if at all it is downs or trisomy 18 it is a genetic disorder and you dont have any control in rectification. the initiall tests will be indicating what it is as only screening test is done. the period or other factors have no influence in the diagnosis of downs or trisomy 18. what will be the next step is they will do a procedure where in they will do testing of the fluid which the baby secretes and do genetic analysis. if it comes positive for downs or trisomy 18 than it is confirmatory, and they might suggest you to discontinue the pregnancy as the child born will be with multiple defects including mental retardation.