Understanding Trisomy 18: A Pregnancy Concern

Question
Thank you for reading my submission.  I am 18 weeks pregnant, 25 years old, and having my second child.  My due date is May 12th. I had the alpha protein screening done a couple of weeks ago.  My doctor called me and informed me that it came back abnormal, and that I had a 1 in 19 chance of having a baby with trisomy 18.  He then suggested I have an amnio, so my husband and I went to our appointment yesterday.  They did an ultrasound, and then we met with the doctor for a consultation.  She told us that the ultrasound looked beautiful, that she could see everything very clearly, and the heart and everything looked great.  She offered us the amnio, but said (when my husband asked for her opinion) that she felt the baby was healthy and fine.  She told us that there is a small percentage of babies that have normal ultrasounds, but are still born with trisomy 18.  We decided to not have the amnio, due to the small risk of miscarriage.  My question is this...  If trisomy 18 is all or nothing (meaning you either have it or you don't, there are not degrees of seriousness) why would no problems show up on the ultrasound?  Wouldn't there be at least one of the common characteristics apparent?  Do these problems (such as heart problems, physical deformities) sometimes develop later in pregnancy?  The doctor told me that I was thin enough that she got nice clear ultrasound pictures.  Are the ultrasound pictures not always clear?  I felt so happy and relieved after I talked to the doctor, but now that I'm home I feel the anxiety coming back.  Please help me to understand.
Thank you,
Mary

Answer
Ultrasound examinations can not always identify abnormalities of the heart or structural anatomy of the fetus. Sometimes abnormalities show up on sonogram and sometimes they do not. If you had an alphafetoprotein screen that gave a 1:19 risk of Trisomy 18, I would also recommend amniocentesis (even if a sonogram was completely normal). Amniocentesis is 100% accurate in identifying chromosomal abnormalities and the risk of miscarriage following an amniocentesis is 1:400. When the risk of the abnormality (1:19) is greater than the risk of miscarriage, I recommend amnio to my patients. Trisomy 18 is quite rare, but I would still have the amnio done. Good Luck.