Monday, 2 January 2012

16 to 20 weeks of pregnancy - visits to your obstetrician and prentatal tests

Q I am now 16 weeks pregnant. What should I expect at my obstetricians?

He may want to do an ultrasound scan. You will see that the baby is now much larger, starting to move its arms and legs. Fingers and toes will be visible and in some instances may be seen to be sucking its fingers and toes!. the sex of the baby can be determined with accuracy at this stage. A triple blood test may be done if the oscar test was missed earlier.

Q What is a triple test and why do I need to have this?

This is done to determine if you are high risk of developing a Down's Syndrome baby ( baby with an extra chromosome resulting in mental retardation). It will also tell us the risk of defects of the spine. It is usually performed if the Oscar test has been missed. It measures 3 hormones present in the mother's blood and correlating this with her age and stage of pregnancy, a risk factor can be determined. This screen is not however as accurate as the Oscar test and like most tests will have false positives and negatives. A positive test would indicate the need for an amniocentesis ( tapping the fluid in the baby's water bag). A positive test for spinal defects can be correlated with a detailed foetal abnormality scan at 20 weeks.

Q I have been told that I need an amniocentesis. How is this procedure carried out and is it risky?

Approximately 15 mls of the baby's water bag is aspirated under ultrasound scan guidance. The ultrasound scan will determine the best site to insert the needle. The needle is inserted with or without local anaesthetic and the needle is watched on the ultrasound scan so that the chance of hurting the baby and its cord are minimised. It is a relatively  painless procedure. The fluid contains the baby's skin cells shed from the baby. The cells are grown in the laboratory and the cells examined. It takes about 2 weeks for the karyotyping, but for an extra cost a PCR may be done and results obtained in 3 days. However it is still advisable to have a full karyotype.

The procedure carries a risk of less than 1%. The main risks are of miscarriage following leakage of the baby's water bag, injury to the baby. The majority will have no complications.

Q My obstetrician advised me to undergo amniocentesis at 16 weeks rather than a chorionic villus biopsy at 9 weeks. Why is this so?

Chorionic villus biopsy (CVB) is a procedure in which a biopsy is taken from the foetal placenta at about 9 weeks gestation.The specimen undergoes a DNA analysis to determine certain genetic disorders. The procedure carries a higher risk of miscarriage than amniocentesis. There are also inconsistencies in the results and maternal contamination of villus samples have led to to errors in gene analysis. More importantly, studies in the UK have shown abnormalities in foetuses born to women who have had this procedure performed.

Both my husband and I are thalassaemia carriers. What tests should I do to determine if my baby is normal?

If both you and your husband are carriers, you have a 1 in 4 chance of having a baby with thalassaemia major, a condition which is almost always fatal in childhood. This can be diagnosed by foetal cord blood sampling at 16 weeks. The foetal cord is identified under ultrasound scan and a needle inserted to aspirate a small amount of blood which undergoes DNA analysis.

Q  I have been told that I should have a detailed scan of my baby at 20 weeks. Why is this necessary?

The aim of every obstetrician is to have as low a perinatal mortality and morbidity rate as possible ie low death rate and low foetal complication. About 20% of perinatal mortality is due to congenital abnormalities. One of the important tools to diagnose congenital abnormalities is the ultrasound scan. The majority of abnormalities are best seen at about 20 weeks. Some abnormalities detected may be so severe as to warrant a decision to have an abortion. Other less severe abnormalities detected can prepare the patient, paediatrician for further treatment at the time of birth.

How accurate is an ultrasound scan in excluding foetal abnormalities? Can I assume that my baby is normal since my ultrasound scan did not show up any abnormalities?

There are 3 levels of ultrasound scanning. The first is a general scan. This scan determines the number of foetus, position of the foetus and placenta, amount of fluid, measurement of the gestational age and growth of the foetus, a general look at the structure of the foetus. The second level would look at a more detailed structure and form of the foetus while the third level includes blood flow studies to exclude cardiac abnormalities. Most obstetricians with sufficient experience would do level 2 scans. The level of accuracy will depend on the experience of the obstetrician performing. An experienced obstetrician should obtain an accuracy of about 80%.

What structural abnormalities may not be picked up by ultrasound scanning?

Small abnormalities are difficult to pick up. Very pften the fingers of the hand are clenched making it difficult to view all of them. Small holes in the heart, the way the major vessels enter and leave the heart are difficult to view, positional deformities of the legs and feet. The accuracy will depend greatly on the position of the baby at the time of scanning, the amount of abdominal fat in the mother, the position of the placenta and amount of fluid.

Q Is a 3 D and 4 D scan of value?

3D and 4D (3D in motion) scans have become almost standard in todays context. With advances in ultrasound machine technology, not only  2D but 3D clarity have improved tremendously. A 3D and 4D scan will complement the 2D scan. It shows the baby in a form more readily identifiable to most patients. If abnormalities are detected at 2D, the 3D and 4D will help confirm or refute the diagnosis. Another great advantage of the 4D is mother/ father and baby bonding which can start now before the baby is born!

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