There have been tremendous recent advances in the field of obstetrical ultrasound as a result of the evolving technology.
Most babies are born normal however, women have a small risk of delivering a baby with an abnormality.
Though we are not 100% accurate in our prenatal diagnosis, we are able to see many of the fetal abnormalities, sometimes as early as the third month.
A special evaluation is now possible in the third month, at 11-14 weeks, to calculate a mother’s risk for having a baby with a chromosomal abnormality such as Down's syndrome (trisomy 21).
At this same time, we can assess all major organ development of the fetus.
This must still be followed by the second trimester scan in the 5th month, at 20-23 weeks, to ascertain the healthy development of the fetus, look for specific "markers" (signs seen on ultrasound), and recalculate the mother’s risk for delivering a baby affected by Down's syndrome (trisomy 21).
Recently, a new test has become available which enables obtaining genetic material belonging to the fetus from his mother's blood. This test, called NIPT, is a very accurate test for the detection of Down's syndrome and can be done from the third month of pregnancy, at 10 weeks, and poses no risk to the pregnancy.
The only definitive confirmatory test is still an invasive test, the CVS which can be done in the third month beyond 11 weeks, or the amniocentesis which can be done in the fourth month at around 15-16 weeks. However, these carry a 1% chance of miscarriage.