What Causes Delayed Periods?

Hi..I got married 9 months back and from 2 months we decided to have a kid. So did consumption during 11-18 days. After that my wife had inrregular periods. Last time after 40th day we did a pg test and it came -ve and gynacologist suggested some tablets and on 48th day she has periods. This month too after 40th day we did pg test and it came -ve. Now again Dr has suggested some medicine. So my Query is why this delay in periods and whats the solution