your story sounds similar to mine, we struggled with having #2 and were successful after many cycles, IVF, etc, finally lucky on an IUI. We were trying sort of half heartedly trying again and I found myself pregnant naturally (first natural pregnancy after 4 years of trying!). I just turned 42 and I'm almost 18 weeks. Here is the run down on testing as I understand it:
1st trimester screen: 11-13 weeks, u/s (measuring nuchal translucency and looking for the presence of the nasal bone) and bloodwork that give you odds for the most common chromosomal abnormalities (including down's)
a new test, called MaterniT21, tests your bloodwork for small amounts of the baby's DNA and can tell you about whether the baby has the 3 most common chromosomal disorders (including down's). i think can be done as early as 9-10wks and has an accuracy of around 99%, but others may know more about this as they are using it it right now (insurance may not cover as it is a new test)
2nd trimester screen: also called AFP--18-21 weeks(?) bloodwork gives you another odds ratio for the chromosomal disorders, also can give you info about spina bifida and soem other things but my OB says these things would be seen on the level II u/s. My understanding is that this test is less reliable than the 1st tri screen (and thus I am not doing it) but others may have different info
level II u/s: 18-20 weeks, give good info about general health and development of the baby, and detects soft markers for down's.