An ultrasound scan sends high-frequency sound waves through the stomach into the uterus. These waves bounce off the baby and a computer translates the returning echoes into an image. The image shows the baby's position and movements. Hard tissues such as the bones show up as white areas on the image and soft tissues appear gray and speckled. Fluids (such as the amniotic fluid that the baby lies in) do not reflect any echoes, so it appears black. It is the contrast between these different shades of white, gray, and black that allows the doctor to interpret the images.
Depending on which stage of pregnancy they are done at, ultrasound scans can:
1. Confirm where the fertilized egg has embedded itself. This is where your placenta will grow.
2. Check if your baby has a heartbeat.
3. Say whether you are pregnant with one baby or more.
4. Detect an ectopic pregnancy, where the embryo implants outside the womb, usually in the fallopian tube.
5. Find out the cause of any bleeding you may be having.
6. Accurately date your pregnancy by measuring your baby.
7. Assess the risk of Down's syndrome by measuring fluid at the back of your baby's neck at 11 to 13 weeks (nuchal translucency scan).
8. Find out why a blood screening test was abnormal.
9. Help to perform diagnostic tests safely, such as CVS or amniocentesis, by showing the position of the baby and placenta.
10. Examine your baby to check that all of his/her organs are developing normally.
11. Diagnose certain congenital abnormalities such as spina bifida.
12. Measure the amount of amniotic fluid and check the position of the placenta.
13. See how your baby is growing over several scans.
14. Check how the blood flows between your placenta and baby.
The most important routinely scans are:
1. Dating and viability scan
It is usually done in the first trimester for the duration of the 6th to 9th week. Although you don't need a scan to confirm a pregnancy, having this scan will:
1. Check if your baby is in the right position inside the uterus.
2. Let you hear your baby's heartbeat, which is one of the signs that the pregnancy is viable. The heart usually starts beating at about six weeks.
3. Find your accurate due date. If your menstrual cycle is irregular or you don't remember the first day of your last period, an ultrasound scan can tell exactly how far into your pregnancy you are.
4. Determine the reason for any spotting or bleeding you may have.
5. Show how many babies you are carrying.
2. Nuchal Translucency Scan
Nuchal translucency (NT) is a collection of fluid under the skin at the back of a baby's neck. The NT scan measures the thickness of this fluid to assess the risk of Down's syndrome and impaired cardiovascular developments. It can be measured using ultrasound. If the scan reveals that there is more fluid than usual, it could be a sign of Down's syndrome.
The NT scan must be taken at a particular time during your pregnancy. It is done between 11 weeks plus 2 days and 13 weeks 6 days when your baby's crown-rump length (CRL) is between 45mm and 84mm. Before 11 weeks, the scan is technically difficult because the fetus is tiny. It would also be too early to combine it with the blood test for Down's syndrome. After 14 weeks, any excess nuchal fluid may be absorbed by your baby's developing lymphatic system. So the test would not be accurate.
3. Anomaly Scan
The anomaly scan is the most common scan of the second trimester. It is done between 18 and 20 weeks. This scan can:
1. Show how your baby is growing and check the fetal movements.
2. Make sure your baby's internal organs are developing well.
3. Detect certain birth defects in your baby.
4. Estimate the amount of amniotic fluid.
5. Check the umbilical cord and position of the placenta.
6. Check for markers of chromosomal abnormalities.
7. Check your cervix and measure the birth canal.
Halfway through your pregnancy, most of your baby's vital organs have already developed. All pregnant women have a scan at this point because if a problem is detected, the necessary precautions can be taken.
4. Growth Scan
A growth scan in the third trimester helps to check how well your baby is growing. The doctor will check the size of your bump by measuring it as well. The doctor doing the scan will check your baby's size by measuring:
1. The circumference of your baby's head.
2. The circumference of your baby's tummy.
3. The length of your baby's thigh bone (femur).
4. The depth of the amniotic fluid around your baby.
This scan is usually done in the 25-32 week n the third trimester.
5. Doppler Scan
This is a form of ultrasound scan that helps assess your baby’s health. It measures the blood flow in different parts of your baby’s body, such as his/her umbilical cord, brain and heart. This helps to show whether your baby is getting all the oxygen and nutrients he/she needs via the placenta.
Doctors may recommend a doppler scan if you need extra care during your pregnancy. Some f the reasons you will need it are if:
1. You’re carrying twins or more
2. The baby is affected by rhesus antibodies
3. The baby is affected by slapped cheek disease
4. The baby isn’t growing at a healthy rate
5. You are experiencing fewer baby movements
6. You’ve previously had a small baby
7. You’ve previously experienced a late miscarriage or suffered the loss of your baby at birth
8. You have an existing medical condition, such as diabetes or high blood pressure
9. You have a low or high BMI
10. You smoke
It is usually done during the 28th-32nd week in the third trimester.
The above scans are the minimum recommended in a 'low-risk' mother during pregnancy. Further scans, either in-between or later, will depend on the presence of any other problems either in a previous pregnancy or in the present one. There is no maximum number of scans that can be performed in a pregnancy. The requirement of the scan should be decided based on the situation at the time of pregnancy.