Corpus luteum in pregnancy has a long life. However, if fertilisation does not take place, it remains active only for $10-12$ days. Explain.

Broken graafian follicle get convert into corpus leuteum and secretes progesterone in large amount that is necessary for care of endometrium. Such endometrium becomes necessary for fertilization ovum (zygote) of implantation and other events of pregnancy. Due to this reason there is a long life of corpus leuteum in pregnancy. But no care of endometrium necessary in absence of fertilization. Therefore corpus leuteum decomposes into $10$ to $12$ days.