hi! intra uterine contraceptive devices or IUCDs as they are called are quite safe to be used after second pregnancy. however as you have undergone a c-section twice i would advice you use it after a period of three months as the scar will have to heal.it starts working within hours of insertions.you have to make sure of it s position however by checking the thread that hangs outside the cervix into the vagina.it is contraindicated in pelvic inflammatory diseases,endometriosis,adenomyosis.