Amniotic fluid will contain fetal skin, urogenital and pulmonary epithelial cells and cells from the extra-embryonic membranes. The cells are first concentrated by centrifug-ing and are cultured for 7-10 days with the result of the karyotype being available in 14-15 days. Culture failure occurs in about 0.5% and can be minimized by taking an adequate volume of fluid and limiting the delay from the sample being taken and sent to the laboratory. There is also the possibility of in vitro mitotic abnormalities occurring as a result of the culture process and therefore multiple cultures are performed for each amniocentesis. Only if a mosaicism or karyotypic anomaly occurs in more than one culture is it likely to be of clinical significance and will need further investigation. This occurs in 0.1-0.2% of samples and may require confirmation on fetal blood sampling.
More recently chromosome-specific probes and fluorescence in situ hybridization (FISH) techniques have been developed to detect numerical aberrations in interphase as well as, non-dividing cells, eliminating the need for prolonged cell culture. Most cytogenetic laboratories will now offer rapid prenatal diagnosis of amniotic fluid using either, fluorescence-based probes for short-tandem-repeat (STR) markers on chromosomes 21, 13 and 18 and poly-merase chain reaction (PCR) amplification of these STRs , or FISH and fluorescence-labelled DNA probes for chromosomes 21,18 and 13 . This allows identification of the three commonest chromosomal anomalies within 3 working days.
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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.