Understanding basic ultrasonography (USG) in pregnancy

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USGUltrasonography is a very important tool in pregnancy to check the baby’s growth, development, position and movements. It also gives parents the first glimpse of their baby. It is very important that you understand the terminologies and other factors in the scan by which your pregnancy will be followed up for 9 months. The doctor will also give you an ultrasound report with a printout of your baby’s images. 3D and 4D color scans are becoming very popular because they offer a more life-like image of your baby.

*Sex determination of the unborn baby (fetus) is strictly prohibited in India under the PCPNDT Act, hence will not be dealt here in this article.

 How does an Ultrasound work and is it safe?

Ultrasound is SAFE during pregnancy

Unlike an X-ray, no radiation passes through the body with an ultrasound.  An ultrasound scan sends high frequency sound waves (NOT harmful radiation) through the tummy into the uterus. These bounce off the baby and a computer translates the returning echoes into an image.

This image shows the baby’s position and movements. Hard tissues such as bone show up as white areas on the image, and soft tissues appear grey and speckled. Fluids (such as the amniotic fluid that the baby lies in) do not reflect any echoes, so appear black. It is the contrast between these different shades of white, grey and black that allows your doctor to interpret the images.



Ultrasound scans have been used in pregnancy for more than 40 years with no known side-effects. Even so, there should be clear medical reasons for doing a scan, and all ultrasound exposure should be justified and limited to the minimum needed to make a diagnosis.

How are the scans carried out?

There are two ways of doing a first trimester scan:

  1. Transvaginal scan (TVS)
  • Vaginal scans give a much clearer picture of your baby, especially at a very early stage of pregnancy.
  • A vaginal scan is done by introducing a probe into the vagina. It can pick up a better image of your baby.
  • If you are having a transvaginal scan (TVS), you will need to empty your bladder first. A full bladder can obstruct the view of your baby.
  • You will need to undress from the waist down so that the probe can be easily inserted into your vagina. The nurse or assisting staff will usually cover your legs with a sheet while the scan is happening.
  1. Abdominal scan
  • It is done AFTER 10 WEEKS and for the rest of your pregnancy.
  • During this scan, the doctor will put some (usually very cold) gel on your tummy. She will then move a small hand-held probe or transducer over your skin to get views of the baby.
  • If you’re having an abdominal scan, you will need to have a full bladder, so it’s best to drink lots of water before you arrive. You need a full bladder to push your uterus higher up in your abdomen so that the scan will get a better image of your baby.
  • You will need to expose your tummy for an abdominal scan. It’s a good idea to wear loose or two-piece clothing such as a salwar kameez so you won’t need to get fully undressed.

Does an ultrasound hurt?

An abdominal scan is painless since it is done over the abdomen. Many women are vary of a vaginal scan, thinking it to be a painful one and may hurt the baby. But a vaginal scan is not that painful if you are relaxed, which inturn relaxes the muscles so that the transducer can slide in easily. The probe lies in the vagina and doesn’t cross the cervix, hence it doesn’t come into contact with the uterus which holds the baby and hence is safe.

When are the scans done?

This can vary from person to person. If yours is a low-risk pregnancy, you may have four or five scans during your entire pregnancy.

You will need more scans if:

  1. You are above 35 years.
  2. You are carrying twins or more.
  3. You have a medical condition, such as cysts or tumours or an ageing placenta, which may cause some complications.

What does each scan at each trimester mean?

First let us find out what are trimesters:

  1. First Trimester: Starts with the first day of your last menstrual period (LMP) and ends with the last day of the 13th week.
  2. Second Trimester: Starts at the beginning of the 14th week after your LMP and lasts through the 27th week of pregnancy.
  3. Third Trimester: Starts at the beginning of the 28th week after your LMP and ends with labor.

Experts recommend having your first scan when you are six weeks pregnant. Trans-vaginal scan is the most preferred scan at this stage. Although you don’t need a scan to confirm a pregnancy, having one in the early weeks 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. (will detect if it’s a twins or multiple pregnancy)
  6. The doctor will also measure the Crown-rump length (CRL): The CRL measurement is the distance between the top of the embryo and it’s rump. The CRL can be measured between 7 to 13 weeks and gives an accurate estimation of the gestational age.



This is also called as an ANOMALY SCAN and is the most common scan of the second trimester. This scan will:

  1. Show how your baby is growing
  2. Make sure your baby’s internal organs are developing well
  3. Detect certain health problems or malformations in your baby
  4. Estimate the amount of amniotic fluid (AF)
  5. Check the position of the placenta
  6. During the scan, the doctor will measure parts of your baby’s body, to see how well the baby is growing. The measurements should match up to what’s expected for your baby at this stage of his development. If there is more than two weeks’ difference between the age of your baby as revealed by this scan and what is expected based on your last monthly period (LMP), your doctor might ask you to get further tests.She will measure your baby’s:
    1. Head circumference (HC)
    2. Head diameter (biparietal diameter or BPD)
    3. Abdominal circumference (AC)
    4. Femur or thigh bone length  (FL) USG
  7. Estimated Fetal Weight: The weight of the fetus at any gestation can also be estimated with great accuracy using polynomial equations containing the BPD, FL, and AC.
  8. It is natural to be curious about your baby’s sex but prenatal sex determination is prohibited in India. This is because of an alarming rate of female abortions.  It is also an offence for you to ask about the sex of your baby. Some hospitals and diagnostic centers require you to sign a document before your ultrasound that states that you will not ask the doctor about the sex of your unborn baby.
  9. There are some conditions which can be detected early by the “Anomaly Scan”. These are:
    1. Absence of the top of the head (anencephaly)
    2. Cleft lip
    3. Defect of the abdominal wall, where the bowel and liver protrude (exomphalos)
    4. Defect of the abdominal wall, where the intestines protrude (gastroschisis)
    5. Missing or very short limbs
    6. Defect of the spinal cord (spina bifida)
    7. Major kidney problems (missing or abnormal kidneys)
    8. Hole in the muscle separating chest and abdomen (diaphragmatic hernia)
    9. Excess fluid within the brain (hydrocephalus)
    10. Edwards’ syndrome or Patau’s syndrome (chromosomal abnormalities)
    11. Major heart problems (defects of chambers, valves or vessels)

