What Causes Excessive Clotting During Periods?

Dear Mam

I am having excessive clotting while periods, is it indication of any problem.Me & my husband are trying for a baby for last 3 months but no success. will these periods have any affect on my conception chances?

My last period dates are as follows:-
1) 13-Feb-14
2) 16-Jan-14
3) 20-Dec-13
4) 15-Nov-13

My BMI is 19.6, taking healty diet, exercise regularly.
Should i have any prebatal vitamin???

Please reply ?

Regards