If we look at the risks in the whole population, if you are twenty years old and having a baby, then your risk is about 1/1600 of conceiving a baby with Trisomy 21 or Down's Syndrome as it is known. And at age 20 your chance of having a pregnancy with any chromosomal problem is about 1/500. Many of these cases will actually miscarry, so it is a different statistic to ask what is your chance of having a live birth with one of these disorders. By age 35 the risk of Down's is about 1/350, and by age 45 the risk is 1/30, and by age 45 the risk of having a pregnancy with any chromosome disorder is about 1/20. Your gyno can help you with the statistics for your specific age.
There are many ways at each stage of pregnancy to determine sex and the health of the baby, and it is so complex that two organizations have issued a joint position on all the complicated testing during the pregnancy. In truth some testing cannot occur until after delivery, but currently we have many ways of making very accurate diagnoses before delivery. Getting a pelvic ultrasound is a genetic screening test. It is not very sensitive however and it only picks up about 60% of the abnormalities in pregnancy, even if done after all the organs are both formed and large enough to visualize completely. It was discovered 30 years ago that certain birth defects are associated with abnormal blood levels of certain proteins and certain hormones. For many years now we have used this test, combined with more intense testing of older mom's to try to determine which babies have some of the most common serious birth defects.
The Human Genome Project in 2001 brought forward the technology that improved prenatal testing. But testing ultimately, for many women, will mean invasive testing through sampling the amniotic fluid with a test called amniocentesis, especially to confirm other tests that may come up abnormal . Now we have NIPT by testing cfDNA! Newer tests can go beyond testing for Down's syndrom and test the whole set of chromosomes of the baby. While this many not on the surface sound "simplified", this testing of mom's blood during early.
After a woman's 10th week of pregnancy, about 10-15% of all of her blood free fragments of DNA are from the placenta. This test does not pick up all abnormalities, and should not be done without early ultrasound tests of fetal health. This has the ability to revolutionize care, as currently about 80-90% of serious birth defects are not picked up until about 18-20 weeks of pregnancy due to the tests that are most frequently used. But it gives parents an option of knowing the health of the baby in a much safer way than amniocentesis. Several companies are now doing this test, and in fact, one option is to find out the sex of your baby, very reliably, and much earlier than ever before!
pregnancy can determine if your baby's chromosomes are normal. It stands for "None Invasive Prenatal Testing" and it's looking for cell-free fetal DNA circulating in mom's blood.