The ultrasound scan is a safe, non-invasive and accurate obstetric tool that plays an important part in the care of pregnant women.
The new mom is excited. She is three months pregnant and has been scheduled for an ultrasound examination. She and her husband will actually be able to see the baby! At the same time she is a tiny bit nervous. Will the baby be normal? Will the doctor be able to tell her that the baby is growing normally? Does she have twins?
What is an obstetric ultrasound scan?
In the 70s, ultrasound scanning was introduced, to image the foetus growing inside the uterus. Ultrasound scans literally threw new light on the growing foetus, giving us a glimpse into the hitherto hidden world of the foetus. We can now measure its growth, and watch and make sure that most of its organs are functioning well. We can see it moving, blinking its eyes, making breathing movements, swallowing, waving its fingers, sucking its thumb and even urinating inside!
Ultrasound scan is a safe, non – invasive, accurate and cost-effective investigation in a pregnancy. It has increasingly become an indispensable obstetric tool and plays an important part in the care of every pregnant woman.
The role of ultrasound scanning in pregnancy
Confirmation of pregnancy: Ultrasound can confirm a pregnancy by 5 weeks from the last period. By 6 weeks of pregnancy a heartbeat can be confirmed.
Diagnosing a miscarriage: When there is vaginal bleeding in early pregnancy, ultrasound examination is very important in confirming the viability of the foetus. If the heartbeat is identified, the chance that the baby will go to term is almost 95 per cent. In case the pregnancy is not growing well, the gestational sac may be irregular and the foetus may not be identified. This is called a blighted ovum. If the foetus is seen but there is no heart beat, a diagnosis of missed abortion will be made.
Occasionally, the pregnancy may not be growing inside the uterus but may actually have formed inside the tube. This is called an ectopic pregnancy.
Dating of pregnancy: Foetal body measurements give us the gestational age of the foetus. A scan done between 6 – 12 weeks of gestation will give us the most accurate age of the foetus. In women who have irregular periods or who do not remember the first day of the last period, such measurements must be made as early as possible in pregnancy to arrive at a correct dating of the pregnancy.
In the second half of pregnancy measuring body parameters will allow assessment of the size and growth of the foetus. The measurements help in determining if the baby is growing well or not.
Confirming that the baby is normal: Many physical abnormalities in the foetus can be reliably diagnosed by an ultrasound scan. A few abnormalities can be seen even in the early weeks of pregnancy but the best time to look for most defects is between 20 – 24 weeks of pregnancy. It is important to remember that even with the most advanced ultrasound technology; 15 – 20 per cent of foetal defects may be missed. A normal scan report is not a guarantee that the baby will be completely normal. Some abnormalities are very difficult to find or to be absolutely certain about.
Diagnosing twins: Ultrasound scanning is invaluable in the diagnosis of twins. Ultrasound scanning is also used to monitor the growth of the twins.
Screening and diagnostic procedures: Ultrasound scanning is a very important part of screening for chromosomal abnormalities like Down syndrome. Ultrasound can also assist in other diagnostic procedures in prenatal diagnosis such as amniocentesis, chorionic villus sampling and foetal blood sampling.
How often is a scan done?
An early scan is done at 7 – 8 weeks to confirm pregnancy, exclude ectopic or molar pregnancies, confirm the presence of a heart beat and measure the crown rump length for dating. A scan between 11 – 14 weeks of it’s useful to check nuchal translucency and can be combined with blood tests to rule out Down syndrome.
A scan at 20 – 22 weeks is important to rule out foetal abnormalities. Growth can be assessed and placental position determined. Further scans may be needed if abnormalities are suspected. Your obstetrician may ask for a scan in the third trimester to assess foetal growth and evaluate foetal size and weight. It might also be done to follow up on possible abnormalities seen in an earlier scan.
Special ultrasound scans.
Doppler ultrasound. This imaging technique is used to measure the flow of blood in the blood vessels of the placenta and the foetal blood vessels.
3-d Ultrasound: This technology uses especially designed probes and software to generate three-dimensional images of the developing foetus. It helps in clearly defining certain foetal abnormalities for example cleft lip.
Fetal echocardiography: this technology is extremely useful to assess the baby’s heart structure and function. This is used to help assess suspected congenital heart defects.
(The author is a Chennai – based obstetrician and gynecologist with a special interest in women’s health issues).