Just adding my 2 cents here. First, look at it this way, it's a 399:1 that the baby is healthy.
What are the ages of the parents? Is the basis for that statement "1 in 400 with Down Syndrome", based on an Ultrasound or did they just do the screening bloodwork, MSAFP (Maternal Serum Alpha Fetal Protein)? The only way to confirm if the baby has Down Syndrome is to do a chromosome study. In uetero it is done with an amniocentisis. They draw fluid out of the womb and look at the baby's cells. Match up all 23 pairs. Usually it is #21 that is fragile and splits off and connects somewhere else that causes Down Sydrome. Even after the baby is born, they have to do a chromosome study to confirm if the baby is Down Syndrome.
I asked about the ages of the parents because unless the mother is over 35 will an amniocentesis be done automatically. It has been proven that the older we get the more chance of the chormosomes not lining up right. So the usual group of women who have Down Syndrome babies are over 35 and young teens.
Please be reassured that what you have described to be so far sounds like one of the screening test came back abnormal. Remember it is a screening test and you need to move to the next test (amniocentisis) to prove whether the baby has Down's Syndrome or not. I recommend doing an amniocentesis because if it is negative (there are lots of false postive screening tests out there) then you don't have the added worry for the rest of the pregnancy. If it is positive, then you have a few months to adjust to the situation, as that is the only option available to JWs.
Let us know how things turn out.
edited to add a PS. These days after a postive screening test comes back, most patients are referred to a Maternal-Fetal Medical Specialist. (an OB with extra training/credentialing in abnormal pregnancies, both babies and moms), they used to be called Perinatalogist. Anyway, a referal to a MFM gives you counseling and a better Ultrasound evaluation. If a MFM thinks you still need a amniocentisis, then do it. Those guys are much better than the average ultrasound scanner and OB. And many times they can determine that the baby doesn't have the charateristics of Down's without the risk of an amniocentisis.