Antenatal Scans

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When it comes to antenatal scans, most know they can expect ultrasound scans. But what other scans and tests can you expect during your 40-week journey? MH finds out.

WORDS ANGEL DREWGUS

Prenatal testing can be a valuable tool for diagnosing and treating conditions that threaten the health or the life of the mother, the baby, or both. They can detect the possibility of trouble very early in a pregnancy so that couples can investigate further if they choose to do so. Furthermore, on the medical front, it’s getting easier to pick up genetic conditions earlier.

Want to find out more about the scans you’ll be going through and some which are optional? Read on.

Ultrasound

An ultrasound scan is an imaging test which uses high-frequency sound waves produced and received through an ultrasound probe to capture live images in the body.         

In obstetrics, you may have two types of ultrasound scans: transabdominal or transvaginal. A transabdominal scan involves an ultrasound probe that is placed on your abdomen. You are usually requested to have a full bladder for these scans. A transvaginal scan uses a probe that is inserted into your vagina. This is performed with an empty bladder and is most commonly performed in very early pregnancy. A transvaginal scan has higher resolution, and sometimes when it is too early to see any details with an abdominal scan, your doctor may recommend a transvaginal scan. Many women are concerned that having a transvaginal scan may hurt them or their baby. Dr Lim Min Yu, consultant, NUH Women’s Centre says that it is actually very safe to have such a scan, and there is no evidence that having a transvaginal scan leads to any harm.

Doppler Ultrasound

Doppler ultrasound monitors blood flow through blood vessels, explains Dr Lim. Doppler ultrasound uses changes in velocities of blood flow in the body to assess waveforms of the major arteries and veins. For pregnant women, the maternal uterine arteries, major foetal arteries (e.g. umbilical artery, middle cerebral artery) and major foetal veins (e.g. umbilical vein, ductus venosus) are often measured. The measurements of the indices for these waveforms help in confirming the well-being of the foetus and affects the decision on when to deliver the baby.

Anatomy Ultrasound

A foetal Anatomy scan is usually performed at 18 to 21 weeks gestation, explains Dr Lim. The scan checks for major physical abnormalities in your baby, although it can't pick up every problem. The scan looks in detail at the baby's bones, heart, brain, spinal cord, face, kidneys and abdomen. Most of the time, the scan will show that the baby appears to be developing normally. However, sometimes the sonographer will find or suspect a problem. According to Dr Lim, some problems can be seen more clearly than others. Rarely, some very serious problems are found — for example, the baby's brain, kidneys, internal organs or bones may not have developed properly. In some very serious, rare cases where no treatment is possible, the baby will die soon after it is born or may die during pregnancy.

Genetic Screening

Genetic screening refers to the tests done to assess the risk of having a genetic condition. A common genetic condition is thalassaemia. Genetic screening methods may include blood tests for the mother. To assess whether the foetus has a genetic condition, invasive testing such as amniocentesis, chorionic villus sampling, or foetal blood sampling may need to be done as well, explains Dr Watt Wing Fong, specialist in obstetrics and gynaecology & consultant, Raffles Women’s Centre.

Amniocentesis

Amniocentesis is an invasive test whereby the doctor inserts a needle into the water bag under ultrasound guidance to obtain some amniotic fluid. The foetus will shed cells into the amniotic fluid and by collecting the amniotic fluid, foetal cells can be isolated and cultured so that the chromosomes can be analysed. It is usually done from the fourth month of pregnancy. It is done if there is a risk of a chromosomal abnormality. It can also be used to assess for foetal infection. Dr Watt informs us that since this is an invasive test, there is a very small risk of miscarriage.

Non-Invasive Prenatal Testing for Down Syndrome

This is a blood test on the mother that can be performed as early as 10 weeks (so technically a first-trimester test). It uses a sorting technology to sort out the cell-free foetal DNA from the maternal DNA that is circulating in the mother’s blood, and to analyse for an excessive amount of cell-free foetal DNA that comes from chromosome 21 (which would then indicate a very high risk for Down syndrome). This is a very effective screening test for Down syndrome. It is very effective because it detects in excess of 99 per cent of Down syndrome at a false positive rate of <0.3 per cent. It is, however, still a screening test, emphasises Dr Tan. Hence, an amniocentesis or a chorionic villus sampling may be performed to confirmed if the foetus has Down syndrome.

Chorionic Villus Sampling (CVS)

Chorionic villus sampling (CVS) is a prenatal test that involves taking a tiny placental tissue sample from outside the sac where the foetus develops. The tissue is tested to diagnose or rule out certain birth defects. Both the baby and placenta (afterbirth) originate from the same cell, so the chromosomes present in the cells of the placenta are the same as those of the baby, says Dr Lim.

An ultrasound is performed before CVS to confirm the gestational age of the foetus. The doctor guides a needle from the patient's abdomen into the placenta under ultrasound guidance. It is usually carried out between 11 and 13 weeks of pregnancy.

Glucose Tolerance Test

An oral glucose tolerance test (OGTT) is used to determine whether a person has diabetes mellitus. In pregnancy, some women may develop diabetes mellitus which they were not known to have before they were pregnant, explains Dr Lim. This is known as gestational diabetes mellitus (GDM). Even mild GDM can have adverse consequences on the health of both mother and child during pregnancy and during the early postpartum period.           

Pregnant women doing the test should not have anything to eat or drink the night before. A fasting blood glucose level is checked in the morning, then she will drink a liquid that contains a known amount of glucose, usually 75 grams. Her blood glucose will be checked an hour later and again after two hours. If the result is raised, then a diagnosis of GDM is made. These women are then usually co-managed with a diabetes specialist.

Group B Strep Culture

Group B streptococcus culture is a swab test done to determine if the woman has Group B streptococcus. Group B streptococcus is a common bacteria that may be found in the intestines in about 25 per cent of women. It usually does not cause problems for the adult. However, if the bacteria colonises the vagina, then during labour and the delivery, the baby may be exposed to the bacteria. Some babies may get an infection from the bacteria and develop a serious lung or brain infection. The risk of foetal infection can be reduced by giving the mother antibiotics when she is in labour if she is tested positive, explains Dr Watt.        

Childbirth is as old as human existence, but perhaps one of the most important changes has been in the area of prenatal testing. Today, we have the opportunity to know vital facts about a baby before he or she is born. Perhaps, now, more than ever, women are using prenatal testing for the purpose of preparation, both medically and emotionally rather than as a route towards termination.

Thanks for sharing!