Dating and screening scan
You will usually be offered an ultrasound scan before 15 weeks of 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 wish, your baby can also be screened for Down's syndrome at this scan. This involves having an ultrasound scan to measure the thickness of the back of your baby’s neck (nuchal translucency). You will also be asked to have a blood test. If you’re more than 14 weeks pregnant, you will be asked to have a blood test only, and not a scan. See our FAQs section for more information about this.
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 major organs, arms and legs. They will also measure the size of their head, waist and thigh bone.
Your sonographer will also check the position of the placenta, which provides vital nutrients and oxygen- 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’s called placenta praevia. If you have placenta praevia, you will be offered further scans later on in your pregnancy. You may also need to have a caesarean delivery – an operation to deliver your baby through your abdomen (tummy).
A Doppler ultrasound monitors circulation and flow in your blood vessels. It can check the flow of blood in your uterus, or how well your placenta is functioning, which can affect your baby's growth and development. This isn’t a routine test. You may be offered this test if your obstetrician thinks there might be a problem with the placenta.
Fetal echocardiogram is a type of Doppler ultrasound to examine your baby's heart before birth. It's usually carried out after 17 to 18 weeks of your pregnancy and involves scanning through your abdomen. It’s a safe test for your baby and is pain-free for you. You will only be offered a fetal echocardiogram if a routine scan shows abnormalities. You may also be offered a fetal echocardiogram if your baby is at risk of having heart problems, such as congenital heart disease. The ultrasound involves a careful look at your unborn baby’s heart and will take longer than a standard ultrasound. For example, on average it takes at least 45 minutes to perform.
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 carry out an amniocentesis test. This involves using ultrasound to guide a fine needle through your abdomen to collect a sample of amniotic fluid that surrounds your baby. Or it may be used to help collect tissue samples from your placenta for chorionic villus sampling. For more information about these tests, see our FAQs section.
Other ultrasound scans in pregnancy
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 high blood pressure
- your baby appears to be small for his or her age
Your midwife will arrange your ultrasound scans. You usually have the scan in an outpatient department in hospital.
Your sonographer or obstetrician 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 sonographer or obstetrician will discuss with you what will happen before, during and after your procedure, and any discomfort you might have. This is your chance to understand what will happen. You can help yourself by getting questions ready 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.
An ultrasound scan usually takes 10 to 15 minutes. A Doppler scan or fetal echocardiogram may take longer depending on what is being looked at.
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 pictures are constantly updated so the scan can show your baby's movements.
You may have the ultrasound scan through your vagina or abdomen (tummy) depending on how many weeks pregnant you are. Both the routine dating scan and the fetal anomaly scan are usually abdominal scans.
This method may be used if the scan is being done in early pregnancy when your baby is still very small. It’s sometimes called a transvaginal ultrasound. A vaginal scan gives a better view compared to an abdominal scan at early stages of your pregnancy. Vaginal scans can also be used later in pregnancy.
You may be asked to change into a medical gown. You will be asked to lie on your back with your knees bent and your legs slightly apart (a similar position to having a smear test). 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 protective cover. You may feel slight discomfort as the sensor is inserted, but it is only temporary and is usually minimal.
This method is usually used for routine scans.
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.
Sometimes it might not be possible to have a clear view of your baby. It depends on your baby’s position in your womb. If you have previous scarring on your abdomen this may affect how clear the pictures are.
You can go home when the scan is finished. Permanent copies of your scan will be stored by the hospital. Your obstetrician or sonographer may give you a picture of your baby to take home with you after having routine scans.
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.
An ultrasound examination is painless and considered safe for you and your baby.
What happens if my ultrasound scan shows an ectopic pregnancy?
If an ultrasound scan shows that you have an ectopic pregnancy, you will need to have treatment immediately. It is a life-threatening condition where the embryo (your developing baby) is outside of your womb and will be unable to develop properly.
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 is the name given to your developing baby before eight weeks of pregnancy. Over seven in 10 ectopic pregnancies are identified during the first ultrasound scan.
Sometimes you may be referred for an earlier scan if you have symptoms which could suggest an ectopic pregnancy. These include symptoms such as one-sided severe abdominal (tummy) pain, abnormal bleeding or fainting. An ectopic pregnancy can show on an ultrasound – particularly a transvaginal ultrasound – as early as five weeks. 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 you’ll need to have treatment using either medicines or surgery.
Some ectopic pregnancies are treated using a medicine called methotrexate. 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 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.
What’s the difference between amniocentesis and chorionic villus sampling?
Although amniocentesis and chorionic villus sampling can both check for genetic disorders, the tests are slightly different from each other.
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. You will usually only be offered this test if you have a high risk of having a baby with a genetic or chromosomal disorder. For example, Down’s syndrome, sickle cell disease or thalassaemia.
Chorionic villus sampling, however, 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 your choice whether you have chorionic villus sampling or an amniocentesis or not. It’s important to discuss the benefits and risks of screening tests with your doctor or midwife before you make a decision.
What’s the purpose of my first ultrasound pregnancy scan?
You will usually be offered an ultrasound scan before 15 weeks of 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. The sonographer will also check whether you’re having one baby or whether you‘re having twins. During this scan, your baby can also be screened for Down's syndrome if you wish.
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.
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. You may also be asked to have a blood test. 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. These tests can tell you if your baby has Down's syndrome.
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