The main purpose of any prenatal screening or diagnostic test is, first, the identification of fetuses at risk of serious structural, genetic, metabolic or haematologic abnormalities before the application of a confirmatory diagnostic test. The difference between screening and diagnostic tests is that the former are designed to have high sensitivity but may also have a high false positive rate. In contrast, diagnostic tests are highly accurate on which management and clinical decisions can be made. Screening tests are safe and applicable either universally or to low risk populations whereas diagnostic tests may involve invasive procedures and only applicable to a high risk group or those with a positive screen result.
It is now possible to diagnose a range of conditions ante-natally that previously could only be detectable after birth. This has allowed parents the ability to make much more informed choices about the pregnancy than was hitherto possible. Parents have a myriad of reasons as to why they want prenatal diagnosis. In some instances, it is because of a previously affected child or a family history of a serious genetic condition, in others a raised risk on a screening test may be the impetus to seek a definite result. Whatever their reason, prenatal diagnosis for the vast majority of conditions will require a fetal sample and hence carry a risk of fetal loss. This chapter will concentrate on some of the developments and techniques currently available for prenatal diagnosis and the implications of these advances on clinical practice.
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The first trimester is very important for the mother and the baby. For most women it is common to find out about their pregnancy after they have missed their menstrual cycle. Since, not all women note their menstrual cycle and dates of intercourse, it may cause slight confusion about the exact date of conception. That is why most women find out that they are pregnant only after one month of pregnancy.