Non-invasive prenatal testing for Down syndrome in general maternity services
Since its discovery in 1997, the presence of cell-free fetal DNA in the maternal bloodstream has been put to clinical use to detect variety of fetal conditions, in the antenatal period. The use of fetal DNA can offer a highly accurate screen for the presence of Down syndrome (trisomy 21). This has numerous advantages over standard first trimester combined screening for Down syndrome; for example, a reduction in miscarriages due to its non-invasive nature. This article considers a number of issues that need to be resolved before widespread implication of this type of screening into standard NHS practice.
For a woman hoping to start a family, discovering two lines on her urine pregnancy test may initiate profound excitement at a future vision of motherhood. This is followed in the subsequent weeks by a realisation that pregnancy is a potentially difficult journey, with many choices and challenges to navigate. One important decision the woman faces is whether to have first trimester screening to assess her risk of having a baby with Down syndrome. Currently, in the UK, approximately two thirds of all pregnant women opt for Down syndrome screening, with about one third declining the test for various reasons including religious, cultural or personal factors.
Down syndrome is a condition caused by an extra (and so three in total, hence ‘trisomy’) chromosome number 21 in all cells of the body. At least 95% of cases are not inherited, instead occurring due to a chance error in cell division called meiosis, which happens before the time of conception. First meiosis occurs during fetal life in females, and second meiosis occurs with formation of the ovum during each menstrual cycle. Although the risk of Down syndrome increases with advancing maternal age, more babies with Down syndrome are born to younger women (because more babies in total are born to younger women). With more than 37 000 babies born in the UK with Down syndrome in 2011 (Wu and Morris, 2013), this condition is one of the most common causes of disability. However, the clinical characteristics of Down syndrome are variable. Many people with Down syndrome have some degree of learning disability, around 50% have a congenital cardiac anomaly, and early-onset dementia and leukaemia are more likely (Polk et al, 2015). Despite these potential health risks, many people with Down syndrome attend mainstream school during childhood and are in work as adults (Jensen and Bulova, 2014).
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