You will be offered one or two routine ultrasound scans in the first half of pregnancy (usually at 20 weeks). There are no known risks to the baby or you from having a scan, but it is important to think carefully about whether to have a scan or not.
The scan may provide information that means you may have to make some difficult decisions. For example, you may be offered further tests if you at risk of having a miscarriage. Some people want to find out if their baby has problems and some do not. Further information can be found in the ‘Screening Tests for you and your baby’ leaflet.
It is important to be aware of what the scans are intended for. Most scans fall into one of three categories:
- Early scans to: date the pregnancy, check the number of babies, look for possible physical problems and take specific measurements of the baby if you have agreed to first trimester screening.
- Anomaly scans are: recommended to be performed between 18 – 20+6 weeks of pregnancy to look for possible physical problems with the baby.
- Scans later in pregnancy: carried out to monitor the baby’s wellbeing and development.
Reason for scans
► Dating pregnancies
It is important to knoe the sixe of the baby in your womb so that we know how mature the fetus is. Scan dates are more accurate than menstrual dates if done before 22 weeks. This is because it looks at the actual age of the fetus, whereas menstrual dates are based on the first day of the last period which assumes fertilisation occurred 14 days later. This is not always the case. Most babies are NOT born on their expected due date, but during a 4 week period around it. Usually babies come when they are ready.
► First trimester (early pregnancy)
All pregnant women are offered an ultrasound scan between 8-14 weeks of pregnancy. This is called the dating scan. It is done to confirm the pregnancy and number of babies in the womb, calculate the expected date of delivery and to check for major problems with the baby that may be detected at this early stage. You may also be offered screening for Down’s, Edwards’ and Patau’s syndromes at this time. This will depend on whether you have agreed to have the screening test done and how many weeks pregnant you are at the time of the scan.
► Mid-pregnancy (anomaly)
You will be offered a scan between 18 weeks and 20 weeks and 6 days. The purpose of this scan is to look for structural problems in the way the baby is developing (sometimes called anomalies). The scan will look in detail at the baby’s head, spinal cord, limbs, abdomen, face, kidneys, brain, bones and heart. In most cases the baby will be developing well, but sometimes a problem is found. If a problem is suspected, you will be referred to a specialist team to discuss the options available to you. However, it is important to know that ultrasound may not identify all problems. Detection rates will vary depending on the type of anomaly, the position the baby is lying in, previous surgery to your abdomen and maternal size.
► Later pregnancy
Scans can be performed in later pregnancy to check the baby’s wellbeing. This may be required if there are concerns about how the baby is growing, or if you have any risk factors identified early in your pregnancy that may affect the growth and wellbeing of the baby e.g. high blood pressure/diabetes. The main measurement for this is the abdominal circumference, which includes the size of the liver and the abdominal wall thickness. An assessment of fluid and Doppler Flow can be done if there are any concerns with the baby’s growth. Doppler Flow indicates how well the placenta is managing the blood supply needed for the baby). If the scan suggests any concerns/problems, you will be referred to a specialist doctor to discuss the options available to you. Scans are sometimes also done to identify the position of the placenta, which may have been low in the womb at an earlier scan. A low placenta increases the risk of heavy bleeding later in pregnancy.
► Sex of the baby
Although we can sometimes tell the sex of the baby, they are not done for personal requests to find out what the sex of the baby is.