The third trimester scans will look at the following:

  1. Your baby’s wellbeing:
    Your baby’s general wellbeing is measured by his biophysical profile. A healthy baby:

    1. stretches and flexes
    2. moves his arms and legs frequently
    3. opens and closes his hands
    4. moves his fingers and toes, lips and tongue
    5. makes breathing movements
  2. Baby’s head (HC) and abdomen (AC) circumference:
  3. The position of the placenta and maturity: A placenta that is too close to the cervix is known as placenta praevia. If you are diagnosed with placenta praevia, you will need to have a caesarean section
  4. Check the position of your baby and weight:  The position doesn’t matter at all up until about 37 weeks. If your doctor thinks your baby is in the breech position (head up, bottom down) at this stage, you will likely have a scan to confirm this. Most breech babies are delivered by caesarean
  5. Check the position of umbilical cord
  6. Measure the amount of amniotic fluid: Excessive or decreased amount of liquor (amniotic fluid) can be clearly depicted by ultrasound. Both of these conditions can have adverse effects on the fetus.
  7. You might get additional scans in your third trimester if you are carrying twins or more

Following is a pdf document which you can download for your reference in which I have noted down the average fetal and obstetric measurements attained at gestational week each week : Fetal and Obstetric USG Measurements in Pregnancy

Is the ultrasound scan in a twin pregnancy different?

How many scans will you need in twin pregnancy?

If you’re expecting twins or more, your doctor will offer you more than just the three routine ultrasound scans that all pregnant women have. The routine scans that all pregnant women have bee discussed before in this article.
The extra scans that you’ll be offered if you’re having twins are:

  • A “chorionicity scan” at the same time as your dating scan (i.e 1st trimester scan) or you may be asked to come back for a scan after some days, if it isn’t clear enough. This is to check whether your twins share a placenta, or have one each. Knowing this helps your doctor to monitor your babies, especially if they may need extra care.
  • Growth scans in your third trimester will check that your babies are growing normally. Twins sharing a placenta will be scanned every two weeks from 16 weeks. Those with their own placenta will be scanned every four weeks from 20 weeks. You may have scans more often if there are any complications.

What happens during the scan in twin pregnancy?

  • Scanning twins is more of a challenge than scanning one baby, because one twin is often behind the other. This means that the person performing the scan (sonographer) may not be able to see your babies clearly.
    If one baby’s head is low in your pelvis, the sonographer may not be able to get any measurements. The scans will take more time than they would for a single pregnancy, usually at least twice as long.
  • Try not to lie too flat when you have your scan, as the extra weight of the babies pressing on your blood vessels can make you feel faint. Either sit up a bit or ask to be tilted slightly to one side to avoid this happening.
  • Your sonographer may ask you to change position several times during the scan. This is just so that she can get all the images and measurements she needs to check your babies’ progress.
  • There’s always some difference in the size of twins, whether identical or not, and this is normal. It’s only when one twin is more than 25 per cent bigger than the other, or growing much faster, that there may be a problem.
  • As well as checking the growth and fluid around your babies, your sonographer will check both twins’ umbilical arteries using a Doppler scanner. This will take place during your routine scans from 24 weeks onwards. It measures the blood flow from the placenta through the umbilical cord to your babies, and will show if they’re getting enough food and oxygen.
  • Scans can also help to show how your twins are lying, and identify which one is which.
  • This may seem like a lot of scans, but there’s no risk to you or your babies. Rest assured that ultrasound scans are a safe and effective way of checking your babies are growing well.

What if there are signs of a problem on my scan?

About 15 per cent of scans will need to be repeated for one reason or another. Most problems that need repeat scanning are not serious.  The most common reason is that the doctor has not seen everything she needs to see. This may be because your baby is not lying in a good position, or that you are a bit overweight. If your doctor finds or suspects a problem, you are most likely to be told straight away.  If any scan reveals a serious problem, you should be given plenty of support to guide you through all the options. Though serious problems are rare, some families are faced with the most difficult decision of all, whether to continue or not with the pregnancy.  Other problems may mean that your baby needs surgery or treatment after birth, or even surgery while he is still in your uterus (womb).

*Thanks a lot for visiting my blog. Please do ‘★ LIKE’ the post if you found it to be useful.Your comments, suggestions, criticism and all opinions are very much appreciated. Please do write your queries in the Reply/comments section and I will try to get back to you asap.

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