Every pregnant woman looks forward to the birth of a healthy baby. Foetal malformation is a common complication. Prenatal diagnosis of foetal anomalies has gained rapid acceptance over the last three decades. Unfortunately, despite the rapid advancement in diagnostic capabilities and aggressive management, significant reduction in prenatal mortality associated with congenital malformations is yet to be achieved.

The main objective of prenatal screening is to identify treatable health problems in the mother that can affect the baby's health, characteristics of the foetus and congenital, genetic or chromosomal problems, or foetal abnormalities.

ROUTINE PRENATAL TESTS

Blood tests determine the blood type and Rh factor. If the patient's blood is Rh positive and the partner's is Rh negative, she may develop antibodies that prove dangerous to the foetus. This can be treated through a course of injections. Blood tests also measure the level of iron in the blood and check for hepatitis B, syphilis and HIV.

Urine tests check for kidney infections and signs of gestational diabetes and pregnancy-induced high blood pressure.

Cervical tests check for STDs, cervical cancer, and Group B streptococcus infection.

Around the 16th to 18th week of pregnancy, the maternal blood screen test is performed. Also known as a "triple-marker" test, it measures the levels of a protein produced by the foetus and two pregnancy-produced hormones in the mother's blood. This test can reveal the chances that a mother is carrying a foetus with neural tube defects or Down syndrome.

The first ultrasound test is also performed around this time. It helps to identify the position of the baby, as well as helping to detect Down syndrome, other chromosome abnormalities, structural defects such as spina bifida and anencephaly, and inherited metabolic disorders.

Around the 24th week of pregnancy, an additional screening test may be performed.

ADDITIONAL PRENATAL TESTS

These tests are performed in high-risk pregnancies or if the doctor suspects an abnormality in the foetus.

1. Amniocentesis is done under continuous sonographic guidance, after 14 weeks gestational age for determination of foetal karyotype (the chromosome characteristics of an individual) and all biological information.

The main disadvantages is that results of the prenatal diagnosis are not available until 17 to 20 weeks of gestational age. Foetal loss after amniocentesis is also seen to be one in every 100-200 procedures. Risk of infection, foetal injury and other minor complications are also seen.

2. Chorionic Villus Sampling (CVS)
This new alternative to amniocentesis removes some of the chorionic villi and tests them for chromosomal abnormalities, such as Down syndrome. Its advantage over amniocentesis is that it can be performed earlier (10 to 12 weeks).

The main disadvantage of this investigation is that it is less accurate (97.5%) compared to amniocentesis. False positive results (2%-3%) due to placental mosaicism and maternal cell contamination is also seen.

3. Maternal Blood Screening
This investigation is performed to test the mother's blood only for alpha-fetoprotein (AFP). This test calculates the woman's individual risk of birth defects based on the levels of the three (or more) substances plus her age, weight, race, and whether she has diabetes requiring insulin treatment. It's important to note that this screening test determines risk only. It is performed around 16 to 18 weeks of pregnancy.

4. Ultrasound
In addition to showing the foetus age, rate of growth, position, movement, breathing and heart rate, it shows the number of foetus and the amount of amniotic fluid in the uterus. The test is used most often to detect Down syndrome, other chromosome abnormalities and structural defects. Ultrasound can indicate the position of the placenta in late pregnancy. It can also be used to detect pregnancies outside the uterus.

5. Glucose screening
Glucose screening checks for gestational diabetes. Gestational diabetes occurs in one per cent to three per cent of pregnancies and can cause health problems for the baby. It is generally performed around 24 weeks of pregnancy.

6. Non-stress Test
This test uses external foetal monitoring to determine foetal movement in post-dated pregnancies. The non-stress test is performed one week after the due date.

7. Contraction Stress Test
This test stimulates the uterus and determines the effect of contraction on foetal heart rate. It is usually recommended when a non-stress test indicates a problem. This test is done after 40 weeks of pregnancy.

8. Percutaneous Umbilical Vein Sampling (PUVS)
This test obtains foetal blood by guiding a needle into the umbilical vein. It is primarily used in addition to ultrasound and amniocentesis in emergency situations where the doctor quickly needs to check the baby's chromosomes for defects or disorders. If the foetus is suspected to be anaemic, this test is the only way to confirm this. It also allows transfusion while the needle is in place. This test is usually performed around 18 to 36 weeks of pregnancy.

CONCLUSION

The goal of prenatal diagnosis is to prevent the severe consequence of genetic mutations. In the absence of prenatal diagnosis of malformations, many opportunities for optimal management can be lost. Whether the news is good or bad, prenatal screening is particularly valuable.