While prenatal screening and diagnostic tests can be performed at various stages of pregnancy, it is advisable to conduct them during specific gestational weeks to ensure the highest accuracy and reliability. For instance, the 'Combined (Dual) Test' is ideally performed between weeks 12 and 14. The 'Triple' and 'Quadruple Tests' are typically recommended between weeks 16 and 20. The Cell-Free DNA (cfDNA) test, however, can be safely administered from the 9th week of gestation onwards.