Why menstrual cycle does not take place before puberty and during pregnancy ?

Explanation

(i) The menstural cycle begins only after the complete development of the gonads .
During puberty , the gonads become functional and increases sex harmone and hence menstrual cycle begins .
(ii) Pregnancy is initiated by fertilization and zygote formation .The zyote gets implanted in the uterus . The ruptured follice develops into a temporart endocrine hormones Uterine wall is thickened to receive the zygote .
IF fertilization does not take place , corpus luteum degenerates , uterine wall ruptures and unfertillized egg is expelled resulting in menstruation . hence menstrual cycle does not occur during pregnancy .