How To Regularise Menstrual Cycle?

hi i am 35 years old and i hav a 5yrs old daughter.my husband and i want another addition to our family but my period timings are not very normal.my period cycle keeps changing sometimes arrives 1 to 2 days ahead and sometimes 2 to 5 day late.my last month period was on 26thnov. and this month is still due.i did pregnancy test this morning but the result is negative. can u pliz help me