There is a blood test that is done between 16 and 18 weeks that gives you a risk factor for Down Syndrome (trisomy 21), Trisomy 18 and also neural tube defects (like spina bifida).

It will give you a risk figure (like 1 in 2000 or 1 in 285) for each of the conditions. There are not false positives for this test because it doesn't say that the baby definately has the condition, just calculates the probability that the baby will based on the enzymes in your blood.

The benefit of second trimester blood tests over first trimester blood tests is that it can also give you a risk figure for neural tube problems.

Based on your age, the risk is low, but if are interested the second trimester testing is available to you.

If the risk factor came back high, you would then have to consider if you wanted to go ahead with amniocentesis to check for one of the trisomies. neural tube problems can also be detected through ultrasound later in the pregnancy.