Ultrasound – The Dating Scan


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An ultrasound scan uses high frequency sound wave to scan the baby in the womb, creating a picture on a monitor screen. Since it uses sound wave instead of radiation, it is safe in pregnancy.

1. What are the types of ultrasound scan?

(1)   Transvaginal scan
A specially designed probe transducer is placed inside the vagina to show the image of the baby. This is usually done in early pregnancy
(2)   Standard Ultrasound
This is the traditional ultrasound scan which uses a probe transducer over the tummy to generate 2D images of the baby
(3)   3D Ultrasound Scan
This ultrasound scan uses specially designed probes and software to create 3D images of the developing baby
(4)   4D Ultrasound Scan
This ultrasound scan uses specially designed scanners to see the movement of the baby
2. How is an ultrasound performed?
The obstetrician/ sonographer will place some gel onto your tummy. This gel works as a conductor for the sound waves. Then a transducer will be placed onto your tummy and produce high frequency sound wave which cannot be heard by human ears. The sound wave goes through your womb and being bounced back by the tissue and bones of the baby. The machine will then generate back and white images of the baby.
3. When are ultrasound scan performed?
Ultrasound scan may be performed at any stages of pregnancy. However, there is no recommended number or no fixed time for ultrasound examination of your baby. Ultrasound scan is used when it is medically indicated. Additional scans may be indicated if your obstetrician suspects a problem or complication during your pregnancy. The number of scans may also vary from one healthcare provider to another.
4. Will ultrasound scan harm my baby?
Ultrasound scan uses high frequency sound waves and is non-invasive. There has not been any evidence to suggest that ultrasound scan will harm your baby. However, it is generally recommended that ultrasound scan should be performed when medically indicated. The long term effect of repeated ultrasound exposure on the baby is not fully known.
5. What is a dating scan?
An early scan is sometimes called a dating scan. The purpose of performing this scan is to confirm the viability of the baby by detecting the baby’s heartbeat. It is important to determine the gestational age of the baby for the ongoing care for your pregnancy. Please refer to our article on When is my due date
An early scan is also important to determine the number of babies and to differentiate the type of twins (identical or non-identical). It is also important to determine whether the pregnancy is inside your womb to exclude an ectopic pregnancy. A gross abnormality such as anencephaly can be detected in on early scan.
6. What if an ultrasound scan does not detect pregnancy or baby’s heartbeat at 6-7 weeks of gestation, does it mean that I am not pregnant?
Do not panic….it is not unusual to have such findings during a scan at this gestation. There are a few conditions whereby the baby’s heartbeat may not be easily detected at this stage of pregnancy. If your womb is tipped backward (retroverted uterus), or if your abdomen is too thick, or if your last menstrual period date was not accurate are some of the reasons why the baby’s heartbeat cannot be visualized. The detection of the baby’s heartbeat may be improved by performing a transvaginal scan.
However, if your obstetrician could not visualize the presence of your baby when the pregnancy sac is more than 20mm, or if there is absence of baby heartbeat when the measurement of baby (Crown-rump length) is more than 6mm, there is a possibility that you are having a miscarriage. A repeat transvaginal scan 7 days later should confirm the diagnosis.
When your obstetrician does not detect any presence of pregnancy sac after 6-7 weeks of pregnancy, he/she may be concerned about the possibility of ectopic pregnancy. Serial blood test (Beta-hCG) coupled with repeated scan may be necessary to differentiate between an early pregnancy and an ectopic pregnancy.

 Picture 1: Scan showing a gestational sac


Picture 2: Scan showing a fetus with the measured CRL of 12 weeks gestational size

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1. Chia P, Raman S. Is My Pregnancy Normal? 2006. Discern Publishing House Sdn Bhd. Kuala Lumpur, Malaysia.
2. Smith NC, Smith PM. Obstetrics Ultrasound Made Easy. First edition. Churchill Livingstone. Edingburgh.

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