Ultrasounds are not able to reveal a baby's genetic 'make-up' (this can only be obtained through genetic tests). However, up to 40 to 50% of babies with genetic disorders will be picked up by an ultrasound examination. Even so, it is important to remember...
........ultrasounds cannot exclude every possible problem in an unborn baby and not all birth defects can be detected by an ultrasound.>
The baby must be at least 18 weeks gestation for the ultrasonographer to adequately examine all the baby's body systems to screen for abnormalities. However, some abnormalities become more obvious as the baby becomes larger and more developed. For example, the baby's heart is easier to examine thoroughly at around 22 weeks. If the ultrasonographer suspects a possible problem, then a more detailed ultrasound of the baby is usually scheduled at a later time, to specifically examine the baby for the abnormality in question.
The routine '18 to 20' week timing came about because caregivers wanted to strike a balance between picking up as many possible abnormalities as early as possible during pregnancy and hopefully allowing parents more choice, particularly regarding whether they wish to proceed with the pregnancy if an abnormality is detected. You may wish to read abnormality in an unborn baby.
NOTE:Be aware that ultrasounds can have 'false positive' results. This means the ultrasound may indicate the baby has a problem, but is in fact normal and healthy. Depending on the condition 'identified', the experience of the operators and the equipment used, up to 1:10 pregnant women (10%) can be told that their baby has a birth defect, when in fact their baby does not. You can read more in worrying concerns.
As a summary, an anomaly scan involves the ultrasonographer looking at: