Many women today choose to delay childbearing until later in life for a variety of reasons. Many of us are completing advanced degrees, building a career or choosing to marry later in life. In fact, compared to 1970 (when 1 out of 100 moms was over the age of 35), today's number is 1 in 7. So, although mom's today are older when they begin childbearing, you are correct that as a woman's age increases, the risk of having a baby with Down syndrome, as well as many other chromosomal abnormalities increases as well.
In addition to chromosomal issues, there are more risks in general when having a baby over the age of 40. Before listing all of them, the important thing to remember is that most women having babies in their late 30's and early 40's, have perfectly normal pregnancies and babies! Here are a few of the risks that increase with increasing maternal age:
1. Gestational diabetes
4. Preterm delivery
5. Problems with the placenta (placenta previa, placental insuffieciency)
6. Stillbirth (only slightly higher: 4/1000 vs 7/1000)
Regarding autism, maternal age does not appear to be a factor, but advanced paternal age can and there does appear to be a slightly increased risk of autism in babies born to fathers over the age of 40. Keep in mind however, that the risk of autism is also increased with a family history and a few other factors. Unfortunately, we have no way to test fetuses for autism or other neurologic issues prior to birth.
I do want to mention, however, something new and really exciting called non-invasive prenatal screening (NIPS). Historically, we relied upon something called a first trimester screen (some blood work and an ultrasound) to screen for Down syndrome, Trisomy 18, Trisomy 13 and a few other chromosomal abnormalities. A woman's chance of having a false positive result (meaning the test says your baby has Down syndrome when in fact he or she really doesn't), greatly increases with age. This would then alarm everyone and a CVS (chorionic villus sampling) or amniocentesis would be recommended. These are both invasive tests with miscarriage rates of 1 in 200 and 1 in 500, respectively. Over the last few years, we have realized that we can pick up fetal DNA in mom's circulation. So with a simple blood test, we can now actually pick up fetal chromosomes in mom's blood and the best part is, this test is much more sensitive than our previously used first trimester screen. This is a great option for women over the age of 35 to get results on chromosomes as early as 10 weeks.
The bottom line is that there is never any guarantee in life, and if you and your wife want to have another child, chances are that things will be fine. Talk to your practitioner about your concerns so that your wife's past medical and obstetric histories are taken into the picture. And good luck!