An ultrasound scan is an assessment tool commonly used by doctors during your pregnancy. It is a safe and useful way of obtaining images of your baby and his/her surrounding environment to provide useful information on his/her health (Figure 14.1). Show
Dating Scan (Week 6–12)Your first ultrasound scan should be performed in your first trimester of pregnancy. In a usual situation, the scan will show the early pregnancy sac in the womb with the fetus within the sac. This can be seen as early as 5 to 6 weeks of pregnancy. Visualising the pregnancy in the womb is reassuring, because it confirms that it is in the right location. Although very uncommon, pregnancy can sometimes be implanted in the wrong place like in the fallopian tubes (known as ectopic pregnancy). This happens to 1– 2% of all pregnancies. The baby’s heartbeat is normally visible by 6 weeks of pregnancy. Sometimes, your doctor may use a vaginal probe for the scan when your pregnancy is about 5 to 6 weeks. Be assured that vaginal scanning is safe and will not harm your pregnancy in any way. The current gestation (age) of your pregnancy can be calculated by taking measurements of your baby during the scan (Table 14.1). Your expected date of delivery (EDD) can then be estimated accurately. This is especially important for your doctor to manage your pregnancy well. It also helps you to plan ahead like informing your employer about maternity leave and making all the necessary arrangements for baby care before the arrival of your baby. Indeed, the accuracy of the first trimester ultrasound scan is within a week. The first trimester ultrasound also allows you to know if you are expecting twins or even triplets. Nuchal Translucency Scan (Week 11–14)In recent years, ultrasound scans in the first trimester can be used to assess the risk of your baby being affected by Down Syndrome (read the article on Down Syndrome Tests). This is done by a measurement of the skin fold thickness (nuchal translucency) at the back of the baby's neck. If the neck fold is unusually thick, it may indicate that your baby may have Down syndrome. Other causes may include heart abnormalities or even other rarer genetic syndromes. This test is quite accurate to detect Down Syndrome as the detection rate is 80% in the hands of experienced doctors. So do consider this test, which is perfectly safe for your baby. If your baby is in an optimal position, ultrasound examination after 11 weeks may be able to visualize the baby’s nose bone (known as nasal bone). The absence of the nose bone is a worrying sign, which increases the risk of Down Syndrome. Screening Scan (Week 18–22)Being an expectant mother, you will find it reassuring to know that your baby will be born healthy with no congenital deformities. An ultrasound scan can be used for screening to look for abnormalities in the physical structure of the baby, for example in the heart, lungs, spine, brain, long bones in legs or arms, organs in the abdomen like liver, stomach, intestines, kidneys, bladder and even check for cleft lip and palate. Finding of certain abnormalities during an ultrasound scan may also alert doctors to the possibility of Down Syndrome or other genetic abnormalities in the baby. Further testing can then be done to exclude these. Ultrasound screening for physical abnormalities is usually done during around the fifth month or 20th week of your pregnancy. It is performed by a sonographer or obstetrician, using a fairly advanced ultrasound machine that provides excellent resolution for imaging. The accuracy of scans is about 70% in detecting all abnormalities. Note:Although a structural screening scan is undertaken, detection of structural problems will never be 100% accurate. Detection rates may vary and be reduced by factors such as maternal obesity, abdominal scars, fetal position and reduced amniotic fluid.
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