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1st Trimester

 

Early OB First Trimester Ultrasound

With the introduction of endovaginal transducers, the resolution of first trimester imaging has greatly improved.  In the first trimester, ultrasound exams are done to demonstrate viability of the pregnancy.  A heartbeat can be detected at approximately 6 weeks menstrual age.  This is also the most accurate time to date the pregnancy.

A new genetic screen is now available called an Ultrascreen that is employed during weeks 11-14 of pregnancy.  This test utilizes ultrasound in conjunction with a maternal blood sample and provides a risk assessment of the likelihood of the baby having a chromosomal abnormality.  It has a detection rate of 91-95% in the hands of an experienced sonographer.

Fun Facts:  6 weeks-baby is equivalent to a lentil, 7 weeks-a blueberry, 8 weeks-kidney bean, 9 weeks-grape 

First Trimester Serum Biochemistry

A blood test taken from the mother can be used to measure the levels of two hormones (free ß-hCG and PAPP-A).  In pregnancies affected by Downs syndrome there is a tendency for the levels of free ß-hCG to be increased and PAPP-A to be decreased. 

The combined first trimester screening test

  • Screening by maternal age alone picks up about 30% of the babies with Downs syndrome.
  • Screening by nuchal translucency picks up about 80% of the babies with Downs syndrome.
  • Screening by free ß-hCG and PAPP-A picks up about 60% of the babies with Downs syndrome.

The combination of these three methods is the most effective, picking up about 90% of the babies with Downs syndrome.
At the end of the combined screening test you will be given a risk that the baby has Downs syndrome; it will not tell you definitively whether or not your baby has Downs syndrome, but can be used to help you to make an informed choice about whether to have an invasive test.  The wish to proceed to one is a purely personal decision, but as a guide we normally offer an invasive test if the risk of Downs syndrome is 1 in 300 or worse.  This of course means, that even with a high-risk result most women will still have a completely normal baby.

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