National Insurance contributions you make contribute to you being able to claim incapacity allowance, maternity allowance and basic state pension.
When you are self employed you pay Class 2 contributions of £2.65 (20112/13) per week, but they don’t have to be paid if you earn below £5,595 as a sole trader (2012/13). However, if you don’t pay these contributions make sure you understand that you will be giving up rights to incapacity allowance and maternity allowance, and possibly basic state pension.
Currently, you need 30 years of NI contributions to make the state pension, so be careful that you will reach this if you have a year or two out of the NI system. If you are claiming Child Benefit for children under 12, you should still receive credits towards your state pension.