered tests, such as amniocentesis or chorionic villus sampling, which can tell you if your baby has Down's syndrome.