Thursday, 7 July 2011

12 to 16 weeks. Visits to your obstetrician

I am 12 weeks pregnant. What can I expect at my visit to my obstetrician?

He may want to do an ultrasound scan. Your baby is now fully formed and is about 10 cms long. This is an important scan as he will probably want to determine if there is visible structural abnormalities, presence or abscence of nasal bone, nuchal fold thickness, and whether there is more than one foetus. Both height and weight, blood pressure and urine tested for sugar, blood and protein.

What is nuchal fold thickness (NT) and why is it done?

Between 11 and 14 weeks the foetus will show fluid behind the nape of the neck. This is from the developing lymphatic system. The measurement is taken with the foetus lying on its back. It has been found that a thick fold may indicate a chromosomal abnormality and the greater the NT the greater the likelihood of an abnormality such as Down's Syndrome. Generally a NT of >2.5 cms would be taken as being abnormal.

Why is it necessary to determine the presence of the nasal bone?

The nasal bone is absent in Down's Syndrome and many other chromosome abnormalities. A nasal bone measurement can be taken between 14 and 20 weeks. A measurement greater than 4.6mm would be considered as normal, while a measurement of less than 3.5mm would likely be a Down's. The ratio of the diameter of the head (BPD) to the nasal bone should by less than 8 and if > 10 is suggestive of Down's Syndrome.

Q  What is the Oscar Test?

This is a prenatal screen for Down's syndrome and other chromosome abnormalities. It measures the risk of having a Down's baby. It it is a combination of the nuchal fold thickness, and 2 blood parameters, free beta HCG and PAPP-A. The accuracy of the test is over 90%, however with any screen there are both false positives and negatives.The test is done from 11 to 14 weeks. If the test is positive, your obstetrician will advise you to have an amniocentesis (tapping of the baby's water bag to obstain foetal cells for culture and examination)

What is the triple test and how does it differ from the oscar test?

The triple test is done at 15 weeks and above. It is also a prenatal screen for Down's syndrome and measures the 3 factors in the blood, alpha feto protein, free beta HCG and unconjugated estriol. The accuracy is over 80% and like the oscar test has both false positives and negatives. As with the oscar test, if positive, your obstetrician will advise an amniocentesis.

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