11w3d: Exhausted

In exactly four days time I will be making my way to the hospital for my NT scan. I am scared of all the things that can go wrong, and excited about what a milestone it will be if everything looks good.

So in order to get me into the spirit of the occasion, I have been doing what any good scientist would do, and reading through the literature surrounding the tests that will be conducted at this appointment, mainly, the NT scan and the blood tests.

The nuchal translucency (NT) test measures the thickness of the sack of fluid behind the neck of the fetus. In general, a fetus with a trisomy (three copies of a chromosome, rather than two) has a thicker NT than a chromosomally normal fetus.

The first figure below illustrates this observation. Each black dot represents two measurements (the crown-rump length and the NT thickness) of a single fetus that was subsequently found to have trisomy 21 (Down's Syndrome). The dark grey rainbow is the distribution of 90% of chromosomally normal fetuses. As you can see, in normal fetuses the NT thickness gradually gets thicker as the fetus gets larger. However, in trisomy 21 fetuses, the thickness remains relatively constant during this gestational period, with a mean thickness of 3.4 mm.

(from Kagan, et al., Ultrasound Obstet Gynecol 2008; 31: 618-624)

The second test is a blood test of the mother to measure our old friend beta-human chorionic gonadotropin (beta-hCG) as well as pregnancy-associated plasma protein-A (PAPP-A). The values on the axis are multiples of the median (MoM), that is, by how many-fold does a result differ from the median value from all normal women. By definition, the median beta-hCG for unaffected pregnancies is 1.0, and the median PAPP-A for unaffected pregnancies is also 1.0.

This second figure below shows how the blood tests from women with trisomy 21 fetuses varies from women with chromasomally normal fetuses. Each dot represents the values from a single patient with a trisomy 21 fetus. The unfilled oval represents the lab values of 90% of women with chromasomally normal fetuses. The median beta-hCG values were 2-fold higher and the the PAPP-A values were 2-fold lower in women with trisomy 21 fetuses.

(from Kagan, et. al., 2008, Ultrasound Obstet Gynecol, 31: 618-624)

So here's hoping that all my dots fall in the normal range, and I can pass this test and start to look forward to moving into the second trimester. It is still feeling unreal, so I hope that seeing a wiggling little fetus on the ultrasound next Tuesday will put some of my anxiety to rest.

References:

Kagan, K.O., Wright, D., Baker, A., Sahota., D., & Nicolaides, K.H. 2008. Screening for trisomy 21 by maternal age, fetal nuchal translucency thickness, free beta-human chorionic gonadotropin and pregnancy-associated plasma protein-A. Ultrasound Obstet Gynecol., 31: 618-624