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| The timeline above shows when an ultrasound scan is performed. The type of scan done will depend on the indication (see below for detailed schedule). |
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Ultrasound uses high frequency sound waves to show an image of the baby in the womb on a monitor screen. As this technique uses sound waves instead of radiation, ultrasound is safer than X-Rays. Since its introduction in medicine more than thirty years ago, ultrasound has become an increasingly important tool in antenatal care. It provides essential information that can guide the doctor plan for a pregnant woman and improve her pregnancy outcome.
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| How does ultrasound work? |
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A special transducer or probe produces high frequency sound waves which bounce off the developing fetus. Very high frequency sound waves between 3.5 to 7.0 megahertz (i.e. 3.5 to 7.0 million cycles per second, which is not audible to the human ear) are used in obstetrics. Echoes from the waves are then received by the same probe and analyzed by a computer to produce a moving (real-time) or still picture on a monitor. This technique is called ultrasonography.
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| How is ultrasound performed? |
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| Ultrasound scan is a painless procedure that requires no special preparation. However, the woman may be asked to have a full bladder earlier in pregnancy. The abdomen is first covered with a thin layer of colorless gel to provide good contact. The probe is then placed on the abdomen and moved around to look at the baby. In early pregnancy (< 10 weeks), a transvaginal probe is used to obtain a clearer image of the baby. A disposable probe cover (like a condom) is rolled over the probe and covered with gel. The probe is then inserted into the vagina and an image of the fetus is captured on the monitor. |
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| How many ultrasound scans should I have in pregnancy? |
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There is no definitive rule as to the number of scans a woman should have during her pregnancy. The schedule for ultrasound scan in pregnancy may vary from centre to centre. When there is an indication, such as in the case of vaginal bleeding, an ultrasound scan ought to be performed. Some doctors will perform an ultrasound when an abnormality is suspected on clinical grounds, while others advocate routine screening ultrasound scans.
Most doctors would recommend an early scan to confirm fetal viability and the gestational age of the baby. This can be done as early as 6 weeks when the fetus can be seen with a trans-vaginal probe. A second scan is recommended at about 11-14 weeks to detect any gross fetal abnormalities. The nuchal translucency can be measured at the time and the presence of the nasal bone can be verified.
Between 18-23 weeks gestation, a detailed fetal survey from "head-to-toe" is done, in particular to look at certain parts of the fetus, which may be difficult to view earlier, such as the face and the heart. This scan will also verify the gestational age, assess the liquor volume and localize the placenta.
A third scan may be performed in the third trimester (from 26 weeks onwards, usually between 30-42 weeks) to assess the fetal health by evaluating the fetal growth and size, liquor volume and Doppler blood flow. The doctor may need to localize of the placenta where placenta praevia (low lying placenta) is suspected.
The total number of scans in a pregnancy varies, depending on whether a previous scan has detected a particular abnormality that requires further assessment.
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| The Recommended Ultrasound Schedule |
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| Fetal Viability scan: [6-10 weeks gestation] |
| • Confirm a normal intra-uterine pregnancy |
| • Assess fetal age gestation |
| • Exclude abnormalities such as ectopic pregnancies or threatened miscarriage |
| • Detect the presence of fetal heart activity |
| • Determine the presence of multiple pregnancies |
| • Identify early fetal abnormalities |
| • Identify abnormalities of the placenta, uterus, and other pelvic structures |
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| Nuchal scan: [11-13+6 weeks gestation] |
| • Estimate fetal gestation |
| • Detect the presence of fetal heart activity |
| • Assess risk of Down's syndrome by measurement of the nuchal translucency and presence of nasal bone |
| • Detect fetal abnormalities |
| • Determine number of fetuses (multiple pregnancies) |
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| Detailed Fetal Anomaly scan: [18-23 weeks gestation] |
| • Assess fetal age, growth and position |
| • Identify fetal structural abnormalities |
| • Exclude multiple pregnancies |
| • Evaluate the placenta, amniotic fluid, and remaining structures of the pelvis |
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| Fetal health/wellbeing scan: [from 26 weeks gestation, usually at 30-32 weeks] |
• Assess fetal health - growth (estimating fetal weight), amniotic fluid volume ±
Doppler blood flow studies |
| • Rule out fetal abnormalities |
| • Assess fetal presentation & position |
| • Localization of placenta |
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| Is ultrasound safe in pregnancy? |
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Ultrasound has been used in obstetrics for more than thirty years. To date, no deleterious or harmful effects on the human fetus had been found. However, some investigators have found greater numbers of lighter babies and non-right-handedness but recent studies have not confirmed the earlier findings.
Similarly, there had been no significant differences in birth weight or length, childhood growth, intelligence, hearing or visual ability or neurological deficits. Hence, many authorities have concluded that the ultrasound scan is safe to use in pregnancy. However, all have agreed that the ultrasound scan should only be used when there is an indication and that the fetus should be exposed to as short duration a scan as possible.
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